Enhanced Preconcentrator for the Analysis of Vapor Phase

Enhanced Preconcentrator for the Analysis of Vapor Phase Volatile Organic Compounds

AbstractAuthors

Analysis of volatile organic compounds (VOCs) in ambient air or other vapor

phase matrices, such as soil gas, product o� gassing, or large volume headspace has

been enhanced by improvements in design of a new automated, preconcentration

system. Many advancements have been made to previously available technology

for the analysis of vapor phase samples to improve productivity and data results.

An increased dynamic range of calibration, allowing sample volumes from 1 to 1000

milliliters to be concentrated, increases productivity by analyzing samples with

varying concentrations without having to perform manual sample dilutions. A new

trap design and improved coating technology using Silonite-D® creates a very inert

sample �ow path, allows higher molecular weight compounds to be recovered, and

adds the ability to analyze highly reactive species such as formaldehyde and

hydrogen sul�de. A three stage trapping design provides a matrix insensitive

concentration technique due to the ability to remove water and all major air

components including oxygen, nitrogen, carbon dioxide. Data quality has been

improved using Accu-Sample™ technology which measures volume by pressure

di�erential of a �xed volume vacuum reservoir. This feature creates calibration

curves with excellent relative standard deviations. Data is presented to show

improved linearity of calibration curves and method detection limits for a wide

range of VOCs.

Thomas X. Robinson, Daniel B. Cardin,

Entech Instruments, Inc.

2207 Agate Court

Simi Valley, CA 93065

USA


7200 Preconcentrator | GC/MS Analysis

Application Note

I N S T R U M E N T S I N C .

IntroductionThe 7200 Preconcentrator with Accu-Sample™ technology

(Entech Instruments, Inc., Simi Valley, CA), represents the next

generation in GC and GC/MS sample preparation systems for

the analysis of vapor phase volatile organic compounds. The

7200 features improvements in 3-stage preconcentration and

water management technology and many past limitations

seen in rotary valve based devices are resolved by combining

digital valve isolation with direct inlet canister autosampling –

which drops potential carryover and cross-contamination far

below what was previously possible.

A Silonite-D® coating throughout the �ow path virtually

eliminates unwanted chemical reactions, ensuring complete

recovery of volatile and light semi-volatile compounds. The

7200’s advanced water and CO2 management technologies

provide superior analysis of polar and non-polar organics and

an inert, heated �ow path recovers hydrocarbons in the range

of C2-C18 (depending upon trapping conditions). the 7200

Preconcentrator includes four built in inlets available for direct

sample introduction, or as inlets for multi-position

autosamplers such as the 7600, 7650, and 7016D (Entech

Instruments, Inc., Simi Valley, CA).

Internal traps feature an optimized geometry that improves

trap temperature consistency during analysis. Accu-Sample™

technology, (patent pending) combines digital valve control

with direct volume measurement – rather than indirect time

integrated �ow measurements – to allow better small volume

accuracy down to 10cc over a wider pressure range and

permits both air / non-air matrices to be analyzed accurately.

A loop injection valve can also be added to further reduce

quantitative sample volume analysis down to as low as 1cc.

System Hygiene:

Like the previous generation 7100A system, the new 7200

Preconcentrator includes the ability to back�ush lines out to

the sample ports to eliminate any previous sample in each line

prior to connecting to the next sample. However, with

laboratories now faced with analyzing both ambient air and

soil gas samples routinely, system hygiene must go beyond a

simple back�ush. Isolation to prevent undesired exposure to

traps, tubing, and valve rotors during the analytical process

has become much more important. The 7200 features a

shorter sample path than its predecessor, and also includes

the ability to prevent any cross-contamination when moving

its Stream Select Valve to the next desired port. This is

accomplished by isolating the downstream �ow path using

Entech’s new digital rotary valve technology. In addition, new

autosampling inlets are available for the 7200 that make only a

brief contact with a sample, just long enough to withdraw the

requested aliquot, rather than for hours or days as with rotary

valve based autosamplers. This further reduces the possibility

of cross-contamination when analyzing higher concentration

soil gas samples.

Quality Assurance:

The 7200 comes equipped with tools for validating system

performance, including the ability to perform automated leak

checking and matrix spiking. The 7200 also records important

parameters during each sample preconcentration to verify

proper system operation, including trapping �ow rates, �ow

volumes, trap pressure drop, trapping temperatures, water

management parameters, desorption temperatures / �ows,

autosampler position, and sample transfer times. Data is

saved into a SQL Database for easy integration into LIMS

systems, and an automated summary is generated that

provides critical run time parameters in an easily interpretated

one page report.

Leak Checking:

A signi�cant source of errors in many GC inlet systems is the

presence of leaks that go undetected. The 7200 performs

automated leak checking using both pressure and vacuum

techniques to ensure a secure leak-tight system exists before

any samples are analyzed. A report is generated giving the

starting and pressure during the monitoring period. Leak

checks can be done by either by selecting individual sample

ports or by selecting a sequence table (which de�nes a group

of samples on the autosampler). Leak checks for canister

samples on autosampling inlets such as the 7600 or 7650 are

not required since all samples remain closed and isolated until

accessed for analysis.


ExperimentalMatrix Spiking:

GC or GC/MS calibration is performed using carefully

prepared standards in a clean matrix (Nitrogen or Zero Air). It

is easy to assume that no interferences exist in actual samples

that will change response factors or detection limits, but this

may not always be the case. The true detection limit of

benzene, for example, may be altered if it co-elutes with a

high concentration interferent that was not present in the

standard. The only way to determine whether interferences

are changing response factors for target compounds in more

complicated matices is to spike low levels of the analytical

standard right into the sample matrix. For example, by

adding 1 ppb of target compounds to the sample being

analyzed, all responses should go up by about 1 ppb.

The 7200 simpli�es this process of interference determination

by allowing a co-preconcentration of sample and calibration

standard. This capability ensures analytical accuracy on the

most critical of samples and can help to uncover any matrix

interferences if they exist.

Trapping Sweep M1 – M2 M2 – M3 Bakeout

Module 1 – Empty Trap -40˚C -40˚C 10˚C N/A 230˚C

Module 2 – Tenax® Trap -40˚C -40˚C -40˚C 230˚C 230˚C

Module 3 – Open Tube N/A N/A N/A -150˚C 1 Minute

Volume (cc) 250cc 75cc 50cc 20cc N/A

Flow Rate (cc per minute) 100 100 10 6 N/A

The GC Analytical data was generated using a 7200

Preconcentrator interfaced with a Shimadzu® QP2010 Ultra

GC/MS. GC oven temperature started at 35˚C (5 min) ramped

at 6˚C/min to 120˚C, then 10˚C/min to a �nal temperature of

220˚C (5 min). The MS acquisition was from 28 to 280 amu (it

is necessary to start at 28 if you are including Formaldehyde in

the analysis, otherwise 30 amu would be used at least for the

�rst 8 minutes for some of the lighter compounds).

Calibration standards were obtained from Linde Gas® and

Scott Specialty Gases®. Three cylinders at 1 ppmv were

blended together using a 4600A Dynamic Dilution System

(Entech Instruments, Inc., Simi Valley, CA), then diluted to 10 ppbv

for the calibration curve and 2 ppbv for the Method Detection

Limit Study. Calibration was performed by picking a nominal

volume to be trapped and used for all samples needing

quantitation, and the curve was obtained by varying the

volume of the 10 ppbv standard. A nominal volume of 250cc,

was used. This volume being equal to 10 ppbv volumes of

10cc, 25cc, 50cc, 100cc, 250cc, 500cc, and 1000cc, yields a

calibration range from 0.4 ppbv to 40 ppbv. The MDL was

analyzed with seven 100cc replicates of the 2 ppbv standard,

this equates to an amount of 0.08 ppbv when the nominal

volume is 250cc. The results of these analyses are

summarized in table 2. Sample trapping conditions of the

7200 Preconcentrator are shown below in table 1.

7200 Preconcentrator

Table 1 - 7200 Preconcentrator trapping conditions for Cold Trap Dehydration.

7200 Preconcentrator, shown with 7600 Autosampler.


Figure 1 - GC/MS shown with 7200 Preconcentrator, 7600 Autosampler, and the 7016D Autosampler.

Figure 2 - 7200 Preconcentrator Flow path featuring Silonite-D® coated tubing.

7200 | 7600 | 7016D System Connections

7200

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MS GC 7600 Autosampler7200 Preconcentrator 7016D Autosampler

Analyze up to 18 Additional Canisters using the 7600’s Expansion Ports!

7200 Preconcentrator

CNC NO

Pump He/SP

VolumeDetermination

Reservoir

Internal StandardCal. Standard

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Reservoir Isolation ValveReservoir Evacuation Valve Pump Isolation ValveFlow Control Valve

GC Carrier Gas

Sample 1

Helium In

Dehydration Module 1 Cold Tenax® Module 2 Focusing Module 3

5

7

Smart Lab™ 2A The 7200 is controlled by Entech’s SmartLab™ 2A control network operating under Microsoft Win XP® or Windows 7® using the latest high-speed USB interface technology.

Sample 2

Sample 3

Sample 4


DiscussionExtended Cold Trap Dehydration (ECTD) is the concentration

technique utilized by the 7200 Preconcentrator, as illustrated

in Figure 1 and Figure 2. The sample �rst �ows through an

empty Silonite-D® coated trap at -40˚C and then through a

second trap at -40˚C containing Tenax® TA. Water is removed

by direct conversion from the gas to the solid phase in trap 1,

eliminating the potential loss of highly polar VOCs (HPVOCs)

into any liquid water. This allows recovery of compounds

such as formaldehyde and hydrogen sul�de that would not

be properly recovered using other water management

techniques. Although some of the heavier VOCs may also

temporarily condense in the �rst cold trap, a secondary step

is used whereby the �rst trap is heated to +10˚C, this enables

another 30 to 50cc of nitrogen to purge any remaining VOCs

to the cold Tenax® trap, with only a minimal transfer of water

vapor. Cooling the Tenax® trap to -40˚C makes the Tenax

100x stronger than Tenax® at 30˚C, and allows quantitative

trapping of the lightest EPA Method TO-15 compounds, while

taking advantage of unreactive nature of Tenax® to recover all

compounds during desorption. Rapid, splitless injection onto

the GC column requires a �nal focusing stage accomplished by

back desorbing the Tenax® trap at 230˚C into a third trap at

-150 ˚C that contains an empty 1/32” Silonite-D® coated

transfer line. Rapid heating of the �nal focusing trap releases

the sample almost instantly, providing unparalleled injection

rates and light-end resolution. Accu-Sample™ technology

developed by Entech for the 7200 Preconcentrator uses a

combination of three new technologies to improve system

performance and reliable quantitation. First, the 7200 isolates

all downstream �ow volumes before rotating the inlet rotary

valve to e�ectively remove about 98% of the downstream

volume. This virtually eliminates the introduction of other

...Discussion continued on page 7.

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1

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6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00

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10

Inlet: 7200Sample Size: 250ml Headspace, TO-15 StandardSplit Mode: Splitless

Column: DB1, 60m, 0.32mm ID, 1μm �lmCarrier: He, 1.5 cc/m constant �owOven Temp: 35˚C 5 min, 4˚C/min to 110˚C, 15˚C/min to 220˚C, 5 min hold.GCMS: Shimadzu GC 2010 Plus and MS QP 2010 UltraMS Operation: 29-180 amu (�rst 9 min),33-280 amu (remaining time). Amu at 3 scans/sec.

1. Freon 114 2. tert-Butyl Alcohol +1,1-Dichloroethene 3. Carbon Disul�de + Freon 113 4. Trichloroethene 5. Tetrachloroethene 6. m,p-Xylene + Bromoform 7. Bromo�uorobenzene 8. 1,2,4-TMB + tert-Butylbenzene 9. Napthalene 10. Hexachlorobutadiene

EPA TO-15 Standard – 250ml Headspace 10 PPB / 82 Component, Splitless

Figure 3 - 250mL, 10 PPBV 82 Component EPA Method TO-15 Standard.


Table 2 - % Relative Standard Deviation and Method Detection Limit (in PPB).

Analyte % RSD MDL 7200 Preconcentrator | EPA Method TO-15 Standard

Formaldehyde 15.02 1.97

Propene 5.34 0.017

Dichlorodi�uoromethane 16.44 0.007

Chloromethane 9.92 0.010

Dichlorotetra�uoroethane 10.51 0.011

Acetaldehyde 11.17 0.079

Vinyl Chloride 6.97 0.010

1,3-Butadiene 7.18 0.011

Bromomethane 8.92 0.008

Chloroethane 6.75 0.013

Ethanol 10.25 0.050

Bromoethene 6.84 0.008

Trichloro�uoromethane 7.70 0.006

Acrolein 4.79 0.012

Acetonitrile 4.02 0.012

Acetone 21.73 0.144

Propanal 5.67 0.068

1,1-Dichloroethene 3.91 0.020

Acrylonitrile 3.56 0.053

Trichlorotri�uoroethane 7.16 0.066

Tert-Butanol 9.31 0.064

Allyl Chloride 4.24 0.023

Methylene Chloride 5.10 0.012

Carbon Disul�de 4.40 0.017

trans-1,2-Dichloroethene 3.78 0.012

Methyl tert-Butyl Ether 5.63 0.015

Vinyl Acetate 4.59 0.008

2-Chloroprene 3.54 0.012

2-Butanone (MEK) 6.69 0.028

1,1-Dichloroethane Hexane 5.11 0.011

Di-isopropyl Ether 4.53 0.013

cis-1,2-Dichloroethene 2.83 0.010

Ethyl Acetate 3.94 0.007

Ethyl Tert-Butyl Ether 4.53 0.013

Chloroform 4.84 0.012

Tetrahydrofuran 3.86 0.014

1,1,1-Trichloroethane 5.19 0.009

1,2-Dichloroethane 4.96 0.015

Benzene 3.16 0.012

Carbon Tetrachloride 6.26 0.012

Cyclohexane 3.41 0.012

Tert-Amyl Methyl Ether 4.47 0.006

Analyte % RSD MDL 7200 Preconcentrator | EPA Method TO-15 Standard

2,2,4-Trimethylpentane 5.02 0.008

Heptane 3.64 0.016

Trichloroethylene 4.54 0.016

1,2-Dichloropropane 3.79 0.017

1,4-Dioxane 3.91 0.074

Methyl Methacrylate 4.45 0.014

Bromodichloromethane 5.64 0.010

cis-1,3-Dichloropropene 4.71 0.012

4-Methyl-2-Pentanone (MIBK) 6.09 0.013

trans-1,3-Dichloropropene 5.10 0.014

Toluene 5.23 0.010

1,1,2-Trichloroethane 4.39 0.012

2-Hexanone 6.85 0.062

Dibromochloromethane 6.59 0.007

Tetrachloroethylene 7.74 0.009

1,2-Dibromoethane (EDB) 5.52 0.010

Chlorobenzene 4.78 0.008

1,1,1,2-Tetrachloroethane 5.41 0.011

Ethylbenzene 5.84 0.047

m-Xylene 7.06 0.112

p-Xylene 6.66 0.086

Styrene 7.53 0.009

o-Xylene 7.75 0.075

Bromoform 9.38 0.014

1,1,2,2-Tetrachloroethane 6.67 0.013

Cumene 8.50 0.009

n-Propylbenzene 9.17 0.011

o-Chlorotoluene 9.18 0.006

4-Ethyltoluene 10.39 0.011

1,3,5-Trimethylbenzene 11.41 0.008

Tert-Butyl Benzene 8.46 0.011

1,2,4-Trimethylbenzene 9.75 0.008

1,3-Dichlorobenzene 15.44 0.007

Sec-Butyl Benzene 16.05 0.012

Benzyl Chloride 10.00 0.007


o-Cymene 10.75 0.010


n-Butyl Benzene 10.12 0.010

1,2,4-Trichlorobenzene 11.40 0.008

Naphthalene 10.71 0.014

Hexachlorobutdiene 18.15 0.009


gases connected to the inlet rotary valve when selecting the

next inlet, avoiding cross-contamination. Secondly, Entech

has developed an Electronic Volume Control (EVC) module

which is used in place of mass �ow controllers to directly

meter in a requested volume, rather than trying to time

integrate the �ow output of a mass �ow controller to

determine volume. This technology yields far better

accuracy, especially when measuring small volumes in the

range of 10–100cc. Finally, the use of Entech’s exclusive

Silonite-D® coatings throughout the instrument results in

much less chemical interaction with tubing surfaces,

providing a more complete transfer through to the GC/MS.

Figure 1 shows a typical GC/MS chromatogram of a TO-15

standard, with a much enhanced backend recovery (TCB,

Naphthalene, HCB) relative to other TO-15 inlet systems on

the market. Table 2 shows the type of calibrations possible

with the 7200 Preconcentrator using Accu-Sample™

technology. Some compounds, such as Acetone, were

elevated due to a slight background in the system, or in the

calibration standards. Excellent MDLs are also shown in Table

2, many of which are down at, or near, 10 part-per-trillion.

These values were obtained by analyzing a 0.2 PPB standard

7 times.

Discussion (continued from pg. 5)

ConclusionThe 7200 Preconcentrator represents the next generation in

laboratory air preconcentrators. The excellent level of

isolation and quantitation o�ered by the 7200’s exclusive

Accu-Sample™ technology provides the ideal platform for

today’s challenging sample types, including soil gas samples

where concentrations could vary from low-PPB to high-PPM

from one sample to the next. Combined with Entech’s latest

robotic autosampler, the 7650, the 7200 provides the air

laboratory with extremes levels of sample isolation which

virtually eliminates any chance of cross-contamination.

Using the 7200’s optional built-in loop for volume

measurements down to 0.25cc, the 7200 also boasts the

largest dynamic range of any system on the market,

maximizing the number of samples that can be analyzed

without prior dilution.

For Further InformationTo learn more about our products and services, visit our web

site at www.entechinst.com.


www.entechinst.com

Entech Instruments, Inc. shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this material.

Information, descriptions, and speci�cations in this publication are subject to change without notice.

© Entech Instruments, Inc., 2013Printed in the USAApril, 2013AM_AN043E

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Enhanced Preconcentrator for the Analysis of Vapor Phase