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TUNGSTEN COIL ELECTROTHERMAL VAPORIZATION FOR ATOMIC SPECTROSCOPY
BY
SUMMER N. HANNA
A Dissertation Submitted to the Graduate Faculty of
WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES
in Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
Chemistry
May 2011
Winston-Salem, North Carolina
Approved By:
Bradley T. Jones, Ph.D., Advisor
Clifton P. Calloway, Ph.D., Chair
Ulrich Bierbach, Ph.D.
Christa L. Colyer, Ph.D.
Willie L. Hinze, Ph.D.
i
DEDICATION
This dissertation is dedicated to my grandmother, Geraldine Gibson, the first person to take me into a lab.
ii
ACKNOWLEDGEMENTS
I would like to first off thank my parents and family for always being a wellspring of enthusiasm, encouragement, and belief in my abilities even and most importantly when I doubted myself. They have been a constant source of compassion and strength throughout my life, and I know that there is no way that I would have ever been capable of “eating other people’s lunch” without them.
To those who have contributed to my education, thank you for encouraging me to constantly challenge myself. To my committee members, Dr. Hinze, Dr. Bierbach, Dr. Calloway, and Dr. Colyer I appreciate all of the time you have taken to direct me through this extensive process. Thank you to Dr. Owens who convinced me that it was infinitely better to be a chemistry major than anything else I could have done in college and then that it would be silly of me to not go to graduate school. You’ve been a steadfast source of enthusiasm and encouragement. Dr. Calloway, thank you for your continued support over the years. You are an inspiring educator and it has been such a pleasure working for you as an undergraduate and now alongside you over the past 7 years. Dr. Bierbach, I appreciate all of the time you’ve taken to counsel me regarding chemistry, my career, and life . You’ve helped me become a better professional and your dedication to students is amazing. Dr. Jones, thank you for your guidance throughout graduate school. Your ability to always see the humor in any situation, enthusiasm for education, and analytical chemistry savvy continues to motivate me to be a better chemist. I appreciate all of the long hours you have dedicated to your students and I am so grateful for the wonderful experience you have provided me.
To my friends, at Wake Forest and beyond, you have each touched my life in a way that I will never forget and I want to take the opportunity to thank you for making my world a better place. Whether you are a friend from childhood, Winthrop, or Salem Hall, I appreciate the time that you have taken to be my friend and the constant support that you have provided. Thank you April and Scott for forcing me to show up for my first day at Wake, even when I wanted to just set up a new life in Rock Hill. A special thanks to my lab group—Carl, Jiyan, and George. I’m pretty certain that I’ve spent more time with you guys than anyone else I know in the last four years. It’s been fantastic to have you all to bounce ideas off of and I’m certain that you’ve helped me become a better scientist. To the lunch group—Chris, Julie, Junker, Erika, and Tanya, I’m so thankful for you all and all the laughs and stories we’ve shared. You all are a fantastic group of friends and I’m so lucky to have you. Amanda—I’m glad that we’ve had the opportunity to become close and thankful for all of our OG days to celebrate, commiserate, and sometimes just get away. Thank you Leigh Ann for being my first friend in Winston—I’ll never forget our special bond and the ability to become immediate friends. I’ll miss dinner nights, OG, accidental twin days, and having such an amazing support system throughout graduate school. Rachel, I’ll always be grateful that I was lucky enough to find a hetero-life mate at 8 years old and even when we’re hundreds of miles apart our relationship is still as close as it was when we were cruising around in your Escort drinking Cherry Cokes. Additionally, thank you to all of the chemistry department graduate students. You all add to a fantastic program that feels more like a family than colleagues and each have made every day in Salem a pleasurable experience.
A special thanks to Daniel for being capable of handling all of the craziness that graduate school has brought out in me. You’ve been the one daily who encourages me and also reminds me that I should take time to enjoy life. I’m amazed by everything that we’ve encountered over the past eight years and your constant talent, humor, and enthusiasm. I’m so thankful for your presence in my life and I am excited to begin a new journey with you.
To anyone I may have left out, thank you for your contribution to making me the person I am today. I love you each, and I am blessed to have you in my life.
iii
TABLE OF CONTENTS
List of Illustrations and Tables ...................................................................................................... VI
List of Abbreviations .................................................................................................................. VIII
Abstract .......................................................................................................................................... IX
Chapter 1: Introduction: A review of tungsten coil electrothermal vaporization as a sample
introduction technique in atomic spectrometry ................................................................................ 1
Introduction .................................................................................................................................. 2
Atomization Theory ..................................................................................................................... 4
Atomic Absorption Spectrometry ................................................................................................ 7
Atomic Emission Spectrometry.................................................................................................... 9
Atomic Fluorescence Spectrometry ........................................................................................... 11
Conclusion .................................................................................................................................. 15
References .................................................................................................................................. 16
Chapter 2: Design of a portable electrothermal vaporization flame atomic emission spectrometry
device for field analysis ................................................................................................................. 28
Introduction ................................................................................................................................ 30
Experimental .............................................................................................................................. 32
Instrumental ............................................................................................................................ 32
Visual Basic Program ............................................................................................................. 37
iv
Reference Solutions and Standards ........................................................................................ 44
Safety Considerations ............................................................................................................. 44
Results and discussion ................................................................................................................ 45
System Optimization .............................................................................................................. 45
Limits of Detection ................................................................................................................. 45
Conclusion .................................................................................................................................. 50
References .................................................................................................................................. 51
Chapter 3: An electrothermal vaporization flame atomic emission spectrometer ......................... 54
Introduction ................................................................................................................................ 56
Experimental .............................................................................................................................. 59
Instrumental ............................................................................................................................ 59
Reference Solutions and Standards ........................................................................................ 66
Safety Considerations ............................................................................................................. 66
Results and Discussion ............................................................................................................... 67
System Optimization .............................................................................................................. 67
Gas Phase Excitation Temperature Mapping of ETV-FAES Flame ...................................... 70
Analytical Figures of Merit .................................................................................................... 71
Conclusion .................................................................................................................................. 77
References .................................................................................................................................. 78
Chapter 4: Design of a compact, aluminum, tungsten-coil electrothermal vaporization device for
inductively coupled plasma-optical emission spectrometry .......................................................... 80
Introduction ................................................................................................................................ 82
v
Materials and Methods ............................................................................................................... 84
Reagents.................................................................................................................................. 84
Instrumental ............................................................................................................................ 84
Results and Discussion ............................................................................................................... 91
Optimization of Experimental Conditions .............................................................................. 91
Optimization of Torch Coolant Gas ....................................................................................... 91
Optimization of Radio Frequency Power ............................................................................... 91
Optimization of Carrier Gas ................................................................................................... 91
Limits of Detection ................................................................................................................. 96
Validation of Method .............................................................................................................. 98
Conclusion ................................................................................................................................ 100
References ................................................................................................................................ 101
Conclusion ................................................................................................................................... 104
Curriculum Vitae ......................................................................................................................... 106
vi
LIST OF ILLUSTRATIONS AND TABLES
Chapter 1 .......................................................................................................................................... 1
Table I. Limits of detection of various W-coil sample introduction methods ............................ 13
Chapter 2 ........................................................................................................................................ 28
Table I. Vaporization program for the ETV-FAES Device ....................................................... 35
Figure 1. Illustration of the ETV-FAES Device ......................................................................... 36
Figure 2. ETV-FAES control electronics ................................................................................... 39
Figure 3. Screen capture image of the Visual Basic Control Program ....................................... 41
Figure 4. ETV-FAES spectrum for 5.0 mg L-1 barium and 2.0 mg L-1 rubidium with sodium
present as a contaminant ........................................................................................................... 42
Figure 5. ETV-FAES emission profile for 5.0 mg L-1 cesium, 5.0 mg L-1 barium, and 2.0 mg L-1
rubidium ..................................................................................................................................... 43
Table II. Optimized operating parameters for ETV-FAES Device ............................................ 47
Figure 6. Illustration of 6-MTH-N Cutting Tip .......................................................................... 48
Table III. ETV-FAES Limits of Detection ................................................................................. 49
Chapter 3 ........................................................................................................................................ 54
Table I. Heating program for the tungsten-coil vaporizer .......................................................... 61
Figure 1. Photograph of the ETV-FAES Device ........................................................................ 64
Figure 2. End-on view of the cutting tip during the vaporization cycle ..................................... 65
Figure 3. Variation in emission signal level with orifice diameter for a 1 µg/mL Mn solution . 68
vii
Figure 4. Variation in emission signal level with ETV gas flow rate for a 1 µg/mL Mn solution
.................................................................................................................................................... 69
Figure 5. Axial temperature map of the ETV-FAES flame ....................................................... 72
Figure 6. Radial temperature map of the ETV-FAES flame ...................................................... 73
Figure 7. ETV-FAES emission spectrum for NIST SRM 2711 ................................................. 74
Table II. Figures of merit for nineteen elements of interest ....................................................... 75
Table III. Elemental Recoveries for WPS-1 and SRM 2711 ...................................................... 76
Chapter 4 ........................................................................................................................................ 80
Figure 1. Illustration of ETV cell .............................................................................................. 85
Figure 2. Photograph of the ETV cell and ICP torch connection ............................................... 86
Table I. Heating program for the tungsten-coil vaporizer .......................................................... 89
Figure 3. Time resolved emission profile of barium, manganese, and strontium found in SRM
2643e .......................................................................................................................................... 90
Table II. Optimized operating parameters .................................................................................. 93
Figure 4. Effect of Ar coolant gas flow rate on the emission intensity of 0.50 mg/L iron ......... 94
Figure 5. Effect of H2/Ar carrier gas flow rate on the emission intensity of 0.50 mg/L iron ..... 95
Table III. Figures of merit for twelve elements of interest ......................................................... 97
Table IV. Recoveries for seven elements of interest .................................................................. 99
viii
LIST OF ABBREVIATIONS
Abbreviation Definition AAS Atomic Absorption Spectrometry AES Atomic Emission Spectrometry AFS Atomic Fluorescence Spectrometry
APDC Ammonium pyrrolidine dithiocarbamate CCD Charge Coupled Device CPE Cloud Point Extraction ETV Electrothermal Vaporization
ETV-FAES Electrothermal Vaporization-Flame Atomic Emission
Spectrometry FAAS Flame Atomic Absorption Spectrometry FAES Flame Atomic Emission Spectrometry
FF-AAS Flame Furnace-Atomic Absorption Spectrometry FI-AAS Flow Injection-Atomic Absorption Spectrometry
FOM Figures of Merit GFAAS Graphite Furnace Atomic Absorption Spectrometry
I Current ICP-AES Inductively Coupled Plasma Atomic Emission Spectrometry ICP-MS Inductively Coupled Plasma Mass Spectrometry
K Kelvin LOD Limit of Detection
L-PAD Large Programmable Array Detector LPM Liters per Minute
m Slope MIBK Methyl Isobutyl Ketone NPT National Pipe Thread RF Radio Frequency
SRM Standard Reference Material TRA Time Resolved Analysis
V Voltage VB Visual Basic
W-coil Tungsten Coil
ix
ABSTRACT
Electrothermal vaporization has served as an alternative sample introduction method in atomic
spectroscopy for some time. Typically, electrothermal vaporizers made of graphite, but these
devices do not lend well to mass production, are expensive, and require large power supplies for
operation. In addition to these structural and operational limitations, they encounter significant
matrix interferences when using samples that contain carbide-forming species or have high
salinity or organic concentrations.
Metal atomizers (in particular the tungsten coil) are a good alternative to these graphite
electrothermal vaporizers. Tungsten coils are commercially produced, easy to use, and do not
require the large power sources of their graphite counterparts. They also are much less porous,
which allows them to easily quantify solutions with both high salt and surfactant contents.
Tungsten coils permit a large degree of user flexibility, and they have found use in both
instrumentation and sample introduction techniques.
This dissertation will focus on the use of the tungsten coil electrothermal vaporizer as a sample
introduction technique in both novel and traditional instrumentation. Here the tungsten coil has
been used as a sample introduction technique in flame atomic emission spectrometry using a
novel flame source: a welder’s torch. The torch has been configured to accept samples introduced
as vapor into a high temperature flame where the resulting emission has been collected by a
charge coupled device spectrometer. This potentially portable instrument quantified metals in
both water and soil samples and species spanned a wavelength range of 324-852 nm.
Tungsten coil electrothermal vaporization sample introduction was also used in inductively
coupled plasma optical emission spectrometry by the introduction of a new cell design into the
space formerly occupied by a spray chamber and nebulizer. This arrangement allowed for the
x
multielement determination of metals in the ultraviolet and visible region with an average
detection limit improvement factor of fourteen when compared to traditional nebulization sample
introduction techniques.
1
CHAPTER 1
INTRODUCTION
A Review of Tungsten Coil Electrothermal Vaporization as a Sample Introduction Technique in Atomic
Spectrometry
Summer N. Hanna, Bradley T. Jones
Department of Chemistry, Wake Forest University,
Winston-Salem, NC, 27109 USA
Note: The following manuscript was published in Applied Spectroscopy Reviews
(DOI: 10.1080/05704928.2011.582659). Stylistic variations are due to the requirements of the journal.
The manuscript was prepared by Summer N. Hanna and edited by Bradley T. Jones.
2
INTRODUCTION
Electrothermal vaporization (ETV) has had widespread use throughout spectroscopy for some time. ETVs
are often made of graphite, but these ETVs require large power supplies, often do not produce isothermal
heating, and suffer from the formation of refractory compounds during atomization. In the early 1970s,
Williams, Reid, and Piepmeier identified tungsten filaments for use as atomizers in atomic spectroscopy.
These studies used commercially available, 24-Watt, tungsten light bulbs, which were chosen for their
precise construction, simplicity, and ability to measure nanogram amounts of a single dissolved element.
They were positioned in X-Y optical benches for adjustment and then used in atomic absorption
spectrometry. All viewing was done directly above the coil in conjunction with a hollow cathode lamp,
and researchers were able to obtain relatively interference free spectra from these alternative ETVs. 1, 2
Over the forty years since Williams’ discovery, tungsten coils (W-coils) have been used in a wide variety
of applications. The attributes that caused the initial employment in spectroscopy (ease of use, simple
design, affordability, and commercial manufacture) has made them widely accessible to a variety of users.
In 2002, Hou and Jones cited 179 occurrences where tungsten devices had been successfully employed in
analytical atomic spectroscopy.3 In the decade since that review, there remains significant interest in the
area of tungsten coil ETVs with work spanning atomic absorption, emission, and fluorescence
spectrometry (AAS, AES, and AFS, respectively). Advances have included the development of new
atomization cells and power supplies, the construction of new instrumentation, and the evaluation of
chemical modifiers. Applications cover areas such as the determination of the lanthanides, which are
notoriously difficult to determine, biological and environmental samples, and even the analysis of
materials used in biomedical engineering. 4-46 Their use is wide-reaching across not only the area of
atomic spectroscopy but also with notable interest coming from the developing world, in particular Brazil
and China. 9, 10, 23, 26, 27, 31, 32, 39, 41-44, 46-53
3
In addition, tungsten coil devices are ideal sample introduction sources. They have been hyphenated to
systems including flow injection AAS (FI-AAS), inductively coupled plasma optical emission
spectrometry (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS). 54-57 The
following review will highlight the theory and use of the tungsten coil as a sample introduction method
covering the areas of AAS, AES, and AFS over the past decade since the Hou and Jones review.
4
ATOMIZATION THEORY
The mechanism for atomization in the tungsten coil is a process that is rarely addressed in the literature,
despite the amount of attention given to W-coil ETV techniques. There have only been a few
investigations into the atomization process, but all findings are supportive of one another.
Initial studies into the atomization of metallic surfaces occurred during the early 1980s. Tungsten tubes
were constructed with the design of graphite furnace cylinders in mind so that they could easily be
inserted into a traditional graphite furnace atomic absorption spectrometer with only the simplest
modifications required to the instrumentation. Two tungsten strips were fabricated to form a cylindrical
tube that could be connected to an electrode within the atomizer, which had been enclosed and shielded
with an inert gas (argon). Modifications were made to a traditional graphite furnace power supply so that
it could generate the lower voltages associated with metal atomizers. An optical pyrometer was then used
to determine the surface temperature of the tungsten tube. An atomization temperature of 3250 K was in
agreement with previous temperature studies of graphite furnaces, but a significant increase in the
sensitivity occurred when using the metal atomizer. 58, 59 This increase was due to a lag between the
temperatures associated with the surface temperature and the vapor phase in graphite furnaces, but when
metal atomizers were used this lag was less pronounced. 60, 61 This explanation of the enhancement seen in
tungsten atomizers also characterized the high heating rate, near isothermal atomization, and lack of
interference encountered with graphite-metal compounds.
In the late 1990s and early 2000s Krakovská addressed the theory of atomization 64, 65 and clearly
explained the different types of reactions that occur when using a tungsten atomizer. There are three basic
reaction types that occur when atomization from a tungsten surface occurs. First is the evaporation of the
metallic analyte (1 and 2):
5
M(s) M(l) M(g) (1)
M(s) M(g) (2)
The second is the reduction of metallic oxides by hydrogen from the protecting gas according to (3).
Queiroz supported these findings 62 and also proved that certain metals including sodium, potassium,
calcium, barium, and magnesium depend on the presence of hydrogen for atomization.
MO(s, l, g) + H2(g) M(s, l, g) + H2O(g) (3)
Finally the third reaction is between metallic oxides with tungsten forming tungsten oxides and the
gaseous metal analyte (4-6):
MO(s, l, g) + W(s,g) M(s, l, g) + WO(g) (4)
2MO(s, l, g) + W(s,g) 2M(s, l, g) + WO2(s, g) (5)
3MO(s, l, g) + W(s,g) 3M(s, l, g) + WO3(s, g) (6)
When this third reaction type occurs, the tungsten oxides are immediately reduced by hydrogen to
regenerate the metallic tungsten surface of the atomizer (7-9):
WO(g) + H2(g) W(s, g) + H2O(g) (7)
WO2(g) + 2H2(g) W(s, g) + 2H2O(g) (8)
WO3(g) + 3H2(g) W(s, g) + 3H2O(g) (9)
In addition to the nine reactions seen above (which occur on the tungsten surface), there are also four
additional reaction types that may occur. Reactions that take place at the wall of the atomizer between
residue components and the metallic surface after evaporization; reactions of the metallic surface of the
6
atomizer with gaseous components of the system; reactions of a condensed analyte with gaseous
components of the system, in particular hydrogen; and homogenous reactions between gaseous
components of the protective gas and evaporated analytes. 3, 63, 64
7
ATOMIC ABSORPTION SPECTROMETRY
Using the tungsten coil as a sample introduction method has been popular in conjunction with hydride
trapping. In AAS, this technique takes advantage of the formation of hydrides by As, Se, Sb, Bi, Ge, Sn,
Te, or Pb with NaBH4 when in acidic solutions. The sample is trapped on the tungsten coil and upon
release from the coil, the analyte is freed from the associated matrix and transported to the atomizer as a
gas. The atomizer is often a silica tube in a furnace or flame where the hydride decomposes and the
resulting absorption or emission measurement is acquired. Analytes studied by this method are often very
volatile and extremely toxic, so they are of great scientific interest.65, 66
Cankur 67 utilized hydride generation AAS for the determination of bismuth in both water and geological
materials. Bismuth was studied because of its recently determined antibacterial effectiveness towards h.
pylori, which excretes acid as a waste product and is attributed to ulcer formation within the
gastrointestinal tract.68 On-line trapping of bismuth was completed using a tungsten coil placed within
the arm of a silica T-tube atomizer. This atomizer was heated using an air-acetylene flame with samples
carried by a mixture of HCl and NaBH4. The resulting bismuthine was collected on a heated tungsten coil,
and the trapped species was found to be stable on the coil for at least 15 minutes. In order to release the
analyte, the coil was quickly heated to a temperature of 1200 °C. The quick heating made possible
through the use of the tungsten atomizer produced sharp, reproducible signals which produced limits of
detection (LODs) with an improvement factor of 150 times that of traditional flow injection hydride
generation AAS.
Permanent modifiers were utilized by both de Souza12 and Xi43 in their determination of selenium, arsenic,
and tellurium. The benefits of permanent modifiers, also known as matrix or chemical modifiers, with
tungsten coils are argued to be the same as when using them in graphite furnaces: increased sensitivity for
volatile analytes with a decrease in matrix effects and an increased atomizer lifetime. In the selenium and
8
arsenic studies a rhodium-coated coil was used to analyze samples of animal blood, bovine muscle, and
drinking water.12 The modifier improved the sensitivity of arsenic by a factor of 2. When compared to an
untreated coil, selenium absorbance dropped at temperatures higher than 250 °C, but when a modified
coil was used selenium was thermally stable up to a temperature of 1400 °C.12 Tellurium was quantified
using a platinum coated tungsten coil that vaporized samples of stream sediments and soil into an
electrically heated quartz tube atomizer. Again, improved trapping was seen when using the modified coil,
and gave rise to enhanced sensitivity and LODs (Table I) when a sample volume of only 1.5 mL was
used.43
Flame furnace AAS was studied for its ability to quantify 25 volatile and semi-volatile elements. 49 This
technique used a tungsten coil atomizer connected to a quartz flame furnace positioned above a flame
burner head for heating. Some analyte loss was encountered during the transport to the quartz furnace and
was overcome by reducing the length of transport tubing. This loss is thought to occur because once the
sample is released by the coil, it is carried through the “cool” transport tubing by the carrier gas where it
may condense on the walls of the transport material. Despite this challenge, 15 of the 25 elements in
Table I saw improvements in their limits of detection. Volatile and semi-volatile metals both were
enhanced through the use of the tungsten coil sample introduction system. Part of this enhancement was
that the flame furnace no longer used energy for desolvation of the sample. With the 10 refractory
elements, no enhancement was seen because the flame furnace was not hot enough to excite these
elements as they typically utilize a nitrous-oxide-acetylene flame that burns about 1000 degrees hotter
than the air-acetylene flame used in this experiment.
9
ATOMIC EMISSION SPECTROMETRY
Over the past decade tungsten coil sample introduction in atomic emission spectrometry has focused on
inductively coupled plasma techniques. Historically, sample introduction methods for ICP have been
limited to nebulization, but when using this introduction method approximately 88-97 % of the sample is
lost to waste.69 Multiple techniques have been hyphenated to ICP-OES including liquid chromatography,
ion chromatography, gas chromatography, and capillary electrophoresis, but these methods still use
nebulizers so the sample loss associated with nebulization remains. 70-73 In order to overcome this loss,
electrothermal vaporization has been used as a successful alternative. Laser ablation and graphite furnaces
ETV sample introduction sources allow nearly 100 % of the sample to be introduced to the plasma.74, 75 In
addition to these methods, tungsten coils have been successfully used as alternative sample introduction
systems for ICP analysis. Users often turn to W-coils because they do not require large power supplies or
laser systems needed by other techniques and are easy to use by comparison.
Davis focused her research of W-coil ICP-OES on the urinary excretion of cadmium as a biomarker for
diseases including osteoporosis, prostate cancer, or pancreatic cancer. 76 Three approaches to quantifying
urinary Cd were used: direct determination, chelation using ammonium pyrrolidine dithio-carbamate
(APDC) with a methyl isobutyl ketone (MIBK) extraction, and the use of Pd as a permanent modifier on
the tungsten coil. When using the direct determination method, cadmium content was quantified using
the standard addition method with and without a bismuth internal standard. Analysis was completed
using a glass tungsten coil vaporization cell with three ports; one of which fit to the ICP with an 8-inch
joint socket connected with a Teflon bushing and o-ring. The tungsten coil was heated with a laboratory
constructed power supply that was controlled with a home-written Visual Basic program. All subsequent
studies mentioned here completed by Davis used this instrumental setup. This method produced results
that agreed well with those determined by graphite furnace atomic absorption spectrometry.
10
The use of APDC chelation with a MIBK extraction was then studied for its ability to chelate a variety of
elements in conjunction with the MIBK extraction as a preconcentration step. This method successfully
liberated the cadmium from the complicated matrix of urine and a bismuth internal standard was once
again used to improve precision and accuracy. The emission spectra gave rise to a Cd line at 228.8 nm
and a secondary emission line at 226. 5 nm, which allowed for increased confidence in measurements
since two locations were available to distinguish the signal from background. Chelation and extraction of
the urine samples produced relatively interference free spectra from an otherwise complicated matrix. 77
Davis continued to investigate methods of Cd determination, but focus shifted towards permanent
chemical modifiers for the tungsten coil. 78 Here palladium was used as the modifier and was adhered to
the coil by depositing small volumes of Pd solution onto the coil and drying at low currents. Samples
could then be pipetted onto the coil and the Pd modifier remained effective after 150 coil firings.
Rodríguez analyzed silicon based minerals using ultrasonic slurry sampling with a tungsten coil using
ICP-MS. 79 A quartz ETV cell was constructed and coupled to a quadrupole based ICP-MS. Samples
were sonicated for uniformity and then pipetted onto the tungsten coil and the gaseous analyte was
transported to the plasma via a glass tube. Samples of interest included micronized talc and quartz
samples. Slurry analysis allowed the user to avoid dangerous chemicals such as hydrofluoric acid in
addition to reducing sample preparation time and avoiding possible contamination. Slurry W-coil ICP-MS
did suffer from several drawbacks when compared to nebulization. LODs were 2-10 times higher than
when nebulization was used, which was thought to be because of the reduced sample volumes. Significant
matrix signal suppression also occurred, which contributed to poor LODs making standard additions
necessary for analysis. Limits of detection ranged from 1.3 to 0.002 ng g-1 and were low enough to
determine the elements of interest in the study. Concentrations found in the slurry samples were in
agreement when compared to conventional nebulization ICP-MS. Benefits of avoiding the hydrofluoric
acid through direct slurry analysis were deemed to outweigh the lack of enhancement.
11
ATOMIC FLUORESCENCE SPECTROMETRY
Hydride generating analytes were again of interest in atomic fluorescence spectrometry using a tungsten
coil sample introduction system that had been coupled to an argon/hydrogen flame. As, Bi, Cd, Ge, Pb,
Sb, Se, Sn, and Te in water and rice were quantified by depositing aqueous samples onto a tungsten coil,
which then sent the vaporous sample via carrier gas to an electrically heated wire coiled around a quartz
tube atomizer. 51 This heated wire was used to ignite the sample-containing Ar/H2 carrier gas.
Fluorescence signals were then collected using a photomultiplier tube and transported to a personal
computer for processing. Some scattering interferences were seen in samples that had a naturally high salt
concentration or very complex matrix. These NaCl crystals deposited on the surface of the coil and
scattered incident light. Chemical modifiers could be used to overcome the salinity in the samples, but in
this instance simple dilution was chosen to alleviate the interferences. LODs were superior to both flame
AAS (FAAS) and ICP-OES for all elements of interest except for germanium, as shown in Table I.
Using cloud point extraction (CPE) cadmium was quantified by a W-coil coupled to an Ar/H2 flame for
fluorescence studies. Cloud point extraction is a liquid-liquid extraction and preconcentration technique
that uses surfactants to eliminate matrix effects found in complex samples. This technique is not often
used with FAAS or ICP-OES because the surfactants deposit carbon on surfaces as they are atomized,
damaging the instrumentation. The low porosity of the W-coil makes this carbon deposition less
detrimental to the instrumentation, making it an ideal sample introduction source. CPE eliminates sample
salinity, so scattering of the signal is no longer of concern when the W-coil is coupled to the Ar/H2 flame.
Water and rice samples were studied for their cadmium content, associated with concerns in China about
exposure to Cd through food and water sources from Cd-Ni battery production found within the country.80
When using CPE, faster desolvation times occurred due to the use of the tetrahydrofuran solvent, but
longer pyrolysis steps were needed to remove the high concentration of organics from the surfactant rich
separation. In addition, a Cd-dithizone complex formed during the extraction, making the atomization
12
process different from mechanisms previously mentioned (atomization theory). The complex is broken by
thermal dissociation and free Cd atoms were formed through a sulfide intermediate on the atomizer in the
presence of the reducing atmosphere around the coil. This dithiozone complex enhanced Cd
measurements because it acted as a chemical modifier in conjunction with the Triton X-114 surfactant on
the surface of the tungsten coil. Through the use of this method, absolute LODs for cadmium were found
to be more sensitive compared to both vapor generation AFS and electrothermal AAS.40
A tungsten coil trapping system was coupled to a graphite fiber felt electrothermal AFS system for the
quantification of cadmium in several food materials: cabbage, spinach, tea, chicken, and rice powder. 81
The proposed graphite fiber felt ETV coupled to the W-coil trap allowed the user to place a solid sample
on the graphite ETV and then trap the vaporized sample on the tungsten coil as in hydride trapping. When
this trapping occurred, it released the cadmium free of almost all matrix interferences at a temperature of
2000 °C. The organic matrices were removed through an ashing step on the graphite and afterwards the
inorganic matrix was trapped on a room temperature coil. The complicated matrix was separated from the
analyte, which was swept into the AFS spectrometer by the carrier gas. This method allowed for sub-
picogram levels of cadmium to be determined using AFS.
13
Table I. Limits of detection of various W-coil sample introduction methods
Element(s) LOD Technique Vaporization Temperatures (°C)
Application Ref.
Te 0.08 ng mL-1
Hydride trapping AAS
1630 geological samples 43
Bi 0.0027 ng mL-1
Hydride trapping AAS
1200 geological samples and water
69
Cd 0.5 µg L-1
Flame furnace AAS
2000 aqueous single element standards
52
Ag 0.9 Pb 25 Tl 10 Zn 3 Cu 2 Sb 15 Bi 15 Te 10 Sn 1000 In 10 As 30 Se 100 Au 50 Fe 250 Co 300 Ni 200 Cr 250 Mn 10 Pd 200 As 10 µg
L-1 Ar/H2 flame AFS without chemical vapor generation
1730 water and rice 51
Bi 2 Cd 0.08 Ge 50 Pb 3 Sb 3 Se 30 Sn 20 Te 3 Cd 0.01 µg
L-1 Ar/H2 flame AFS with cloud point
extraction
1900 water and rice 40
Cd 30 ng L-1 Graphite fiber felt vaporizer with
tungsten coil trap AFS
1000~2300 cabbage, spinach, tea, chicken, rice
powder
81
As 0.5 ng g-1
Ultrasonic slurry sampling ETV-
ICP-MS
NR Micronized talc, quartz
80
14
Ce 0.8 Cd 0.9 U 0.3 V 0.5 Pb 1.3 Li 0.9 Rb 0.5 Sb 0.6 Ba 3.3 Cd 0.15 µg
L-1 ICP-AES NR urine 77
Cd 228.8 nm: 0.04
226.5 nm: 0.2 µg L-1
ICP-AES with ammonium
pyrrolidine dithio-carbamate chelation
NR urine 78
Cd 0.2 µg L-1
ICP-AES using Pd as a chemical
modifier
NR urine 79
NR= not reported
15
CONCLUSION
Tungsten coil electrothermal vaporization as a sample introduction method has undergone a variety of
uses since its conception forty years ago. Table I has highlighted the limits of detection found in sample
introduction systems since 2002, and oftentimes W-coil methods are comparable with instrumentation
that is routinely used in the laboratory. Samples span a wide range with both biological and industrial
significance, illustrating the versatility of the technique. This body of work will focus on the use of a
tungsten coil electrothermal vaporizer as a sample introduction system for both flame atomic emission
spectrometry and inductively coupled plasma optical emission spectrometry. The accessibility of the
tungsten coil ETV allows continued advances in atomic spectroscopy, a field which some argue has
reached maturity. However, the use of this particular device illustrates that innovation from relatively
benign sources can easily open doors in scientific advancement that others assumed closed.
16
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113. Maestre, S. d. L.-V., M.T.C., Plasma behavior during electrothermal vaporization sample
introduction in inductively coupled plasma atomic emission spectrometry. Spectrochimica Acta Part B
2001, 56, 1209-1217.
28
CHAPTER 2
Design of a Portable Electrothermal Vaporization Flame Atomic Emission Spectrometry Device for Field
Analysis
Summer N. Hanna,a Joseph Keene,a Clifton P. Calloway, Jr.,b Bradley T. Jonesa
a Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina, 27109 USA
b Department of Chemistry, Physics, and Geology, Winthrop University, Rock Hill, South Carolina, 29733 USA
Note: The following manuscript was written for publication in Instrumentation, Science, and Technology.
Stylistic variations are due to the requirements of the journal. All of the presented research was conducted
by Summer N. Hanna with the exception of the results for lithium which were generated by Joseph Keene
under the direction of Summer Hanna. Computer programming was completed by Clifton P. Calloway, Jr.
The manuscript was prepared by Summer N. Hanna and edited by Bradley T. Jones.
29
ABSTRACT
A non-traditional, portable flame atomic emission spectrometry device has been constructed using a
welding industry metal cutting torch as the flame source with a tungsten coil electrothermal vaporization
device for sample introduction. Twenty-microliter sample aliquots were pipetted onto a commercially
available tungsten filament and a home-written Visual Basic program was used to automatically dry and
atomize the sample. The sample then traveled into the heart of the air-acetylene flame where it was
excited, and the resulting emission signal was focused onto the entrance of a miniature charge coupled
device spectrometer. Figures of merit were determined for five alkaline and alkali earth metals of
biological and environmental significance. Limits of detection were, on average, 100 times lower than
those for traditional flame emission spectrometers. The capabilities of this portable device make it an
ideal tool for biological field analysis, particularly in areas where traditional laboratories are inaccessible.
KEYWORDS
Electrothermal Vaporization, Portable Instrumentation, Atomic Emission Spectrometry, Welding Torch
30
INTRODUCTION
Most flame atomic emission spectrometry (FAES) devices are inexpensive, require little user training,
and are convenient to use in a wide variety of settings. Even as a mature technique, FAES remains a
standard in trace elemental analysis.82 This method is well-suited to alkaline and alkali earth metals
(which have significant biological and environmental importance) and is capable of analyzing a wide
variety of samples.83, 84 However, conventional flame systems often do not produce the energy necessary
to excite most elements on the periodic table, they are highly reactive and prone to interferences, and they
are limited in their sample introduction capabilities.85 This makes flame atomic spectroscopy methods
well-suited for the investigation of alternative sample introduction approaches.
Electrothermal vaporization (ETV) is a relatively common sample introduction technique for atomic
spectroscopy. ETVs come in a myriad of designs, including tubes, boats, wires, and filaments. ETVs have
been fashioned from an array of materials such as tantalum, graphite, or tungsten, with graphite being the
most popular. 48, 86-88 However, graphite ETVs require large power supplies and often suffer from memory
effects. Metal ETVs are a good alternative because of their lower power requirements, high melting
points, and chemical inertness. Tungsten has the highest melting point of any element at 3422°C, can
support chemical modifiers, and is easily manipulated into different shapes and arrangements. Tungsten
filaments have been used as ETVs since the early 1970s. 89-91 These filaments are commercially available
as incandescent light bulbs, and hence are mass produced to strict specifications. As a result, they are low
in cost and simple to obtain. Similar ETV devices have been reported in numerous applications: flame
furnace atomic absorption spectrometry (FF-AAS), Inductively Coupled Plasma Atomic Emission
Spectrometry (ICP-AES), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). 48, 92, 93 One
advantage of the ETV is that it desolvates the aqueous samples, and provides a dry aerosol plug to the
excitation source. The flame, therefore, only provides energy to atomize the dry aerosol, and then excite
the emission. In the end, more energy is available to excite the emission, and an enhanced signal is
observed.
31
In this work, a non-traditional FAES source, a welding-industry metal cutting torch, will be coupled to a
tungsten coil ETV. The traditional use of the flame source will be altered, and the ETV will act as the
sample introduction system. An ETV cell is coupled to the torch, which allows a steady stream of 10%
hydrogen / 90 % argon gas to introduce aerosol samples as discrete plugs into the heart of an acetylene-air
flame. This system will allow for the simultaneous determination of several elements, in this case alkali
and alkaline earth metals. These species have dietary and environmental significance, making them ideal
analytes for field analysis techniques.94-96
32
EXPERIMENTAL
Instrumental
A Victor® oxygen-acetylene machine cutting torch is used as the flame source. The torch is designed to
create a heating flame to melt a metal surface. It is composed of a brass body with three gas inlet lines.
The heating flame is supplied via a flashback-protected fuel (acetylene) port and a flashback-protected
oxidant (air) port. These two gases are kept separate throughout the barrel length of the torch, mixing
instead in the cutting tip at the apex of the device. The tip divides the pre-mixed flame gases into ten
equal streams arranged in a circle approximately 1 cm in diameter. The streams produce ten high
temperature flame jets that merge together about 1 cm away from the tip surface. The third gas port is
intended for “cutting” oxygen. This high velocity stream of oxygen is not mixed with the flame gases in
the tip. Instead, it passes through a dedicated central channel, exits the torch at the end of the tip, and
pierces the flame. As it passes through the flame, it is heated and oxidizes the metal being cut, resulting
in a slag jet. Cutting tips vary by the size of their central channel orifice diameter and are industry
standardized. Typically, the smaller the inner channel diameter, the smaller the thickness of the metal that
is to be cut. Larger central channels also require higher cutting gas flow rates.
The ETV-FAES device described here does not use the cutting oxygen. Instead, the central channel is
connected to an aluminum laboratory-designed electrothermal vaporization cell. The tungsten filament
inside the cell is bathed by a stream of 10% hydrogen / 90% argon gas, which prevents oxidation of the
filament and acts as a carrier gas. Samples are deposited onto the filament and then dried automatically
using a variable low current. Once dryness is achieved, a previously optimized vaporization program is
applied (Table I).97 When the sample is vaporized, the detector is triggered, and the carrier gas introduces
the sample into the central channel of the torch and directly into the flame as a concentrated aerosol plug.
Here the atoms in the sample are thermally excited and upon their relaxation, emission occurs through the
release of a photon at a characteristic wavelength.
33
The Victor® MT 310A three hose machine cutting torch (Victor Professional, Part Number 0380-0222, St.
Louis, MO, USA) is used as the flame source. This torch has a total length of 355.6 mm. The barrel,
which houses the three “hoses” for air, acetylene, and the sample carrier gas, has a length of 254.0 mm.98
Different fuel and oxidant gases may be employed depending upon the specific torch acquired. The torch
is secured on an adjustable rail, positioned axially with respect to the detector, and connected to air,
acetylene, and sample gases. Historically, axial viewing improves the figures of merit for ICP
spectrometry, so this viewing arrangement was chosen for the ETV-FAES device.99
The ETV is connected to a solid state constant current power supply (BatMod, Part Number VI-LU1-EU-
BM, Vicor, Andover, MA, USA). The supply provides up to 200 W of power (15 V, 13.3 A max), and
may be configured to run from 110 VAC or from 12 VDC (for portable applications). The supply is
small: 235 x 65 x 35 mm (including cooling fins). The supply is controlled with a home-written Visual
Basic program and a digital-to-analog converter (Measurement Advantage MiniLab 1008, Measurement
Computing, Norton, MA, USA). The tungsten filament is carefully extracted from a commercially
available 150 W, 15 V projector light bulb (Osram HLX 64633 Xenophot, Augsburg, Germany). The W-
coil filament is exposed by removing the fused silica envelope while leaving the glass base intact. The
filament is coiled in ten turns and when viewed from the side, the turns are 3 mm high and together they
occupy a width of 6 mm. Viewed end-on, the coil encircles an internal area approximately 1 mm in width
and just less than 3 mm in height. The prongs of the bulb base are 7 mm apart and 10 mm long, and fit
into a ceramic base. The 19 mm outer diameter ceramic socket is sunk into a 25 mm o.d. aluminum rod
and sealed with epoxy so that the coil aligns with an 8-32 tapped hole on the adjustable cap, centered with
the line of sight of the coil (Figure 1). The cap of the cell also has a 1/2 NPT thread fitted with a
connector that attaches to the central channel of the torch. The bottom of the 88 mm long rod is tapped
(1/4-20) for mounting. The electrical leads to the socket pass through an electrical feed-through on the
side of the rod. Purge and carrier gases enter through the gas ports on either side of the rod, pass through a
34
mixing chamber below the tungsten filament, and then escape through a small orifice in the ceramic base.
This flow carries the analyte to the torch during vaporization.
35
Table I. Vaporization Program for the ETV-FAES device. The sample has been dried prior to the first step. The detector is triggered 5 s prior to the beginning of Step 2. Step 3 allows sufficient cooling before the introduction of the next sample.
Current (Amps)
Time (seconds)
0 10 8 10 0 60
36
Figure 1. Illustration of ETV-FAES Device a) ½ NPT b) 8-32 tapped hole centered and in the line of sight of coil c) opening in ceramic base for purge gas throughput d) two 1/16 NPT gas fittings [one front and one rear of the rod e) electrical line feed through f) ¼-20 tapped hole for mounting
25 mm
19mm
40 mm
c
e
f
d
19 mm
88 mm
b
a
37
The optics are simple. The tip of the torch is positioned two focal lengths (f = 75 mm) from a fused silica
lens that is two focal lengths from the detector. This forms a one-to-one image of the flame on the
entrance slit of a 2048-element CCD array spectrometer (Ocean Optics, Dunedin, FL, USA).
Visual Basic Program
A Visual Basic (VB) program was designed to automatically dry and vaporize solutions so that
they move into the flame as an aerosol plug. Previous iterations of this program required a painstaking
manual optimization of the drying parameters. This was often a cumbersome process and had to be
repeated throughout the lifetime of a coil (~400 firings) due to changes in the resistance of the dry coil
(either from buildup of oxides along its surface or from a decrease in the filament wire diameter with age).
Variations in sample matrix also caused similar changes in resistance. When a drop of liquid was applied
to the W-coil, it spanned the loops of the filament, and acted as a short circuit. Thus, as the drop
evaporated the resistance across the coil changed. All of these effects caused changes in coil temperature
upon the application of a constant current, resulting in large variations in the drying time. The present
version of the Visual Basic program eliminates the need for this manually optimized drying program.
Each of the effects mentioned above result in a variation of the potential that must be applied across the
W-coil in order to maintain a constant current. By monitoring this potential, one may predict when the
W-coil is dry by measuring the change in potential per unit time (slope, m) for a given constant current.
The coil is determined to be dry once an empirically determined threshold slope is attained. The amount
of time necessary to reach dryness is therefore variable, and need not be previously determined. The
approach corrects for sample volume, filament age, and sample matrix by allowing the drying to continue
only up to the threshold value.
When the Visual Basic program is initiated, a series of warning messages appears to ensure that the user
is following all safety guidelines associated with the equipment. Then a digital 5V is applied through the
minilab 1008 to close a DC controlled, solid-state, normally open, optical relay switch (Crydom, Part
38
Number D1D12, San Diego, CA, USA). Current passes through the W-coil only when the relay is closed
(Figure 2). The constant current power supply is programmed via analog control voltages (0-5 V) sent
from the D/A output of the minilab 1008 to the power supply’s trim pin. The desired current is generated
from the input control voltage (V) using the formula:
4 1 (1)
where I is the desired constant current, and Imax is the maximum current generated by the power supply,
13.3 Amps.
The automated dry step is applied first at 3.4 A. The VB program runs a variable time loop, and the
voltage across the coil is read every 0.5 seconds. The slope (m) is calculated using the most recent
voltage (V2) and the previous voltage (V1):
m V – V
. (2)
The two latest slope values are averaged and displayed on the program screen. Once the minimum slope
value of 0.38 (the threshold value seen in Figure 3) has been exceeded, the program terminates the drying
loop and proceeds to the vaporization stage (Table I). First the coil is cooled for 10 s at 0 A. Then the
dry sample is vaporized at 8 A, followed by a 60-second cooling period prior to the next sample injection.
The currents and times are user selectable during this stage, and up to 8 steps may be programmed (Figure
3).
39
Figure 2. ETV-FAES control electronics.
40
The spectrometer is triggered for automatic data collection by the VB control program (Figure 3). In the
Visual Basic code, a command reads to the “Trigger Step?” entry and the associated amount of time to
“measure background before trigger step” (step 2 and 5 s in the Fig 3). In this case, a 5 V signal is sent to
the spectrometer’s external trigger pin 5 s prior to the beginning of the vaporization step (Step 2). The
detector then collects a user-selected number of spectra each having a specified integration time. In this
case, upon receiving the trigger signal, 50 consecutive spectra are collected each having a 0.2 s
integration time. These 50 spectra span a 10 s period, beginning 5 s prior to the vaporization step. This
allows for observation of the background signal prior to the high temperature vaporization stage, to ensure
that no sample is lost prematurely. Figure 4 shows a typical ETV FAES spectrum. In Figure 5, the
signals at the peak wavelengths for Rb, Cs, and Ba for each of the 50 spectra are plotted versus time. The
analytical signal may be recorded as the maximum value from this plot (peak height) or the sum of
several successive spectra (from time 6-9 s for example) in a peak area mode. In this instance, all
analytical signals were recorded as the maximum value from the plot.
41
Figure 3. Screen Capture Image of the Visual Basic Control Program
42
Figure 4. ETV-FAES spectrum for 5.0 mg L-1 cesium, 5.0 mg L-1 barium, and 2.0 mg L-1 rubidium with sodium present as a contaminant.
0
10
20
30
40
50
60
70
80
90
350 450 550 650 750 850
Rel
ativ
e E
mis
sion
In
ten
sity
Wavelength (nm)
Na
Cs Rb
Ba
43
Figure 5. ETV-FAES simultaneous emission profile for 5.0 mg L-1 cesium, 5.0 mg L-1 barium, and 2.0 mg L-1 rubidium. Monitored at 852.1 nm, 553.6 nm, 794.8 nm, respectively. Time 0 (not shown) occurs 5 s prior to the vaporization stage.
-5
0
5
10
15
20
25
30
5 6 7 8 9 10
Rel
ativ
e E
mis
sion
In
ten
sity
Time (seconds)
Rb
Cs
Ba
44
Reference Solutions and Standards
All standard solutions were prepared from their metal salts using Trace Metal Grade Nitric Acid (Fisher
Scientific, Fair Lawn, NJ, USA) digestion or distilled deionized water (Milli-Q Systems, Millipore,
Billerica, MA, USA). Standard solutions for calibration curves were prepared by serial dilution using
distilled deionized water.
Safety Considerations
Material safety data sheets were consulted prior to the use of all chemical reagents. All indicated safety
precautions were taken during each step of analysis, and all waste was stored in separate containers prior
to disposal. Necessary precautions were also taken with regards to working with a high voltage power
supply.
45
RESULTS AND DISCUSSION
System Optimization
For all system optimization studies, 20.0 µL samples of 20.00 µg/L aqueous lithium standard solution
were distributed onto the tungsten coil using a micropipette (Eppendorf, Hauppauge, NY, USA), dried
using the Visual Basic program, and vaporized using previously optimized parameters (Table I). Viewing
of the flame image was initially done in a radial, side-on fashion, but after a comparison of lithium
emission intensities, axial, end-on viewing was chosen for all analyses.99 The peak height values were
recorded, and all optimization studies were repeated in triplicate. The parameter operating value that
gave rise to the highest signal was determined to be the optimal setting (Table II).
The cutting tip designation 6-MTH-N (Victor Equipment, Part Number VIC0333-0370, St. Louis, MO,
USA) is designed to cut through metal at a maximum depth of 252 mm using natural gas or propane as
the fuel source. The tip is 8.0 cm in length with a diameter of 1.5 cm, and is composed of two brass pieces:
a hollow cylindrical barrel and a solid insert with a 3 mm diameter bore through the length of the insert as
seen in Fig. 6 a and b. The end of the insert has six, 1.0 mm openings in a circular pattern that allow the
fuel and oxidant to pass through into the cutting tip body. These gases are then directed into ten 1.0 mm
openings that are arranged circularly around the front end of the insert illustrated in Fig. 6 part d. While
the fuel and oxidant are mixed through the length of the tip as they exit the front end, a flame occurs in a
“flower-like” pattern. In the center of this flower, the sample enters the flame.
Limits of Detection
Calibration curves for five group I and II elements of interest were constructed using the previously
prepared standard solutions: Ba, Cs, K, Li, and Rb. These elements were chosen because of their ease of
excitation, which was well suited for the lower temperatures of the air-acetylene flame. A similar device
using an oxygen-acetylene flame has been validated in a laboratory environment and was found to be
capable of the analysis of less volatile species such as copper and lead with great success. In Table III, all
46
LODs were calculated by multiplying the standard deviation in the blank signal by three and dividing by
the slope of the element’s calibration curve. To produce a reasonable guide for the FAES LOD, the LODs
from five sources were averaged and the results are used as a basis of comparison for ETV-FAES85, 100-103.
The ETV sample introduction provided a 25-fold enhancement of the LOD of lithium compared to a
standard flame atomic emission device, while Ba saw a 250-fold improvement. On average, the
improvement in LOD was a factor of 100.
47
Table II. Optimized Operating Parameters for ETV-FAES Device
Parameter Optimized
Value Integration Time (msec) 180
Cutting Tip 6-MTH-N Acetylene Gas Flow
(LPM) 1.4
Air Flow (LPM) 10.6 H2/Ar Flow (LPM) 1.0
48
Figure 6. Illustrations of 6-MTH-N Cutting Tip. a) solid insert including 10, 1.0 mm gas flow channels around the tip end b) outer copper sheath of cutting tip c) inner and outer pieces joined together for use d) end-on view of cutting tip with flow openings shaded.
a)
b)
c)
d)
49
Table III. ETV-FAES Limits of Detection
Element Wavelength
(nm)
ETV-FAES LOD
(µg/L)
FAES LOD
(µg/L)
Improvement Factor
Li 670.8 0.04 1 25 K 766.5 0.8 3 4 Rb 794.8 0.3 60 200 Cs 852.1 4 200 50 Ba 553.6 2 500 250
50
CONCLUSION
The ETV-FAES device described here can be easily constructed using commercially available materials.
It requires little user training, is rugged, portable, and capable of achieving LODs comparable to modern
instrumentation such as the inductively coupled plasma. The device is ideal for mobile labs, teaching,
industrial applications, and laboratories that cannot make significant monetary investments in analytical
instrumentation.
51
REFERENCES
1. Klaic, P. M. A. N., A.M.; Moreira, A.S.; Vendruscolo, C.T.; Ribeiro, A.S., Determination of Na,
K, Ca and Mg in xanthan gum: Sample treatment by acid digestion. Carbohydrate Polymers 2011, 83,
1895-1900.
2. Sokolnikova, J. V. V., I.E.; Menshikov, V.I., Determination of Trace Alkaline Metals in Quartz
by Flame Atomic Emission and Atomic Absorption Spectrometry. Spectrochim. Acta, Part B 2003, 58,
387-391.
3. Miller-Ihli, N. J., Atomic Absorption and Atomic Emission Spectrometry for the Determination
of the Trace Element Content of Selected Fruits Consumed in the United States. J. Food Comp. Anal.
1996, 9, 301-311.
4. Ingle, J. D. C., S.R., Spectrochemical Analysis. Prentice Hall: Upper Saddle River, NJ, USA,
1998.
5. Pinto, F. G. L., F.G.; Saint'Pierre, T.D.; da Silva, J.B.B.; Costa, L.M.; Curtius, A.J., Direct
Determination of Dy, Sm, Eu, Tm, and Yb in Geological Samples by Slurry Electrothermal Vaporization
Inductively Coupled Plasma Mass Spectrometry. Anal. Lett. 2010, 43, (6), 949-959.
6. Lee, Y. I. K., J.K.; Kim, K.H.; Yoo, Y.J.; Back, G.H.; Lee, S.C., Design and Critical Evaluation
of Improved Electrothermal Vaporization Flame Atomic Absorption/Emission Spectrometry for Direct
Determination of Trace Metals in Microliter Samples. Microchem. J 1998, 60, 231-241.
7. Okamoto, Y. K., H.; Kataoka, H.; Tsukahara, S.; Fujiwara, T., Direct determination of bromine in
plastics by electrothermal vaporization /inductively coupled plasma mass spectrometry using a tungsten
boat furnace vaporizer and an exchangeable sample cuvette system. Rapid Commun. Mass Spectrom.
2010, 24, (9), 1265-1270.
8. Wu, P., Wen, X., He, L., He, Y., Chen, M., Hou, X., Evaluation of Tungsten Coil electrothermal
vaporization-Ar/H2 flame atomic fluorescence spectrometry for determination of eight traditional hydride-
forming elements and cadmium without chemical vaporization generation. Talanta 2008, 74, 505-511.
52
9. Williams, M. P., E.H., Commercial Tungsten Filament Atomizer for Analytical Atomic
Spectrometry. Anal Chem 1972, 44, (7), 1342-1344.
10. Reid, R. D. P., E.H., Horizontal Tungsten Coil Atomizer and Integrated Absorbance Readout for
Atomic Spectrometry. Anal. Chem. 1976, 48, (2), 338-341.
11. Hou, X.; Levine, K. E.; Salido, A.; Jones, B. T.; Ezer, M.; Elwood, S.; Simeonsson, J. B.,
Tungsten coil devices in atomic spectrometry: absorption, fluorescence, and emission. Analytical Sciences
2001, 17, 175-180.
12. Barth, P. H., S.; Krivan, V., Analysis of Silicon Dioxide and Silicon Nitride Powders by
Electrothermal Vaporization Inductively Coupled Plasma Atomic Emission Spectrometry Using a
Tungsten Coil and Slurry Sampling. J. Anal. At. Spectrom. 1997, 12, 1359-1365.
13. Bings, N. H. S., Z., Development of a tungsten filament electrothermal vaporizer for inductively
coupled plasma time-of-flight mass spectrometry and its possibilities for the analysis of human whole
blood and serum. J. Anal. At. Spectrom. 2003, 18, 1088-1096.
14. Spaargaren, D. H., Presence and Significance of Lithium in the Brown Shrimp, Crangon crangon.
Comparative Biochemistry and Physiology, Part A: Molecular and Integrative Physiology 1998, 91, 457-
461.
15. Koji, M. K., T.; Yamaya, H.; Kobayashi, E.; Nogawa, K., Urinary Excretion Levels of Sodium
and Potassium in Envrionmental Cadmium-Exposed Subjects. Toxicology 1998, 127, 187-193.
16. Shiraishi, K. K., S.; Zamostyan, P.V.; Tsigankov, N.Y.; Los, I.P.; Koruzun, V.N., Dietary intakes
of alkaline metals, alkaline earths, and phosphorous in Ukrainians. Biomedical Research on Trace
Elements 2009, 20, 62-68.
17. Donati, G., Gu, J., Nobrega, J.A., Calloway, C.P. Jr., Jones, B.T., J. Anal. At. Spectrom. 2008, 23,
361-366.
18. Nakamura, S.; Kobayashi, Y.; Kubota, M., Temperature of a W Ribbon Furnace in
Electrothermal Atomic Absorption Spectrometry. Spectrochimica Acta Part B 1986, 41, 817-823.
19. Alavosus, T. J.; Murphy, R.; Schatzlein, D., Am. Lab. 1995, 23, 31-34.
53
20. Christian, G. D., Feldman, F.J., A comparison study of detection limits using flame-emission
spectroscopy with the nitrous oxide-acetylene flame and atomic-absoroption spectroscopy. Applied
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21. Ramirez-Munoz, J., Spectrochemical Methods of Analysis. Wiley-Interscience: New York, NY,
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22. Bruno, T. J., Svoronos, P.D.N., Handbook of Basic Table for Chemical Analysis. CRC Press:
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23. Robinson, J. W., Undergraduate Instrumental Analysis. 5th ed.; Marcel Dekker: New York, NY,
1995.
54
CHAPTER 3
An Electrothermal Vaporization Flame Atomic Emission Spectrometer
Summer N. Hanna and Bradley T. Jones
Department of Chemistry, Wake Forest University,
Winston-Salem, NC, 27109, USA
Note: The following manuscript was published in the Journal of Analytical Atomic Spectroscopy (DOI:
10.1039/c1ja10060b). Stylistic variations are due to the requirements of the journal. All of the presented
research was conducted by Summer N. Hanna. The manuscript was prepared by Summer N. Hanna and
edited by Bradley T. Jones.
55
ABSTRACT
An inexpensively built, light-weight, potentially transportable flame atomic emission spectrometry device
has been designed and developed. A tungsten coil is employed as an electrothermal vaporization source
coupled to an oxygen-acetylene flame. The tungsten coil is simple in design and commercially available
as a 150 W, 15 V light bulb, while the flame source is generated by a welder’s metal-cutting torch.
Nineteen elements have been determined with limits of detection as low as 0.9 ng L-1 for Ca and 8.0 ng L-
1 for In. The accuracy of the method is verified by determining the concentration of thirteen elements in
two reference materials: Montana soil and a water pollution standard solution. No statistically significant
difference is observed between the certified and determined values at a 95 % level of confidence.
Sample volumes as low as 10 µL may be analyzed. The best torch tip design is identified, and gas flow
rates are optimized. The temperature profile of the flame is also discussed. The electrothermal vaporizer
and cutting torch are designed to be portable. Incorporation of a simple hand-held CCD spectrometer
would render the entire system usable in the field.
KEYWORDS
Tungsten coil, electrothermal vaporization, atomic emission spectrometry, portable instrumentation
56
INTRODUCTION
The state-of-the-art for analyzing complicated samples by atomic spectrometry includes graphite furnace
atomic absorption spectrometry (GFAAS),1 inductively coupled plasma optical emission spectrometry
(ICP-OES),2 and inductively coupled plasma mass spectrometry (ICP-MS).3 These techniques are quite
powerful, providing detection limits at the part per billion (ppb) level and below. For the most part,
however, they involve instrumentation that is large, fragile, and expensive. A single element GFAAS
device may cost $50,000 at the lower end, while multi-element ICP-MS systems may be priced at
$200,000 or more. Even the least expensive of these devices is out of reach for small businesses,
undergraduate teaching institutions, and laboratories in third world countries. Another shortcoming of
state-of-the-art equipment is its limited utility in the field. Portability and analytical performance are often
diametrically opposed in environmental field applications.4 An Electrothermal Vaporization Flame
Atomic Emission Spectrometry (ETV-FAES) device will address these concerns. The ETV-FAES device
is a portable flame atomic emission detector. The novelty of the device is in the design of the excitation
source and the sample introduction apparatus. The emission source will be the oxygen-acetylene flame
generated by a typical sheet metal cutting torch, mass produced at low cost for the welding industry, and
designed to be portable for use in scrap metal yards. The cutting flame will burn at a temperature in
excess of 3000 °C, hot enough to excite the emission of almost every metal and semimetal on the periodic
table.5 Since the torch is not produced specifically for analytical instrumentation, ETV-FAES will benefit
from the lower cost associated with the demand for the product in the welding industry. Almost every
small town will have a welding supplier who carries such torches.
The cutting torch, on the other hand, is not designed for liquid sample introduction. The ETV-FAES
device, therefore, will use an electrothermal vaporizer (ETV) to introduce dry analyte-containing aerosol
into the oxygen “cutting” central channel of the torch. Again, the ETV will employ a heating element
designed and produced for a different purpose: the tungsten coil (W-coil) or filament contained in a
57
common bulb, such as an automobile headlamp.6 The bulb is portable, mass produced to strict
specifications, costs less than $5, and may be powered by a 12 V battery, such as a car battery. Liquid
sample aliquots of 10 μL or more will be placed on the coil, dried at low current, and subsequently
vaporized at higher current. The low mass of the filament will eliminate the need for water cooling. A
simple flow of Ar with 10% hydrogen, a common welding gas, will protect the coil from oxidation, and
rapidly cool the system once the current is removed. During the vaporization step, a dry aerosol
containing the analyte atoms will be carried to the flame, where excitation resulting in atomic emission
will occur. For true portability, the emission signal could easily be collected with a hand-held charge
coupled device spectrometer. This will allow for trace level, simultaneous, multi-element analyses with
very small sample volumes.
Electrothermal vaporizers have been used only sparingly with flame systems. In 1993, a graphite furnace
ETV was employed with an air-acetylene flame in a laser enhanced ionization experiment.7 Ar was the
carrier gas between the two devices. Later, the same system was employed for the determination of Pb in
blood.8 In each case, the ETV provided sensitivities approaching the theoretical maximum. In 1995 a
tantalum filament was employed as an ETV for flame atomic absorption spectrometry.9 Again, Ar gas
transported the sample vapor to the air-acetylene flame. More recently, a W-coil ETV device was used to
provide sample vapor for a hydrogen-air flame in an atomic fluorescence experiment.10 Detection limits
were in the low ppb range. Each of these experiments probed analyte atoms in the ground state, so high
temperature emission sources were not necessary.
Most ETV applications for atomic emission spectrometry involve inductively coupled or microwave
plasmas. In fact, a very recent review with more than 300 references focuses largely on these
applications.11 Most ETV devices were composed of graphite, but some employed metal tubes, cups,
loops, and filaments. In 2002, a review article was published with 179 references to the use of W devices
in AAS and ETV systems.12
58
The specific W-coil proposed for ETV-FAES has also been employed as an ETV sample introduction
system for ICP-OES. 13 14 15 16 On average, replacement of the traditional nebulizer with the W-coil ETV
device resulted in an LOD reduction factor near 300. The conventional reasoning for this improvement
involves three factors: introduction of the sample into the ICP as a dry aerosol (thus requiring no energy
for desolvation in the plasma), production of very fine particulate matter (or even atomic vapor) from the
ETV, and introduction of the analyte as a concentrated plug in a brief time period. These processes should
translate equally to the flame, so similar improvement factors are anticipated.
In the work described below, sample aliquots as low as ten microliters will be deposited onto a tungsten
filament commonly used in a 150 watt projector bulb. The filament will then be dried at low currents
supplied by a constant current power supply. The dried sample will be vaporized at a slightly higher
current and the resulting aerosol carried through the central channel of a standard metal-cutting torch.
This will guide the vapor into the heart of a flame, where the metal atoms will be excited at high
temperatures and emission will occur.17 This atomic emission will be collected using a spectrometer
connected to a personal computer. Multiple elements will be detected simultaneously at the parts per
billion level. The sensitivity and analysis time will rival modern commercial instrumentation but at a
fraction of the cost. The device will be small and robust enough to transport for field applications.
59
EXPERIMENTAL
Instrumental
A Victor® oxygen-acetylene machine cutting torch is used as the flame source. The torch is designed to
create a heating flame to melt a metal surface. It is composed of a brass body with three gas inlet lines.
The heating flame is supplied via a flashback-protected fuel port and a flashback-protected oxidant port.
These two gases are mixed within the body of the torch. The third gas port is for “cutting” oxygen. This
high velocity stream of oxygen is not mixed with the flame gases in the torch. Instead, it passes straight
through the torch, pierces through the flame where it is heated and used to oxidize the metal in the plate
forming a slag jet. The ETV-FAES device will not produce the slag jet, rather the ETV gas will be excited
by the heating flame formed at the end of the cutting tip. The tip divides the pre-mixed flame gases into
six equal streams arranged in a circle approximately 1 cm in diameter. The six streams produce six high
temperature flame jets that merge together about 1 cm away from the tip surface. The ETV stream is
injected into these merging flame jets, through a seventh central channel. Cutting tips vary by the size of
their inner channel orifice diameter and are industry standardized. Typically, the smaller the inner channel
diameter, the smaller the thickness of the metal that is to be cut and likewise larger diameters are used to
cut thicker pieces of metal. This difference in size also requires a change in gas flow rates, with larger tips
requiring higher flow rates.
In the case of ETV-FAES, the design has been changed so that acetylene and oxygen continue to form a
flame in the six concentric circles of the cutting tip, but the center channel has undergone much change.
Connected to this outlet is a laboratory-designed electrothermal vaporization cell. Inside the vaporization
cell is a tungsten filament bathed by a stream of 10% hydrogen 90% argon gas, which prevents oxidation
of the filament and simultaneously acts as a carrier gas. The carrier gas may be mixed with auxiliary
oxidant (oxygen or nitrous oxide) outside the ETV cell, but prior to introduction to the torch. This allows
for higher flow rates through the central channel. Samples are pipetted onto the filament and then dried
60
using a low current supplied by constant current power supply. The optimization method for the heating
program was developed elsewhere and used here to simultaneously vaporize the sample and trigger the
detector at the beginning of the vaporization cycle. The progressively lower drying currents prevented the
coil from heating to a red glow, mitigating the chance for a loss of analyte prior to vaporization.18 When
the sample is vaporized, the carrier gas introduces the sample into the center channel of the torch and
directly into the flame as a concentrated aerosol plug. In Figure 2 this sample plug can easily be seen
flowing through the center of the flame. Here the atoms in the sample are thermally excited and upon
their relaxation emission occurs through the release of a photon at a characteristic wavelength.
61
Table I. Heating Program for the Tungsten-Coil Vaporizer.
Applied Current (Amps) Process Time (Seconds) Signal Read 4.1 Dry 30 No 3.6 Dry 25 No 3.3 Dry 20 No 3.1 Dry 20 No 0 Cool 10 No 8 Vaporize 10 Yes 0 Cool 20 No
62
The Victor® MT 1065A three hose machine cutting torch (Victor Professional, Part Number 0399-1121)
is secured using an aluminum base onto an adjustable rail system. The torch is positioned axially and the
oxygen, acetylene, auxiliary oxidant, and sample gas inlet lines are securely connected as seen in Figure 1.
Axial viewing increases the sensitivity for inductively coupled plasma emission, so this viewing method
was chosen for ETV-FAES as well. The end-on viewing method reduces the background emission of the
oxygen-acetylene flame by focusing on the center channel where analyte emission occurs, rather than on
the bright blue peripheral flame jets (Figure 2). 19 The aluminum electrothermal vaporization cell is
connected to a solid state constant current power supply (BatMod, Victor, Andover, MA) and the power
supply is controlled with a Visual Basic program and a digital to analog converter (Measurement
Advantage MiniLab 1008, Measurement Computing, Norton, MA). The tungsten filament is carefully
extracted from a commercially available 150 W, 15 V projector light bulb (Osram HLX 64633 Xenophot,
Augsburg, Germany). The bulbs are mass produced to strict specifications. The W-coil filament is
exposed by removing the fused silica envelope while leaving the base intact. The filament is mounted in
the ETV housing using the two-pronged base and placed in the vaporization cell’s ceramic base. The 19
mm o.d. ceramic socket is countersunk into a 25 mm o.d. aluminum rod so that the upper surface is flush.
The bottom of the 7 cm long rod is tapped for mounting. The electrical leads to the socket pass through
feedthrough on the side of the rod. Purge/carrier gases enter through the other side of the rod, pass
through a mixing chamber, and then escape through a small orifice in the ceramic base. The torch is
positioned two focal lengths (f=75 mm) from a fused silica lens that is two focal lengths from the detector.
This forms a one-to-one image of the flame onto the 25 µm entrance slit of a small Czerny-Turner
monochromator (MonoSpec 18, Scientific Measurement Systems, Inc. Grand Junction, CO). The
monochromator has a 2400 gr mm-1 grating (110 x 100 mm) giving a reciprocal linear dispersion of 2 nm
mm-1 at 400 nm with a 156 mm focal length and optics of f/3.8. The analytical signal is detected by a
thermoelectrically cooled CCD spectrometer (Spec-10, Princeton Instruments, Roper Scientific, Trenton,
NJ) with a linear 2-dimensional array of 1340 x 100 pixels. Each pixel has a dimension of 20 x 20µm
which translates to a image area of 26.8 x 2 mm. This provides an approximately 55 nm spectral window
63
depending on the central wavelength. Elements are observed using an integration time of 0.025 seconds
while collecting 20 spectra during a ten-second exposure period as triggered by the Visual-Basic coil
control program.
64
Figure 1. Photograph of the ETV-FAES Instrument.
Flame Cutting Tip
Torch Flashback
Arrestor
Acetylene
Oxygen
Nitrous
Oxide
ETV
16.50 cm
65
Figure 2. End on view of the cutting tip during the vaporization cycle of a) distilled deionized water b) tap water.
a b
66
Reference Solutions and Standards
All standard solutions were prepared from their metal salts using the appropriate digestion method. 20
Standard solutions for calibration curves were prepared by serial dilution using distilled deionized water
(Milli-Q Systems, Millipore). Water Pollution Standard 1 (WPS-1) (VHG Labs, Manchester, NH) was
analyzed using the standard addition method and was also diluted before being spiked with the element of
interest. NIST standard reference material (SRM) 2711—Montana Soil B (NIST, Washington, D.C) was
prepared by weighing out 1.9935 g and digesting in 4.00 mL of 14 mol L-1 trace metal grade nitric acid
(Fisher Scientific, Pittsburg, PA) and 4.00 mL distilled deionized water (to prevent evaporation) on a
heating block at 100 º C for two hours. This solution was allowed to cool, filtered using a Büchner Funnel,
and diluted to 25.00 mL with distilled deionized water.
Safety considerations
Material safety data sheets were consulted before using each chemical reagent. Necessary safety
precautions were taken in each step of the analysis, and all aqueous waste was stored separately prior to
disposal. All personnel working with the flame source were educated in the potential hazards associated
with flammable gases, and were also trained in the safe handling and transportation of compressed gases
by National Welders Supply Company.
67
RESULTS AND DISCUSSION
System Optimization
For optimization studies, 40.0 µL samples of an aqueous manganese standard solution were deposited
onto the tungsten filament using a micropipette (Eppendorf, Germany) and dried using a previously
determined heating program (Table I). The samples were then vaporized and passed through the torch into
the flame. The optimization parameters were then confirmed by repeating each experiment with a
different element, copper. Gas flow rates were optimized by altering the gas flow rates for each inner
channel orifice diameter and measuring the resulting emission intensities. Each gas flow was increased in
0.5 L/min increments from 0-2 L/min and then in 1.0 L/min increments until a flow rate of 10.0 L/min
was reached.
When gas flow rates were optimized, an inner channel orifice diameter of 3.58 mm (cutting tip size 7)
was the most suitable for the largest variety of elements. The optimal flow rates for acetylene, oxygen,
and auxiliary gas were 2.5, 4.5, and 3.0 L/min, respectively. 21The optimal H2/Ar ETV flow rate was
determined once the flame gases were set. The resulting emission intensities were higher when a flow
rate of 1.5-3 L/min was used for the 10 % H2, 90 % Ar carrier gas. For all reported work, an ETV carrier
gas flow of 2.0 L/min was used.
68
Figure 3. Variation in emission signal level with orifice diameter for a 1 µg/mL single-element Mn standard solution.
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5
Rel
ativ
e E
mis
sion
Sig
nal
Inner Orifice Diameter (mm)
69
Figure 4. Variation in emission signal level with ETV gas (10% H2/Ar) flow rate for a 1 µg/mL Mn solution.
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12
Rel
ativ
e E
mis
sion
Sig
nal
Carrier Flas Flow Rate (LPM)
70
Once a cutting tip size was determined and operational variables were optimized, viewing parameters
were identified by adjusting the height and width of the viewing area in a 1x1 mm grid pattern. The flame
image was measured and found to have a diameter of 5 mm with the center channel at a diameter of 1mm.
The ideal viewing position occurred when the 1.0 mm inner center channel diameter image was focused
on the pinhole monochromator entrance. The torch was positioned at a distance of 2f from the end of the
tip of the inner cones of the flame to the focusing lens, which was positioned 2f from the entrance of the
monochromator. 22
Gas Phase Excitation Temperature Mapping of ETV-FAES Flame
In order to determine the temperature gradient across the excitation flame, an alternative method of gas
phase temperature analysis was explored. A derivation of the Boltzman Distribution developed by Donati
23 was utilized to generate gas phase temperature “maps” where the emission intensities of rubidium and
lithium are simultaneously monitored and using the mathematical relationship:
(1)
a gas phase temperature can be generated. 24 The emission intensities of the 780.0 and 670.8 nm lines of
rubidium and lithium, respectively, were measured using a fiber optic connected to the detector. The fiber
optic attachment was used so that a 5x5 mm grid pattern could easily be generated without moving the
apparatus. The results were then used to create a visual “map” of resulting flame temperatures. Using
these “maps” the highest temperature zones of the flame could easily be identified and calculated to be
about 3000ºC at the optimal viewing area. In Figures 5 and 6 there is a lack of symmetry in the
temperature gradient across the flame image that is thought to be due to the pull generated from the
exhaust hood located above the flame. In Figure 5, this drag and resulting lower temperatures were
located from 0-4 mm across and 8-10 mm in height of the flame image, while in Figure 6, the length of
the flame is shown. The area of the flame adjacent to the intake of the exhaust hood is located along the
71
area located along the top of the image at a width of 10 mm. An effect identical to that in Figure 5 can be
seen spanning a width of 6-10 mm and along the length of the flame from the torch tip at 0 mm to about
25 mm down the flame.
Analytical Figures of Merit
Calibration curves were obtained for nineteen elements of interest using previously prepared standard
solutions: Ag, Al, Ba, Ca, Co, Cr, Cs, Cu, Eu, Fe, In, La, Mn, Ni, Pb, Pd, Sr, Tl, and Yb. These elements
were chosen because of either their ubiquity in natural samples and their range of specific properties.
Each was observed at the primary emission wavelength.25 Limits of detection (LODs) are listed in Table
II and were calculated by multiplying the standard deviation in the blank signal by three and dividing the
product by the slope of the element’s calibration curve. A typical emission spectrum is given in Figure 7.
The spectrum demonstrates the relatively low background emission encountered with the ETV-FAES
device. The ETV-FAES figures of merit were compared with those of traditional flame atomic emission
spectrometry (FAES) by connecting a nebulizer and spray chamber to the welding torch to collect
emission signals using continuous sample introduction. A significant enhancement in sensitivity for many
elements was observed. Detection limits for traditional FAES methods are well known, and usually vary
by no more than a 10.5,26,27,28,29The average improvement using the ETV-FAES was 43 and six of the
nineteen elements had improvement greater than fifty fold. 30
Element recovery for WPS-1, Water Pollution Standard, and NIST SRM 2711, Montana Soil, were also
completed using the method of standard additions and all data was reported at a 95 % significance level
(Table III).
72
CEMD Temperature Profile-- Axial View
Flame Image Width (mm)
0 2 4 6 8 10
Fla
me
Imag
e H
eigh
t (m
m)
0
2
4
6
8
10
2200 2300 2400 2500 2600 2700
Figure 5. Axial Temperature Map of the ETV-FAES Flame with torch center located at 5 mm x 5 mm, indicated by the cross-hatched center circle. The entire cutting tip is indicated by the white circular outline as though it was being viewed end-on
73
CEMD Flame Temperature Profile-- Radial Side View
Flame Length (mm)
0 10 20 30 40
Fla
me
Wid
th (
mm
)
0
2
4
6
8
10
0 500 1000 1500 2000 2500 3000 3500
Figure 6. Radial Temperature Map of the ETV-FAES Flame. In this arrangement, the edge of the torch tip spans the width of the flame and is located at the 0 mm length position, indicated by the white line with the center of the torch located at 0 mm x 5 mm as indicated by the white void.
74
Figure 7. ETV-FAES Emission Spectrum for NIST SRM 2711 (Montana Soil).
0
20
40
60
80
100
120
140
160
180
440 445 450 455 460 465
Rel
ativ
e E
mis
sion
Int
ensi
ty
Wavelength (nm)
Cs
Cr Eu
Sr
Cs
75
Table II. Figures of Merit for Nineteen Elements of Interest
Element Wavelength
(nm)
FAES LOD
(ng/mL)
ETV-FAES LOD
(ng/mL)
LDR%
RSD
Improvement
Factor
Cu 324.7 0.1 0.2 1.8 2.7 0.5 Ag 328.1 0.08 0.01 2.1 2.9 8 Co 345.4 0.9 0.009 2.2 3.6 100 Ni 352.4 0.2 0.04 1.9 3.1 5 Pd 363.5 0.3 0.02 2.2 2.1 15 Fe 371.0 0.6 0.007 2.4 1.8 86 Tl 377.6 0.1 0.006 3 5.2 17 Al 396.2 0.06 0.003 1.9 6.9 20 Yb 398.8 0.2 0.003 2.5 2.6 67 Mn 403.3 0.03 0.007 2.8 4.3 4 Pb 405.8 0.8 0.003 1.7 2.3 267 Ca 422.7 0.01 0.0009 3 4.5 11 Cr 425.4 0.9 0.009 2.8 3.0 100 La 441.8 1 0.01 2.4 2.3 100 In 451.1 0.04 0.008 2.1 0.9 5 Cs 455.5 0.2 0.2 2.5 4.4 1 Eu 459.4 0.0005 0.0006 2.4 4.1 0.8 Sr 460.7 0.003 0.001 3 8.4 3 Ba 553.6 0.5 0.02 2.4 3.7 25
76
Table III. Elemental Recoveries for WPS-1 and SRM 2711 VHG WPS-100 SRM 2711
Concentration
Element Wavelength
(nm) Reported (µg/mL)
Found (µg/mL)
% Recovery
Reported (µg/mL)
Found (µg/mL)
% Recovery
Cu 324.7 0.2 0.2 ± 0.2 100 0.8 ± .02 0.9 ± 0.2 112
Ag 328.1 - - - 0.19
± .003 0.2 ± 1.1 95
Co 345.4 0.08 0.08 ± 0.1 100 - - - Ni 352.4 0.2 0.3 ± 0.3 150 - - - Fe 371 0.2 0.2 ± 0.3 100 - - -
Tl 377.6 - - - 0.2 ± .001 0.2 ± 0.5 90
Al 396.2 0.2 0.4± 0.2 200 - - -
Yb 398.8 - - - 0.02* 0.03 ±
1.3 150
Mn 403.3 0.003 0.003 ±
0.3 100 1.27 ± 0.2 1.2 ±0.5 95
Pb 405.8 0.2 0.2 ± 0.6 100 0.94 ± 0.3 0.99 ±
0.2 105 Cr 425.4 0.2 0.1 ± 0.4 50 - - -
Cs 455.5 - - - 0.25* 0.22 ±
0.4 88
Eu 459.4 - - - 0.09* 0.12 ±
0.4 133
*Noncertified Values
77
CONCLUSION
The ETV-FAES system is capable of achieving limits of detection comparable to traditional AES
instrumentation while using significantly smaller sample volumes. The instrument’s design allows for a
high degree of user accessibility and requires little training for use, which has been illustrated by a similar
ETV-FAES instrument used exclusively by undergraduate researchers. It has large selectivity coupled
with compact size, portability, and low cost. This makes ETV-FAES ideal for mobile labs, teaching
applications, and increases accessibility to instrumentation for those incapable of purchasing expensive,
complex equipment.
78
REFERENCES
1 Sturgeon, R.E. Spectrochim. Acta Part B 1997, 52 1451. 2 Hou, X., Jones, B.T., “Inductively Coupled Plasma Optical Emission Spectroscopy,” in Encyclopedia of Analytical Chemistry, Robert A. Meyers, Ed., John Wiley & Sons Ltd., Chichester, 2008. 3 Becker, J.S., Can. J. Anal. Sci. and Spectrosc. 2002, 47, 98. 4 Hou, X. Jones, B.T, Microchem. J., 2000, 66, 115. 5 Ramirez-Munoz, J., “Flame Emission Spectrometry,” in Spectrochemical Methods of Analysis (J.D. Winefordner, Ed.), Wiley-Interscience, New York, NY, 1971. 6 Sanford, C.L.; Thomas, S.E.; Jones, B.T., Appl. Spectrosc. 1996, 50, 174. 7 Smith, B.W.; Petrucci, G.A.; Badini, R.G.; Winefordner, J.D., Anal. Chem. 1993, 65, 118. 8 Riter, K.L.; Matveev, O.I.; Smith, B.W.; Winefordner, J.D., Anal. Chim. Acta 1996, 333, 187. 9 Duxbury, M.; de Mora, S.J., Microchem. J. 1995, 51, 337. 10 Wu, P.; Wen,X.; He, L.; He, Y.; Chen, M.; Hou, X., Talanta 2008, 74, 505. 11 Resano, M.; Vanhaecke, F.; de Loos-Vollebregt, M. T. C., J. Anal. At. Spectrom. 2008, 23,1450. 12 Hou, X. Jones, B.T., Spectrochim. Acta Part B 2002, 57, 659. 13 Davis, A.C.; Alligood, B.W.; Calloway, C.P.; Jones, B.T., Appl. Spectrosc. 2005, 59, 1300. 14 Davis, A.C.; Calloway, C.P.; Jones, B.T., Micorchem. J. 2006, 84, 31. 15 Davis, A.C.; Calloway, C.P.; Jones, B.T., Talanta 2007, 71, 1144. 16 Skoog, D; Holler, F.; Nieman, T; Principles of Instrumental Analysis, Thomson Learning: USA, 1998; Vol 5. 17 Donati, G.L., Gu, J., Nobrega, J.A., Calloway, C.P. Jr., Jones, B.T. J. Anal. At. Spec., 2008, 23, 361. 18 Alavosus, T.J., Murphy, R.; Schatzlein, D., Am. Lab., 1995, 27, 31. 19 Winefordner, J.D., Masfield, C.T., Vickers, T.J. Anal. Chem., 1963, 35, 1611. 20 Victor Professional Industrial Gas Apparatus Catalog, 2008 21 Christian, G.D., Feldman, F.J. Anal. Chem., 1971, 43, 611. 22 Donati, G.L., Jones, B.T. J. Anal. At. Spectrom., 2011, 26, 838-344.
23 Donati, G.L., Calloway, C.P. Jr., Jones, B.T. J. Anal. At. Spectrom., 2009, 24, 1105
79
24 Harrison, G.; Massachusetts Institute of Technology Wavelength Tables, The MIT Press: Cambridge, MA, 1969 25 Ingle, J.D.C.; Spectrochemical Analysis. Prentice Hall: Upper Saddle River, NJ, USA, 1998.
26 Bruno, T.J., Svornos, P.D.N. Handbook of Basic Tables for Chemical Analysis. CRC Press: Boca Raton, FL, 1989.
27 Christian, G.D.; Feldman, F.J. Appl. Spectrosc. 1971, 25, 660.
28 Robinson, J.W. Undergraduate Instrumental Analysis. 5th Ed. New York, NY: Marcel Dekker, 1995.
29 Smith, B.W., Parsons, M.L. J. Chem. Ed., 1973, 50, 679.
80
CHAPTER 4
Design of a Compact, Aluminum, Tungsten-Coil Electrothermal Vaporization Device for Inductively Coupled Plasma-Atomic Emission Spectrometry
Summer N. Hanna,a Clifton P. Calloway, Jr.,b Jason D. Sanders,b Ronald A. Neslon,b Jamaal Cox,b Bradley T. Jonesa
a Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina, 27109 USA
b Department of Chemistry, Physics, and Geology, Winthrop University, Rock Hill, South Carolina, 29733 USA
Note: The following manuscript was published in Microchemical Journal (DOI:
10.1016/j.microc.2011.04.009). Stylistic variations are due to the requirements of the journal. All of the
presented research was conducted by Summer N. Hanna with the exception of the computer programming
completed by Clifton P. Calloway and the autodry program developed by Jason Sanders, Jamaal Cox, and
Ronald Nelson. The manuscript was prepared by Summer N. Hanna and edited by Bradley T. Jones.
81
ABSTRACT
A new compact, aluminum electrothermal vaporization cell was constructed for inductively coupled
plasma atomic emission spectrometry analysis. This cell is compact enough to fit within the space
occupied by the spray chamber and fits directly to the quartz torch without extraneous tubing through the
use of a simple Compression fitting. Sample volumes as low as 10 µL were analyzed with an automated
control program for efficient vaporization. Twelve elements were analyzed utilizing a time resolved
acquisition method so that real-time data could be generated over a period of ten seconds with an average
improvement factor of 14 for elements over a wavelength range of 193-445 nm.
KEYWORDS
Inductively coupled plasma atomic emission spectrometry, tungsten coil, electrothermal vaporization
82
INTRODUCTION
Sample introduction for inductively coupled plasma-atomic emission spectrometry (ICP-AES) typically
relies on large volumes of liquid analytes. Methods historically fall within two categories: pneumatic and
ultrasonic nebulization, both of which depend on the use of a spray chamber to prevent large liquid
droplets from entering the plasma. When using nebulization, approximately only 3-12% of the sample is
introduced into the plasma, with the remainder being lost to waste. 69 This method often encounters
problems due to the autosampler and copious amounts of tubing necessary for sample transport, which
can introduce air bubbles into samples. Poor connections between components can also create problems
in otherwise routine methods. 104
Because of these limitations, all sample introduction techniques will focus on electrothermal vaporization
(ETV). This method has been used extensively for the introduction of slurries, powders, solid materials,
and even complex biological matrices.55, 105-107 ETVs are often made from graphite, which can be
expensive because of the material’s required purity, inability to tolerate mass production, and resulting
low demand for production. 59 A large thermal gradient often occurs and interferences may be seen when
determining carbide-forming elements. Samples with a large concentration of surfactants may also
interfere with measurements by depositing carbon on the torch.40 Because of these disadvantages, metal
atomizers are becoming increasingly popular since they are nonporous, can provide quick and efficient
vaporization of a wide variety of materials, and do not require the elaborate power supplies that are
necessary when using graphite ETVs.
Tungsten is particularly well suited for these types of applications since it has the highest melting point of
any element and is found as a boat, ribbon, or coil. 76, 108, 109 Extensive work has been done with tungsten
ETVs which are regarded as a reliable vaporization source. Rare earth elements, which are notoriously
difficult to analyze because of their chemical similarities and tendency to form oxides, have been well
characterized using tungsten as an ETV in inductively coupled plasma mass spectrometry (ICP-MS)
83
analysis. 110 Complex matrices, such as human urine, have also been successfully quantified using
tungsten coil ETVs when paired with permanent modifiers or extraction techniques. 76, 77
Tungsten coils such as the ones used in the determination of cadmium in urine samples are a great
alternative ETV source. These coils are made for use in projector light bulbs and can be purchased for as
little as two dollars each since they are commercially mass produced, which ensures adherence to strict
structural specifications. Their small size of less than 20 mm3 allows for the use of sample volumes on a
microliter scale. This reduction in volume provides users the opportunity to analyze extremely small-
volume or precious samples while utilizing the multielement capabilities of ICP-OES. Tungsten coil
ETVs in the past have used cells that did not connect directly to the torch of the ICP, but instead relied on
centimeters of tubing. Tubing makes analyte loss possible through condensation of the “hot” analyte
vapor on the walls of “cool” transport tubing.51 To overcome potential loss, a new ETV cell design that
connects directly to the quartz torch of the ICP through a simple Compression fitting has been used. The
reduction in cell size and elimination of tubing provides an ETV sample introduction source that can fit
within the space formerly occupied by a spray chamber and nebulizer. The ETV components can be
replaced with a traditional sample introduction system in less than a minute, which allows the user to
quickly alternate between sample introduction methods.
84
MATERIALS AND METHODS
2.1 Reagents
All standard solutions were prepared from their metal salts using 15.8 N trace metal grade nitric acid
(Fischer Scientific, Pittsburg, PA, USA) and 18 MΩ distilled, deionized water (Milli-Q Systems,
Millipore). Working solutions were prepared from NIST Standard Reference Material (SRM) 1643e,
Trace Elements in Water, by simple dilution using distilled, deionized water and also utilized the standard
additions method to mitigate matrix effects typically encountered with ICP-AES.
2.2 Instrumental
W-coil ETV-ICP-AES is composed of an aluminum electrothermal vaporization cell and the ICP-AES
system. Figure 1 shows a schematic diagram of the ETV-ICP-AES cell. In this system, a tungsten coil is
extracted from a commercially available 150 W * 15 V projector light bulb (Osram HLX 64633 Xenophot,
Augsburg, Germany). The filament is exposed by removing the silica envelope while leaving the base
intact and is then mounted in the ETV cell’s ceramic base. The ceramic base is secured using two screws
to an aluminum mount. This mount has a 1” outer diameter and is 7/8” in length. The mount and socket
fit flushed into an aluminum rod and use a screw to create an airtight seal. This rod is 3” in length with an
11/16” inner diameter to accommodate the socket and mount. The arrangement of the socket on the
aluminum mount allows for the W-coil to align with a 1/8” hole which is used for sample introduction.
This opening is plugged with a screw and rubber o-ring to prevent oxidizing gases to enter the ETV cell.
A 1/16” NPT for gas fittings is located on the back side of the cell and immediately following is a ¼”
NPT at the base of the rod which allows for the connection of a glass outer Compression joint fitting
union (SS-4-UT-6-400, Swagelok, Solon, OH, USA). The compression fitting provides an immediate
glass-to-glass connection to the ICP-OES quartz torch without use of extraneous tubing normally
necessary in ETV-ICP-AES.
85
Figure 1. Illustration of ETV cell. a) tapped hole for riser screw b) 2 screws to attach socket to mount c) 1” aluminum rod with 11/16” inner diameter to fit ceramic socket d) 1/8” hole centered and in line of sight of coil e) 1/16” NPT for gas fitting in back near bottom f) ¼” NPT for glass outer joint compression fitting
a
b
c
d
e
f
86
Figure 2. Photograph of the ETV cell and ICP torch connection
87
Figure 2 depicts the joining of these two parts in a manner identical to the glass to glass connection that
would be used for a spray chamber. This connection allows for the quick and easy replacement of a spray
chamber with the ETV cell in less than a minute.
A Visual Basic program controls the coil drying and vaporization cycle while simultaneously initiating
ICP spectra collection. The program automatically dries the coil by measuring the potential across the coil
at a constant current (3.4 Amps). As the liquid sample evaporates the potential across the coil increases.
The Visual Basic program runs a variable time loop, and every 0.5 seconds reads the voltage across the
coil. Using this measured voltage, a slope is calculated using the formula:
. (1)
The two most recent slope values are averaged and displayed on the screen. Once the voltage potential
slope reaches 0.38 (value determined empirically), the coil is dry. The program then continues to a set of
user defined values which vaporize the sample and trigger data collection. Step 2 in Table I at 8 amps is
vaporization and designated trigger step, which signals the ICP data software to begin collection of the
analytical signal through the time resolved analysis feature.111
The ETV connects directly to the ICP-AES torch. The axially arranged quartz torch, (ML155064,
Meinhard Glass, Golden, CO, USA) is surrounded by three water-cooled radio frequency (RF) coils that
use a frequency of 40.68 MHz to generate an argon plasma. The analytical zone of the plasma is aligned
with the 40 µm x 100 µm entrance slit of the argon-purged optical system. The Prodigy High Dispersion
ICP-OES (Teledyne Leeman Labs, Hudson, NH, USA) uses an 110 mm x 110 mm Echelle grating of
52.13 gr/mm to focus (focal length (f), f=800mm) the analytical signal on a solid-state, Large Format,
Programmable Array Detector (L-PAD). This enables continuous coverage over a wavelength range of
165-1100 nm. The analytical signal is collected using the Prodigy’s Time Resolved Analysis (TRA)
feature, which collects all analytical lines in a method simultaneously over a defined time period (Elapsed
Time) at a defined frequency (Time Slice). The real-time data is displayed graphically, and allows the
88
user to immediately observe analyte emission in the plasma. This capability is advantageous to previously
mentioned ETV-ICP analyses where data acquisition was only capable in 1 second intervals, causing
approximately 90 % of the signal to be lost as dead time.112, 113 TRA reduces the dead time by collecting
100 millisecond time slices acquired over the ten second vaporization period. Each of the wavelengths is
simultaneously observed using the LPAD camera, and a desired wavelength(s) can be exported
individually or as a group of wavelengths for data manipulation (Fig. 4).
89
Table I. Heating program for the tungsten-coil vaporizer.
Step Current (Amps)
Time (seconds)
Signal Read
Drying Autodry variable No Cooling 0 10 No
Vaporization 8 10 Yes Cooling 0 60 No
90
Figure 3. Time resolved emission profile of barium, manganese, and strontium found in SRM 1643e
-5
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9 10
Rel
ativ
e E
mis
sion
In
ten
sity
Time (seconds)
Sr
Ba
Mn
91
RESULTS AND DISCUSSION
3.1 Optimization of Experimental Conditions
Traditional sample introduction was used to optimize the viewing position of the spectrometer by the
connection of a double-pass spray chamber and nebulizer to the torch. The generation of a constant signal,
as opposed to a discrete plug, allowed the viewing of the plasma to be optimized using the manufacturer’s
suggested method. Upon optimization, the spray chamber and nebulizer were removed and the ETV cell
was reattached with special care to avoid jarring the torch, ensuring optimal viewing conditions for the
analytical signal.
To identify the maximum sample introduction efficiency, the operating parameters of the ETV-ICP
system were assessed. 40.0 µL samples of a 0.50 mg/L solution of iron were pipetted onto the W-coil and
subjected to the previously mentioned vaporization cycle. Five emission measurements at each setting
were averaged. Each averaged value within a parameter was plotted to determine the maximum. The
setting which produced the maximum emission signal was determined to be the optimal setting within
that particular parameter. The optimization of the auxiliary flow is not discussed here, as it did not
contribute to signal or plasma stability.
3.1.1 Optimization of Torch Coolant Gas
The torch coolant gas provides a cooling mechanism to the quartz torch while also creating a more stable
plasma. Gas flows from 13.0-20.0 L/min were used to determine the effect on emission signal. 13.0 L/min
of coolant gas caused peak broadening, occasional splitting of emission peaks, and significant plasma
instability. Figure 4 shows that flows in excess of 18.0 L/min provided no observable enhancement in
emission. The highest emission intensities occurred when coolant flow rates of 17.0 L/min were used.
3.1.2 Optimization of Radio Frequency (RF) Power
92
The wattage of the RF coils of the ICP-OES system creates the plasma by generation of a 40.68 MHz
radio frequency field. This field accelerates free electrons in the argon gas provided by the coolant flow.
When these free electrons collide, they generate extensive heating to produce the plasma used for
excitation of the vaporized sample. RF Powers of 0.9, 1.0, 1.1, 1.2, 1.3, and 1.4 kW were studied for their
ability to produce the highest emission signals while also creating a very stable plasma. The
manufacturer’s suggested setting is 1.1 kW, and optimization studies indicated that there was no
significant difference in the emission intensity with respect to the differing radio frequency powers. This
lack of difference allowed all studies to continue at the manufacturer’s suggested setting of 1.1 kW.
3.1.3 Optimization of Carrier Gas
0.5, 1.0, 1.5, and 2.0 L/min flows of the 10 % hydrogen/ 90 % argon carrier gas were studied for their
ability to transport the aerosol plug into the plasma. In addition to transport, this carrier gas creates a
reducing atmosphere around the tungsten coil while simultaneously providing quick cooling to the coil
after the application of high currents. If the flow of the carrier gas falls below 0.5 L/min it is unable to
prevent oxidation of the tungsten coil at high applied currents. This oxidation causes the coil to quickly
become brittle and introduces tungsten oxides into the plasma. The tungsten oxides create a large
emission band that severely interferes with signals of interest. Figure 5 shows virtually no emission signal
when this low carrier gas flow rate is used. Flow rates that are higher than 2.0 L/min create very weak
emission signals. The 1.0 L/min flow of the 10 % hydrogen/ 90 % argon carrier gas provided the best
signals while also preventing the coil from oxidizing at higher vaporization currents.
93
Table II. Optimized Operating Parameters
Parameter Optimized
Value RF Power (kW) 1.1
Coolant Gas Flow (L/min)
17
H2/Ar Flow (L/min) 1.0 Auxiliary Flow (L/min) 0
94
Figure 4. Effect of Ar coolant gas flow rate on the emission intensity of 0.50 mg/L iron
0
5
10
15
20
25
30
35
14 15 16 17 18 19 20
Rel
ativ
e E
mis
sion
Sig
nal
Coolant Flow (L/min)
95
Figure 5. Effect of H2/Ar carrier gas flow rate on the emission intensity of 0.50 mg/L iron
0
50
100
150
200
250
0.5 1 1.5 2
Rel
ativ
e E
mis
sion
Sig
nal
Gas Flow (L/min)
96
3.2 Limits of Detection
Using the optimized conditions in Table II, analytical figures of merit were determined for arsenic,
barium, cadmium, copper, iron, lead, manganese, palladium, platinum, selenium, silver, and strontium.
The standard solutions were prepared as indicated in Section 2.1. Five replicates of each concentration
were taken, and limits of detection (LODs) were calculated by multiplying the standard deviation of the
blank (n=10) by three and dividing by the slope of the calibration curve. For comparison to traditional
ICP-OES systems figures of merit (FOM), the ETV cell was replaced by a standard Scott-style double-
pass spray chamber (Part Number: ML155055, Meinhard Glass, Golden, CO, USA) and a conical
nebulizer (Model Number: TR-30-K2, Meinhard Glass, Golden, CO, USA). Results are listed in Table III
and an average improvement factor of 14 was observed for the twelve elements studied.
97
Table III. Figures of merit for twelve elements of interest
Element Wavelength
(nm)
ETV LOD
(µg/L)
Nebulizer LOD
(µg/L)
Improvement Factor
Arsenic 193.7 70 200 3 Barium 445.4 0.1 4 40
Cadmium 226.5 2 2 1 Copper 324.8 0.5 10 20
Iron 259.9 4 4 1 Lead 220.4 2 90 45
Manganese 257.6 0.2 1 5 Palladium 340.5 10 30 3 Platinum 214.4 10 300 30 Selenium 196.1 8 100 13
Silver 328.1 1 3 3 Strontium 407.7 2 3 2
98
3.3 Validation of Method
The method of ETV-ICP-AES was validated through the use of NIST SRM 1643e, trace metals in water,
using the standard additions method to reduce matrix effects often encountered when using ICP-OES.
Table IV lists the recovered values of seven elements of interest in the standard reference material. There
was a 103 % average recovery for these elements.
99
Table IV. Recoveries for Seven Elements of Interest
Element Reported Value (µg/L)
Recovered Value (µg/L) % Recovery
Ag 0.11 0.11 100 Ba 0.55 0.61 110 Cd 0.7 0.7 100 Cu 0.02 0.03 108 Mn 0.42 0.42 100 Se 1.2 1.1 92 Sr 3.2 3.6 112
100
CONCLUSION
A new electrothermal vaporization device was successfully constructed and used to quantify twelve
elements spanning the ultraviolet region. These elements are often difficult to excite, and an ETV source
was used to allow for improved limits of detection. This ETV allows the user to reduce necessary sample
volumes from several milliliters to only a few microliters. The ETV cell is also easily replaced by a
standard spray chamber, providing a rapid interchange between sample introduction methods. Figures of
merit were improved on average by a factor of 14. Particular success was seen with platinum, which often
relies on inductively coupled plasma-mass spectrometry because of the limited ability to analyze reduced-
volume samples associated with inductively coupled plasma atomic emission spectrometry. Future
outlooks include slurries and solids with anticipation that materials will perform well on the tungsten coil
due to the platform shape of the horizontally oriented tungsten coil.
101
REFERENCES
1. Okamoto, Y.; Komori, H.; Kataoka, H.; Tsukahara, S.; Fujiwara, T., Direct determination of lead
in biological samples by electrothermal vaporization inductively coupled plasma mass spectrometry
(ETV-ICP-MS) after furnace-fusion in the sample cuvette-tungsten boat furnace. Fresenius Journal of
Analytical Chemistry 2000, 367, 300-305.
2. Gaines, P., Sample introduction methods for ICP-MS and ICP-OES. Spectroscopy 2005, (January
2005), 20-23.
3. Amberger, M. A., Broekaert, J.A.C., Direct mulitelement determination of trace elements in
boron carbide powders by slurry sampling ETV-ICP-OES. Journal of Analytical Atomic Spectrometry
2010, 35, 1308-1315.
4. Nickel, H., Zadgorska, Z., A strategy for calibrating direct ETV-ICP-OES analysis of industrial
ceramics in powder form. Fresenius' Journal of Analytical Chemistry 1995, 351, 158-163.
5. Detcheva, A., Barth, P., Hassler, Calibration possiblities and modifier use in ETV-ICP-OES
determination of trace and minor elements in plant materials. Analytical Bioanalytical Chemistry 2009,
394, 1485-1495.
6. Li, S., Hu, B., Jiang, Z., Chen, R., Direct determination of trace refractory elements in human
serum by ETV-ICP-MS with in-situ matrix removal. Analytical Bioanalytical Chemistry 2004, 379,
1076-1082.
7. Berndt, H., Schaldach, G., Simple low-cost tungsten-coil atomizer for electrothermal atomic
absorption spectrometry. Journal of Analytical Atomic Spectrometry 1988, 3, 709-712.
8. Wen, X.; Wu, P.; Chen, L.; Hou, X., Determination of cadmium in rice and water by tungsten coil
electrothermal vaporization-atomic fluorescence spectrometry and tungsten coil electrothermal atomic
absorption spectrometry after coud point extraction. Analytica Chimica Acta 2009, 650, 33-38.
102
9. Davis, A. C.; Alligood, B. W.; Calloway, C. P.; Jones, B. T., Direct determination of cadmium in
urine by electrothermal vaporizer-inductively coupled plasma analysis using a tungsten coil vaporizer.
Applied Spectroscopy 2005, 59, (10), 1300-1303.
10. Nakamura, S. K., Y., Kubota, M., Temperature of a W ribbon furnace in electrothermal atomic
absorption spectrometry. Spectrochimica Acta Part B 1986, 41, 817-823.
11. Nakamura, Y., Takahashi, K., Kujirai, O., Okochi, H., Evaluation of electrothermal vaporization,
emission intensity-tie-wavelength measurement and time resolution combined with an axially viewed
horizontal inductively coupled plasma using an echelle spectrometer with wavelength modultaion.
Journal of Analytical Atomic Spectrometry 1997, 12, 349-354.
12. Shibata, N.; Fudagawa, N.; Kubota, M., Electrothermal vaporization using a tungsten furnace for
the determination of rare-earth elements by inductively coupled plasma mass spectrometry. Analytical
Chemistry 1991, 63, 636-640.
13. Davis, A. C.; Calloway, C. P.; Jones, B. T., Chelation of urinary cadmium with ammonium
pyrrolidine dithio-carbamate prior to determination by tungsten-coil inductively coupled plasma atomic
emission spectrometry. Microchemical Journal 2006, 84, (1-2), 31-37.
14. Wu, P.; Wen, X.; He, L.; He, Y.; Chen, M.; Hou, X., Evaluation of Tungsten Coil electrothermal
vaporization-Ar/H2 flame atomic fluorescence spectrometry for determination of eight traditional hydride-
forming elements and cadmium without chemical vaporization generation. Talanta 2008, 74, 505-511.
15. Hanna, S. N.; Keene, J.; Calloway, C. P.; Jones, B. T., Design of a Portable Eelctrothermal
Vaporization Flame Atomic Emission Spectrometry Device for Field Analysis. Instrumentation Science
and Technology 2011, (in press).
16. Gras, L. d. L.-V., M.T.C., Limiting effects on transport efficiency by matrix load in inductively
coupled plasma optical emission spectrometry with electrothermal vaporization sample introduction.
Spectrochimica Acta Part B 2000, 55, 37-47.
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17. Maestre, S. d. L.-V., M.T.C., Plasma behavior during electrothermal vaporization sample
introduction in inductively coupled plasma atomic emission spectrometry. Spectrochimica Acta Part B
2001, 56, 1209-1217.
104
CONCLUSION
Atomic spectroscopy has been the standard for trace metal analysis for some time, and the incorporation
of novel sources into otherwise routine methods continues to allow for the development of breakthroughs
in spectroscopic techniques. The tungsten coil, despite its forty years of use, continues to produce
improvements in instrumentation development and applications.
Tungsten coil electrothermal vaporization as a sample introduction method continues to be a promising
technique. It generally outperforms its graphite counterparts by being operationally less demanding while
generating improved figures of merit. The electrothermal vaporization flame atomic emission
spectrometer discussed here is constructed for a total of less than five hundred dollars and can be operated
independently by an undergraduate researcher. Limits of detection ranged from 0.6 to 200 ng L-1, which
are on average 100 times better than figures of merit associated with traditional flame atomic emission
spectrometry. This technique is comparable to inductively coupled plasma optical emission spectrometry
but at a fraction of the cost and with the potential for portability.
The tungsten coil electrothermal vaporization sample introduction method for inductively coupled plasma
optical emission spectrometry eliminates the use of peristaltic tubing, nebulizer, and spray chamber
accessories of the sample introduction system. The newly designed aluminum electrothermal vaporization
cell fits directly onto the quartz torch via a glass to glass connection by using a simple compression fitting.
The likelihood of condensation on the sides of the vapor transport material is reduced and the device
introduces the analyte as a discreet plug into the heart of the argon plasma. Sample volumes are reduced
from the milliliter scale associated with nebulization to a microliter scale. The multielement analysis of
small-volume or precious samples is possible in real-time. The time resolved analysis feature allows user
105
flexibility in data processing without the loss of emission data to dead time caused by a lagging
spectrometer. Using the tungsten coil electrothermal vaporization system, an average improvement factor
of fourteen was seen for twelve elements of interest spanning the ultraviolet and visible regions.
While this body of work has only focused on microliter scale, liquid samples, the logical next step would
be to focus on solid and slurry analysis. The ability to analyze slurries and solids eliminates the need for
extensive sample pretreatment—a step that is often the most time-consuming component to experimental
methods in atomic spectroscopy while also reducing the risk of analyte loss or contamination. The
tungsten coil would make an ideal solid sampling technique because of its rapid heating to temperatures
in excess of 3000 °C degrees. The tungsten coil’s design is already similar to that of a L’Vov platform, so
that solid or slurry samples could easily be placed on a horizontally oriented coil for rapid vaporization
into nearly any instrumental system for trace metal analysis. The cell housing for the tungsten coil would
need to be altered so that solid or slurry samples could easily be placed on the surface while ensuring that
samples did not fall off of the horizontally-oriented coil. Stability could be achieved by always
maintaining a sample cell that is parallel to the surface on which the instrument is placed.
Tungsten coil electrothermal vaporization sample introduction provides alternative methodology to
instrumentation routinely used in laboratories. This technique may also be coupled to systems that are not
normally considered for scientific use such as the welder’s torch encountered here. This method will
continue to allow for advances in atomic spectrometry by providing users with creative and flexible
techniques for a virtually infinite assortment of samples.
106
CIRRICULUM VITAE
Birthplace
April 22, 1985 Marion, South Carolina
Education
Wake Forest University Winston-Salem, NC Doctor of Philosophy in Chemistry 2011 Winthrop University Rock Hill, SC Bachelor of Science in Chemistry 2007
Scholastic and Professional Experience
Research Assistant 2009-2010 Wake Forest University Winston-Salem, NC Teaching Assistant 2007-2009, 2010- 2011 Wake Forest University Winston-Salem, NC Graduate Student Instructor July 19-30, 2010 CERTL Summer Outreach Winston-Salem, NC Technology Transfer Graduate Student Intern August 2010-May 2011 Wake Forest University Baptist Medical Center Office of Technology Asset Management Winston-Salem, NC
Publications
Jiyan Gu, Summer N. Hanna, Bradley T. Jones. A Portable Tungsten Coil Atomic Emission Spectrometer for the Simultaneous Determination of Metals in Water and Soil Samples. Analytical Sciences. 2011. (in press) Summer N. Hanna, Joseph Keene, Clifton P. Calloway, Jr., Bradley T. Jones. Design of a Portable Electrothermal Vaporization Flame Atomic Emission Spectrometry Device for Field Analysis. Instrumentation, Science, and Technology. 2011. (in press) Summer N. Hanna, Bradley T. Jones. An Electrothermal Vaporization Flame Atomic Emission Spectrometer. Journal of Analytical Atomic Spectroscopy. 2011, DOI: 10.1039/c1ja10060b
107
Summer N. Hanna, Bradley T. Jones. An Electrothermal Vaporization Source for Inductively Coupled Plasma Optical Emission Spectrometry. Microchemical Journal. 2011, DOI: 10.1016/j.microc.2011.04.009. Summer N. Hanna, Bradley T. Jones. A Review of Tungsten Coil Eelectrothermal Vaporization as a Sample Introduction Technique in AtomicSpectrometry. Applied Spectroscopy Reviews. 2011, DOI: 10.1080/05704928.2011.582659
Poster Presentations
Summer Hanna, Joseph Keene, Bradley T. Jones. 2009. “Cutting Edge Metal Detector:” Presented at the Wake Forest University Graduate Student Research Day. Summer Hanna, Joseph Kene, Bradley T. Jones. 2009. “Cutting Edge Metal Detector:” Presented at the Federation of Analytical Chemistry and Spectroscopy Societies Annual Conference. Summer Hanna, Bradley T. Jones. 2010. “Increasing Accessibility to Instrumentation: A “Cutting Edge Metal Detector” for Field Analysis:” Presented at the Wake Forest University Graduate Student Research Day. Summer Hanna, Bradley T. Jones. 2010. “Increasing Accessibility to Instrumentation: A “Cutting Edge Metal Detector” for Field Analysis:” Presented at the Federation of Analytical Chemistry and Spectroscopy Societies Annual Conference.
Oral Presentations
Summer Hanna. “Chemistry Beyond the Periodic Table.” 2010. Marion High School. Marion, SC Summer Hanna, Christopher MacNeil, Bradley T. Jones, Robert Coffin. 2010. “Quantification of Palladium in Low Bandgap Polymers Using Inductively Coupled Plasma Optical Emission Spectrometry.” Winthrop University, Rock Hill, SC. Summer Hanna, Bradley T. Jones. 2011. “Design of a Compact, Aluminum Electrothermal Vaporization Device for Inductively Coupled Plasma Optical Emission Spectrometry.” Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy Annual Meeting.
Awards Dean’s Fellowship 2007-2008 Wake Forest University Winston-Salem, NC
Professional and Academic Organizations Society for Applied Spectroscopy 2008-present Wake Forest University Graduate Student Association 2007-2010
Reynolda Campus Chair 2009-2010
108
Association of University Technology Managers 2011-present
Related Professional Experience
Panelist for 2008 “Getting into College Alone: The Barriers of a First Generation College Student,” held at Winthrop University, sponsored by Pi Sigma Alpha Guest speaker and lab facilitator for “Opportunities in Science” held in 2009 at Marion High School, Marion, SC
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