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

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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

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

1LECO Corporation

2University of Ghent

- 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

- 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

- 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%

- Do you need simultaneous techniques to measure?
- How is signal to noise ratio improved?
- Examples for solution and for solid material sampling.

- 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.

- 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

Conc (ppm)RSD SignalRSD Ratio

2.3219% 0.8%

38.5710% 0.2%

426 3.5% 0.09%

10 second integration

n = 10

120

100

80

Signal (mV)

Ag107

60

Ag109

40

20

10

15

20

25

30

35

40

45

50

Time (s)

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

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)

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

- 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

Figure6

0.06% RSD, 100 ppb

n = 10

107Ag/109Ag

Figure 7

Figure 9

207Pb/206Pb

Figure 8

107Ag/109Ag

107Ag/109Ag

RSD = 0.29%

- 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