Isotope ratios and abundance sensitivity obtained with an inductively coupled plasma-time-of-flight mass spectrometer

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Abstract
Isotope ratios and abundance sensitivities have been determined with an inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS). Abundance sensitivities are at least in the 106 range for low abundance ions that precede high abundance ions. Three methods of detection for isotope-ratio measurement have been compared. The three systems involve gated detection followed by analog integration, analog averaging, or ion counting. Gated ion counting offers excellent precision—between 0. 64 and 1. 00% relative standard deviation (RSD). These values approach those predicted from counting statistics and are comparable to those reported for other inductively coupled plasma-mass spectrometry (ICP-MS) instruments. In addition, a greater number of accumulated counts or longer analysis times would afford precisions of 0. 1% with stable gating electronics. The accuracy of the counting method is in the 1–10% range if no correction for mass bias is performed. However, this ion counting method suffers from a limited dynamic range due to pulse pileup. Constant-fraction discrimination gated integration and commercial boxcar averager techniques offer a broader dynamic range because of their analog nature, but the attainable RSD values are limited by drift in the detection systems and by the methods employed to calculate an accurate ratio. Overall, mass bias in the ICP-TOFMS is more severe than previous work in ICP-MS due primarily to detection system bias.

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