Charge-mode electrometer measurements of S-isotopic compositions on SHRIMP-SI

Abstract

Measurement of ion currents is fundamental to isotope ratio measurements. For small signals, discrete ion arrivals can be detected in pulse counting systems. This procedure works well until a few hundred thousand counts per second when dead time and gain drift become significant. Where sufficiently high signal is available, ion currents can be measured directly in Faraday cups coupled to electrometers with high ohmic (1011, 1012Ω) resistors. However, electrometers measuring current are limited by Nyquist–Johnson noise inherent in the high ohmic resistors to effective count rates above 500,000c/s. Electrometers can also be operated in charge mode where the feedback resistor is replaced by a capacitor. In this method, charge can be accumulated on the capacitor and the ion current determined by the change in voltage with time. We have implemented this system on the secondary ion mass spectrometer SHRIMP-SI and demonstrate its performance in comparison to the electron-multiplier pulse-counting system and the current mode of the electrometers for S isotope measurements. Of particular interest to us is the measurement of the low abundance 36S isotope (0.015%). From pyrite samples, we have performed 4-isotope analyses with the initial 36S− count rate of 100,000c/s for a total acquisition time of 720s. For analyses of mass-fractionation-corrected 36S/32S ratios on S isotope standards Ruttan and Balmat, we obtain standard deviations of the measurements of 0.21 and 0.27‰, and standard errors of the means of 0.07 and 0.13‰, respectively. These measurements demonstrate that charge mode can extend the range of electrometer measurements to allow isotope ratio determinations at the 0.1‰ level from signals of 100,000c/s. Charge mode offers a fast, responsive system, with little gain drift, and is ideally suited to in situ analysis on an ion microprobe.