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Inductively Coupled Plasma Mass Spectrometry
Dr. Lloyd Allen and Dr. Stuart Georgitis
LECO Corporation3000 Lakeview AvenueSt. Joseph MI 49085
Principles of OperationSpectrometer Componenets
Sample Introduction System
Sample Cell
Optical Bench
Detector
Recorder or Computer
The ICP-MS Spectrometer
• ICP : Source of Ions
– Atmospheric Argon based plasma
– Operated from 2,500 to 8000 Kelvin to produce ions
– Requires interface to vacuum bench
• Mass Spectrometer : Mass Filter or Mass Analyzer
– Quadrupole
– High Resolution Double Focusing
– Ion Trap
– Time-of-Flight
The ICP-MS Spectrometer (2)
• Sample Forms Possible
– Solids : Conductive and Non-Conductive
– Liquids : Aqueous and Organic
– Gases
• Method Advantages
– Very Low Noise = Very High Signal to Noise Ratio
• excellent detection limits (ppt)
– Isotopic Analysis
– Few interferences compare to other atomic techniques
– Speed
The ICP-MS Spectrometer
• Qualitative Uses
– Semi quantitation in the absence of standards, solids.
– Concentration profiling
– Isotopic Ratios = Dating, Finger Printing, Finger Pointing
• Method Disadvantages
– TDS : typically < 0.2%
– High Sensitivity = Contamination during sample prep.
– Sequential systems have elevated RSD’s for Ratios
– High resolution systems : Resolution > = < Sensitivity
– Argide and Matrix Interferences
The Purpose of the Plasma
Desolvation : Drying Droplets
Vaporization : Particles to gas
Excitation : Emission
Atomization : Dissociation
Ionization : Electron Losse-
Radial Emission
Med Energy Required(NAZ) Mn Compromise
High Energy Required Short Wavelengths As, Se
Low Energy Required Long Wavelengths K, Cs, La,Li, Na, Sr
The Plasma of ICP
• AES or MS– Frequency
• 27 Mhz• 40 Mhz
– Matching Network• Crystal Controlled• Free Running
– Solid State– Mini-Torch or
Standard
• ICP-MS Only
– Secondary or Pinch Discharge
• Center Tapped• Interlaced Coils• Torch Shields
– X, Y, Z Control
ICP-MS Components: Interface
Ions must proceed from atmospheric pressure to an area of reduced pressure
required for MS
Plasma 1st stage 2nd stage Analyzer stage
AtmosphericPressure
~ 1 torr 10-4 torr 2 x 10-6 torr
Mechanical Pump(Interface)
Turbo Pump Turbo Pump
Mechanical Pump(Backing)
The ICP-MS Interface
Sampler Cone
Skimmer Cone
Barrel Shock
Mach Disk
SupersonicJet
Zone of Silence
Torch & ICP
SampleIon lenses andMass Analyzer
The ICP-MS Interface
Torch & ICP
Sample
Sampler Cone
Skimmer Cone
Secondary Discharge Oxides, Polyatomics,secondary excitation
Velocity Consideration4 eV, 100 amu ion 1964 m/s
14 eV, 100 amu ion 3674 m/s
A factor of 2 reduction in ions in extraction volume
Ion Energies for Shielded Load Coil
Detection Limits (10s, 3 s)Shielded vs. Top-grounded
ICP Sample Introduction Systems
• Solution Nebulizers
– Concentric
– High Efficiency C.
– V-groove
– Modified Liechte
– Cross Flow
– Burgener
• Spray Chambers
– Scott’s
– Cyclonic’s
– Inert or glass
• Ultrasonic Nebulizer
– Membrane Desolvator
• Direct Injection Nebulizer
• Arc-Spark
• ETV
• Laser
• Direct Insertion Nebulizer
• Hydride Generator
• Discrete Sampling
ICP-AES and ICP-MS
Inductively Coupled Plasma
Simultaneous Sequential
PMT
CID
CCD
PMTPMT’s
Magnetic Sector
Quadrupole
ICP-AES ICP-MSICP-AES ICP-MS
Magnetic Sector
Time of Flight
Ion Trap
The Two Major Approaches to ICP-MS Spectrometry
Sequential : Mass Filters
or
Simultaneous : Mass Analyzer
Quadrupole ICP-MSA Sequential Mass Filter
All m/z Values In
One m/z Value Out
+
-+
-
Separation Based on Stability of the m/z Value in the RF and DC Fields on the Quadrupole Rods
Quadrupole ICP-MS
RF1, DC1
RF3, DC3
Time1
Time2
Sequential Analysis Limitations: ICP-MS
• Sample throughput > = < a function of the number of
m/z values measured
– Transient Signals : very few isotopes analyzed
• ETV
• Chromatography
• Single spot laser ablation
– Can not obtain high precision isotope ratios
– Small volume samples: v. few isotopes analyzed
– Susceptible to cones plugging (TDS) by prolonged sample contact
TOF ICP-MS TheorySimultaneous Mass Spectra
KE = 1/2 mv2 = zV m/z = 2V/v2 m/z = (2Vt2)/(L2)
Velocity v = L / t
Flight Tube Length (L)
(+)
Accelerating Voltage (V)
++++++
Repeated up to 30,000 times per second
Requirements of TOF-ICP-MS
• Continuous ion beam requires modulation
• Detector must respond to fast ion events (ns)
• Data acquisition system must be able to handle TOF
speeds
• Matrix ions must be removed to avoid detector saturation
Right-angle/Orthogonal Injection
Repeller
Acceleration Field
Field-FreeFlight Region
Ion Lenses
Orthogonal TOF ICP-MSDisadvantages
• Transmission Efficiency at best 20%
• Sensitivity/Resolution Tradeoff
• Mass Dependent Optics in TOF due to mass dependent energies
Orthogonal Transmission
Detector Planeor
Ion Mirror
OriginalIon Packet
L= 0.5 to 0.75 m
23 mm dia.ion detector
vy / vx
y
x
Mass-Dependent Energies
Repeller
Acceleration Field
DetectorIon Mirror
Vsteer
Green - PbRed - Li
Ion beam
Orthogonal Mass Bias
0 256M/Z
CTS
Low Mass BiasLow Mass Bias
Mid Mass BiasMid Mass Bias
High Mass BiasHigh Mass Bias
Axial Mass Bias
0 256M/Z
CTS
On-Axis Ion Injection Advantages
• Improved Ion Transmission Efficiency
• Reduced Mass Bias
• Reduced Optical Maintenance
• Reduced Instrument Footprint
Schematic Diagram of Axial TOF ICP-MS
123
Acceleration
Extraction
Skimmer
Sampler
Detector
Ion Mirror
ICP Torch
Vacuum StagesFlight Tube
Simultaneous Mass Spectra Modulation
(+)
++++++
Repeated up to 30,000 times per second
Accelerate to TOF Accelerate to TOF Accelerate to TOF
RejectReject RejectReject RejecRejectt
38 Micro S38 Micro S38 Micro S38 Micro S 38 Micro S38 Micro S
Schematic Diagram of Axial TOF ICP-MS
123
Einzel 2
X-Steering
Y-Steering
Acceleration
Repeller
Modulation
Extraction
Skimmer
Sampler
Third Stage Orifice
Detector
Energy Barrier
Gridded Ion Mirror
Flight Tube
ICP Torch
Ion Optic 1
Einzel 1
Ion MirrorIon Mirror
Acceleration Field
Detector
Simultaneous Advantages• Transient Signals: complete multielement analysis
• High precision isotope ratios : Simultaneous Reads– no additive noise when employing corrections– no sample introduction or plasma noise
• Small volume samples: complete multielement analysis– minimum sample destruction– maximum spatial concentration profile capability
• Sample throughput =delivery and rinse time primarily
• Cone plugging via TDS exposure is minimized
Method Advantage :TOF Means Speed
30,000 Full Mass Spectra per Second
U = Mach 115
Detection LimitsAre They Signal To Background ? Or Signal to Noise?
0 256M/Z
CTS
Detection LimitsAre They Signal To Background ? Or Signal to Noise?
0 256M/Z
CTS
TOF ICP-MS Detection Limits (3)
Element DL (ng/mL) Element DL (ng/mL)
Ba 0.002 Rb 0.004
Co 0.004 Rh 0.002
Cu 0.004 Sr 0.002
Dy 0.009 Ta 0.006
Er 0.008 Tb 0.001
Eu 0.003 Th 0.005
Gd 0.005 Tl 0.008
Ho 0.002 Tm 0.002
La 0.003 U 0.004
Lu 0.002 W 0.004
Nd 0.009 Y 0.003
Pr 0.002 Yb 0.005
Mn 0.003 V 0.003
Short Term StabilityInternal Standard Results
(20 min. 10 ppb)
% RSD % RSD Limit
V 1.04 0.31
V/Y 0.51 0.42
Ba 0.70 0.33
Ba/Tb 0.51 0.43
U 1.44 0.3
U/Bi 0.59 0.46
0
1000
Sig
na
l (cp
s)
0
6000
204 206 208
4
m/z
Sig
na
l (m
v)
204 206 208
4
m/z
Dual-Mode Detection
1 ng/mL 100 ng/mL
Ana
log
Sig
nal (
mV
)Io
n C
ount
ing
Sig
nal (
cps)
Saturation
10 -3 10 -2 10 -1 100 101 102 103 104100
101
102
103
104
105
106
107
209
Bi
Concentration (ppb)
Co
un
tin
g S
ign
al
(cp
s)
10 -3
10 -2
10 -1
100
101
102
103
104
An
alo
g S
ign
al
Dynamic Range
Correlation CoefficientCounting Analog
Co 0.9993 1.0000In 0.9994 0.9999
Cs 0.9994 1.0000Ba 0.9999 0.9997Bi 1.0000 1.0000U 0.9991 0.9999
Dynamic Range (Counting and Analog)
1E-3 0.01 0.1 1 10 100 1000 10000 100000
0.1
1
10
100
1000
10000
100000
1000000
R=0.99954
Bis
mu
th R
ela
tive
Sig
na
l A
rea
Concentration (ppb)
LECO Patented Ion Counting/Analog Detection
Scheme
ETP AF831H 20 dB
gain switchingpre-amp
100 MHz Ion Counter
500 MHzflash A/D
Windowed Buffer
Dual Accumulator VMEBus
High Data Throughput• Data throughput from ICP-MS up to 750 Mbytes/sec
• reduction is necessary for practical analysis
• Buffer retains 2000, 2 ns bins from each spectra
• Individual spectra are summed and the data transferred to the host computer
•Max bandpass 0.75 Mbytes/sec
TOF-MS500 MHz A/D, 100 MHz Counter
30 kHz Spectral Rate
Buffer
Accumulation
Mass Mapping
Summation
Throughput(Mbytes/sec)
750
0.75
120
13
Mass Mapping
Mass
68 69 70
40Ar
14N 2
68Zn (impurity)
70Ge
139La
(2+)69
Ga
2 ppb Ga/Ge, 500 ppb La
Time Bins
Bin Summation
255 Summed to 1
Figures of Merit and Applications
• Spectral resolution and matrix deflection
• Detection limits and speed of analysis
• Multielement transient signal analysis
• Isotope ratios and internal standards
• Solid sample analysis by LA
Quadrupole Resolution
Low M/Z High M/Z
0.3 AMU 1.0 AMU
TOF Resolution
Low M/Z High M/Z
Unit Mass Baseline Resolved 1.0 AMU at Greatest Mass
< 0.3 AMU at Least Masswith No Sacrifice in Sensitivity
TOF Resolution
Low M/Z High M/Z
Lower Mass Resolving Power
66 67 68 69 70 71 72 73 74 75 76 77
40Ar
14N
2
68Zn (impurity)
76Ge
74Ge
73Ge
72Ge
71Ga
70Ge
139La
2+
69Ga 2 ppb Ga/Ge, 500 ppb La
2 ppb Ga/Ge
Mass
Resolving Power at High Mass
204 205 206 207 208 209 210
R50%
= 1270
R10%
= 475
Sig
na
l
Mass
50 ppt Pb, Bi
ResolutionSelected Spectral Regions Expanded
58 60 62 64
65Cu
+
64Zn
+
63Cu
+
62Ni
+
61Ni
+
60Ni
+
134 135 136 137 138
59Co
+
58Ni
+
138Ba
+
137Ba
+
136Ba
+
135Ba
+
134Ba
+
m/z
202 204 206 208
208Pb
+
207Pb
+
206Pb
+
205Tl
+
204Pb
+
203Tl
+
Quadrupole ICP-MSMatrix Filter
RF1, DC1
RF3, DC3
Time1
Time2
TOF and High Matrix
Low M/Z High M/Z
Coulombic Repulsion During Flight
TIME
+
+ +
+
TOF and High Matrix
Low M/Z High M/Z
Selectable Matrix Removal
Flight Tube
T.R.I.P.Transverse Rejected Ion
Pulse
AccelerationRepeller
Modulation
Background Species Deflection (T.R.I.P.)
NO+
Ar+
Ar+
NO+
O+, OH+
ICP-MS Speed Quadrupole vs TOF
* Theoretical 0.3 sec. dwell time, 5 replicates, 60 sec. rinse time
** 3 points/peak, 10 s quadrupole settle time
EPA 200.8 (Analytes, Interference Corrections, Internal Standards)
0.0
100.0
200.0
300.0
400.0
500.0
600.0
1 11 21 31 41 51 61 71 81 91
Number of m/z Values Measured
Tim
e (
se
co
nd
s)
Quadrupole**
TOFMS