Isotope ratio performance of an axial time of flight icp ms
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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

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Isotope Ratio Performance of an Axial Time of Flight ICP-MS

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Isotope ratio performance of an axial time of flight icp ms

Isotope Ratio Performance of an Axial Time of Flight ICP-MS

Stuart Georgitis1, Lloyd Allen1, and Janos Fucsko1, Frank Vanhaecke2

1LECO Corporation

2University of Ghent


Introduction

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

Isotope Ratio Fundamentals

  • Sources of Noise in ICP-MS

    • Flicker Noise: Non Fundamental, Caused by Sample Introduction system and ICP. sa s

    • Shot Noise: Fundamental, Due to the Random Arrival Rate of Particles (photons, electrons, ions) at a detector.

      sa s1/2


Isotope ratios

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%


Isotope ratios1

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 fundamentals1

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 fundamentals2

Isotope Ratio Fundamentals

  • The theoretical shot noise limit can be calculated:

    RSD = (s/s)

    at the Shot Noise Limit s = s1/2

    RSD = s-1/2

    RSD2A/B = RSD2A + RSD2B

    or

    RSD2A/B = sB-1 + sB-1


Solid sample isotope ratios nist 610 glass

Solid Sample Isotope RatiosNIST 610 Glass


Solid sample isotope ratios 206 pb 207 pb in nist glass

Solid Sample Isotope Ratios206Pb/207Pb in NIST Glass

Conc (ppm)RSD SignalRSD Ratio

2.3219% 0.8%

38.5710% 0.2%

426 3.5% 0.09%

10 second integration

n = 10


Transient signal isotope ratio precision 1

Transient Signal Isotope Ratio Precision (1)


Transient signal isotope ratio precision

Transient Signal Isotope Ratio Precision

120

100

80

Signal (mV)

Ag107

60

Ag109

40

20

10

15

20

25

30

35

40

45

50

Time (s)


Transient signal isotope ratio precision1

Transient Signal Isotope Ratio Precision*

Ratio5 ng (%RSD) 50 ng (%RSD)

Ag (107/109) 0.230.04

Ba (138/137) 0.310.10

Cu (63/65) 0.210.12

Pb (208/207) 0.480.04

Pb (208/206) 0.480.10

Pb (206/207) 0.360.12

Zn (64/66) 0.630.07

*10 ml Injection n = 5


Isotope ratio precision

Isotope Ratio Precision

0.75

0.70

0.65

Pb-208 = 1.3 %

Peak Area

Ratio

Pb-206 = 1.3 %

0.30

0.48

0.47

0.46

0.45

Ratio Precision = 0.34%

0.44

0.43

0.42

0

5

10

15

20

Time (min)


Isotope ratio precision rsd

Isotope Ratio Precision(%RSD)

50mg/L208/206208/207206/20763/65

0.07% 0.11% 0.09% 0.10%

500mg/L208/206208/207206/20763/65

0.06% 0.05% 0.02%0.05%

30 Second Integration Time

n=10


Isotope ratio limitations simultaneous techniques

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

Silver Isotope Ratios %RSD vs Concentration

Figure6

0.06% RSD, 100 ppb

n = 10

107Ag/109Ag


Lead isotope ratios rsd vs concentration

Lead Isotope Ratios %RSD vs Concentration

Figure 7


1 m g l steady state solution nebulization rsd vs integration time

1 mg/L Steady State Solution Nebulization, %RSD vs Integration Time

Figure 9

207Pb/206Pb


1 m g l steady state solution nebulization rsd vs integration time1

1 mg/L Steady State Solution Nebulization, %RSD vs Integration Time

Figure 8

107Ag/109Ag

107Ag/109Ag

RSD = 0.29%


Conclusions

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


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