isotope ratio performance of an axial time of flight icp-ms stuart georgitis 1, lloyd allen 1, and...
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Isotope Ratio Performance of an Axial Time of Flight
ICP-MS
Stuart Georgitis1, Lloyd Allen1, and Janos Fucsko1, Frank Vanhaecke2
1LECO Corporation2University of Ghent
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Introduction
• Nature of noise in ICP-MS measurement
• Sequential and simultaneous detection: fundamental
differences in signal ratios
• Axial TOF ICP-MS: Is it really better for isotope ratio
determinations
• Isotope ratios of transient and steady state signals with
liquid and solid sampling methods
• Characterization of TOF ICP-MS performance
• Limitations of measurements
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Isotope Ratio Fundamentals
• Sources of Noise in ICP-MS– Flicker Noise: Non Fundamental, Caused by
Sample Introduction system and ICP. s
– Shot Noise: Fundamental, Due to the Random Arrival Rate of Particles (photons, electrons, ions) at a detector.
s1/2
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0 15 300
25
RSD 109
Ag = 0.17%
RSD 107
Ag = 0.18%
Time (min.)
Sig
na
l (m
V)
Isotope Ratios• 50 ng/mL Ag• 30 min. period• Each point
– 5 repetitions– 10 s integration/repetition
• Relative Standard Deviation (%)– 107Ag: 0.18%– 109Ag: 0.17%– 107Ag/109Ag: 0.02%
0 5 10 15 20 25 30
21.40
21.45
21.50
21.55
21.60
Raw Signal Intensity
Time (min.)
10
7 Ag
Sig
na
l (m
V)
20.20
20.25
20.30
20.35
20.40
10
9 Ag
Sig
na
l (m
V)
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Isotope Ratios
• Do you need simultaneous techniques to measure?
• How is signal to noise ratio improved?
• Examples for solution and for solid material sampling.
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Isotope Ratio Fundamentals
• Flicker Noise can be minimized or eliminated by ratio pairing. Flicker noise elimination is most effectively done using simultaneous acquisition.
• Should Flicker noise be eliminated, shot noise should be the dominant remaining source of noise.
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Isotope Ratio Fundamentals
• The theoretical shot noise limit can be calculated:
RSD = (/s)
at the Shot Noise Limit = s1/2
RSD = s-1/2
RSD2A/B = RSD2
A + RSD2B
or
RSD2A/B = sB
-1 + sB-1
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Solid Sample Isotope Ratios
NIST 610 Glass
20
25
30
35
40
45
50
55
60
65
70
0 20 40 60 80 100
Time (s)
An
alo
g S
ign
al (
mV
)
Ag107
Ag109
Pb206
Pb207
Pb208
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Solid Sample Isotope Ratios
206Pb/207Pb in NIST Glass
Conc (ppm) RSD Signal RSD Ratio
2.32 19% 0.8%
38.57 10% 0.2%
426 3.5% 0.09%
10 second integrationn = 10
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Transient Signal Isotope Ratio Precision (1)
0
50
100
150
200
250
0 50 100 150 200
Time (s)
Sig
na
l (m
V)
Ag107
Ag109
Ba138
Ba137
Cu63
Cu65
Pb206
Pb207
Pb208
Sr86
Sr87
Sr88
Zn64
Zn66
Zn68
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Transient Signal Isotope Ratio Precision
20
40
60
80
100
120
10 15 20 25 30 35 40 45 50
Time (s)Time (s)
Sig
nal
(m
V)
Ag107 Ag109
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Transient Signal Isotope Ratio Precision*
Ratio 5 ng (%RSD) 50 ng (%RSD)
Ag (107/109) 0.23 0.04
Ba (138/137) 0.31 0.10
Cu (63/65) 0.21 0.12
Pb (208/207) 0.48 0.04
Pb (208/206) 0.48 0.10
Pb (206/207) 0.36 0.12
Zn (64/66) 0.63 0.07
*10 l Injection n = 5
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Isotope Ratio Precision
0 5 10 15 20
0.42
0.43
0.44
0.45
0.46
0.47
0.48
Ratio Precision = 0.34%
Ratio
Time (min)
0.30
0.65
0.70
0.75
Pb-208 = 1.3 %
Pb-206 = 1.3 %
Peak Area
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Isotope Ratio Precision(%RSD)
50g/L 208/206 208/207 206/207 63/65
0.07% 0.11% 0.09% 0.10%
500g/L 208/206 208/207 206/207 63/65
0.06% 0.05% 0.02% 0.05%
30 Second Integration Timen=10
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Isotope Ratio Limitations Simultaneous Techniques
• Even at the Shot noise limit, practical limitations arise
– In order to obtain a %RSD of 0.01 on a 1:1 Ratio, 200 Million counts must be accumulate
– In order to obtain a %RSD of 0.001 on a 1:1 Ratio, 20 Billion counts must be accumulated
– Ultimately, detector saturation limits the overall count rate which can be tolerated and integration for infinite time (2000 s/rep) is not possible
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Silver Isotope Ratios %RSD vs Concentration
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 20 40 60 80 100
Concentration (g/L)
%R
SD
of
Iso
top
e R
ati
o
%RSD MeasuredTheoretical Limit
0.06% RSD, 100 ppbn = 10107Ag/109Ag
Figure6
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Lead Isotope Ratios %RSD vs Concentration
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 10 20 30 40 50 60 70 80 90 100
Concentration (g/L)
%R
SD
of
Ra
tio
206Pb/207Pb
Theoretical Limit(206/207)
206Pb/208Pb
Theoretical Limit(206/208)
207Pb/208Pb
Theoretical Limit(207/208)
Figure 7
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1 g/L Steady State Solution Nebulization, %RSD vs Integration Time
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 20 40 60 80 100Integration Time (s)
%R
SD
%RSD MeasuredTheoretical Limit
207Pb/206Pb
Figure 9
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1 g/L Steady State Solution Nebulization, %RSD vs Integration Time
0
0.5
1
1.5
2
2.5
3
0 20 40 60 80 100
Integration Time (s)
%R
SD
of
Ra
tio
%RSD Measured
Theoretical Limit
107Ag/109Ag
Figure 8
107Ag/109AgRSD = 0.29%
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Conclusions
• Fast simultaneous detection provides better element and isotope ratios.
• Precision of signal ratios are primarily controlled by counting statistics if practical (<2000 sec) integration time is used.
• The improved performance helps applications:– isotope ratio analysis from small or heterogeneous samples, using
steady state or transient signals– isotope dilution analysis– internal standardization even for fast changing transient signals:
speciation, chromatography, laser ablation