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