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Isotope Ratio Performance of an Axial Time of Flight ICP-MS. Stuart Georgitis 1 , Lloyd Allen 1 , and Janos Fucsko 1 , Frank Vanhaecke 2 1 LECO Corporation 2 University of Ghent. Introduction. Nature of noise in ICP-MS measurement - PowerPoint PPT Presentation
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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
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
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
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)
Isotope Ratios
• Do you need simultaneous techniques to measure?
• How is signal to noise ratio improved?
• Examples for solution and for solid material sampling.
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.
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
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
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
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
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
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
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
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
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
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
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
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
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%
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