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

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 10(6) 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.

Orthogonal acceleration time-of-flight mass spectrometry

“…In a later study, Hieftje and coworkers investigated the ability of ICP±oa-TOFMS to measure isotope ratios precisely and to yield abundance sensitivity (the dynamic range in measurable signal intensities for isotopes with adjacent mass numbers) (Myers et al, 1995c). Good FIGURE 32.…”

Peer Reviewed: Plasma Source TOFMS

“…Furthermore, alignment of the main velocity of the ions with the direction of the flight tube in aa-TOFMS eliminates turnaround-time errors. Despite the apparent advantages and limitations of these competitive geometries, the experimental performance of both designs is similar and comparable with the figures of merit from other ICPMS platforms (16)(17)(18)(19)(20). Commonly, ICP-TOFMS systems achieve elemental sensitivities of 10 6 -10 7 cps/ppm/isotope and detection limits of 0.1-10 ppt for all elements and isotopes of interest, measured concurrently.…”

Distance-of-Flight Mass Spectrometry with IonCCD Detection and an Inductively Coupled Plasma Source

“…The improvement in isotope-ratio precision compared with the MCP/ phosphor screen detection system is attributed to the directcharge integration and on-line readout of the IonCCD camera system. Isotope-ratio precision for ICP-TOFMS is typically ≥0.05% RSD [33][34][35][36] and requires seconds to minutes of acquisition time, depending on wheteher analog detection or ion counting is used. For the three elements studied at 1-ppm concentration, isotope-ratio precision is comparable to that for TOFMS but were obtained with no more than 1 s integration time.…”