Calibration of Nu-Instruments Noblesse multicollector mass spectrometers for argon isotopic measurements using a newly developed reference gas

Abstract

The greatest challenge limiting 40 Ar/ 39 Ar multicollection measurements is the availability of appropriate standard gasses to intercalibrate detectors. In particular, use of zoom lens ion-optics to steer and focus ion beams into a fixed detector array (i.e., Nu Instruments Noblesse) makes intercalibration of multiple detectors challenging because different ion-optic tuning conditions are required for optimal peak shape and sensitivity at different mass stations. We have found that detector efficiency and mass discrimination are affected by changes in ion-optic tuning parameters. Reliance upon an atmospheric Ar standard to calibrate the Noblesse is problematic because there is no straightforward way to relate atmospheric 40 Ar and 36 Ar to measurements of 40 Ar and 39 Ar if they are measured on separate detectors. After exploring alternative calibration approaches, we have concluded that calibration of the Noblesse is best performed using exactly the same source, detector, and ion-optic tuning settings as those used in routine 40 Ar/ 39 Ar analysis. To accomplish this, we have developed synthetic reference gasses containing 40 Ar, 39 Ar and 38 Ar produced by mixing gasses derived from neutron-irradiated sanidine with an enriched 38 Ar spike. We present a new method for calibrating the Noblesse based on use of both atmospheric Ar and the synthetic reference gasses. By combining atmospheric Ar and synthetic reference gas in different ways, we can directly measure 40 Ar/ 39 Ar, 38 Ar/ 39 Ar, and 36 Ar/ 39 Ar correction factors over ratios that vary from 0.5 to 460. These correction factors are reproducible to better than ± 0.5‰ (2σ standard error) over intervals spanning ~ 24 h but can vary systematically by ~ 4% over 2 weeks of continuous use when electron multiplier settings are held constant. Monitoring this variation requires daily calibration of the instrument. Application of the calibration method to 40 Ar/ 39 Ar multicollection measurements of widely used sanidine reference materials ACs-2, FCs-2, and TCs-2 demonstrate that calculated 40 Ar*/ 39 Ar K can be accurately corrected to yield model 40 Ar/ 39 Ar ages consistent with those reported by Earthtime 40 Ar/ 39 Ar laboratories. Replicate analyses of 8–12 single-crystal sanidine ages are reproduced to within 1–2‰ (2σ standard error) under optimal analytical conditions. This calibration technique is applicable over a wide range of isotopic ratios and signal sizes. Finally, the reference gas has the added advantage of facilitating straightforward characterization of electron multiplier dead time over a wide dynamic range.