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 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.   s 1/2

Isotope Ratios 50 ng/mL Ag 30 min. period Each point –5 repetitions –10 s integration/repetition Relative Standard Deviation (%) – 107 Ag: 0.18% – 109 Ag: 0.17% – 107 Ag/ 109 Ag: 0.02%

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  = s 1/2 RSD = s -1/2 RSD 2 A/B = RSD 2 A + RSD 2 B or RSD 2 A/B = s B -1 + s B -1

Solid Sample Isotope Ratios NIST 610 Glass

Solid Sample Isotope Ratios 206 Pb/ 207 Pb in NIST Glass Conc (ppm) RSD Signal RSD Ratio % 0.8% % 0.2% % 0.09% 10 second integration n = 10

Transient Signal Isotope Ratio Precision (1)

Transient Signal Isotope Ratio Precision Time (s) Signal (mV) Ag107 Ag109

Transient Signal Isotope Ratio Precision* Ratio5 ng (%RSD) 50 ng (%RSD) Ag (107/109) Ba (138/137) Cu (63/65) Pb (208/207) Pb (208/206) Pb (206/207) Zn (64/66) *10  l Injection n = 5

Isotope Ratio Precision Ratio Precision = 0.34% Ratio Time (min) Pb-208 = 1.3 % Pb-206 = 1.3 % Peak Area

Isotope Ratio Precision (%RSD) 50  g/L208/206208/207206/20763/ % 0.11% 0.09% 0.10% 500  g/L208/206208/207206/20763/ % 0.05% 0.02%0.05% 30 Second Integration Time n=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 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 0.06% RSD, 100 ppb n = Ag/ 109 Ag Figure6

Lead Isotope Ratios %RSD vs Concentration Figure 7

1  g/L Steady State Solution Nebulization, %RSD vs Integration Time 207 Pb/ 206 Pb Figure 9

1  g/L Steady State Solution Nebulization, %RSD vs Integration Time 107 Ag/ 109 Ag Figure Ag/ 109 Ag RSD = 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