EGT-A sensitive time-of-flight mass spectrometer for multielement isotope gas analysis.

Abstract

The principles of operation and figures of merit of a novel, compact (324mm×Ø 114mm; volume approximately 1000cm3 ) reflectron-type time-of-flight mass spectrometer designed for simultaneous multielement isotope gas analysis is presented. The system, which consists of a pulsed electron impact ion source, is designed either to directly analyse gas samples collected and stored in a compartment or samples extracted from solids using a CW laser system (fibre-coupled diode laser, <75W, λ=808±10nm). In latter case, laser pulses are focussed onto the sample surface to spot sizes of approximately 400μm in diameter that allows for direct ablation and vaporisation of solid sample material and releasing of trapped gases. A cleaning and trapping system that consists of various cold stages and getters is used before the gas enters the mass analyser. Measurements on various gases were conducted for performance evaluation, ranging from standard gases (Ar, Kr, and Xe) to trapped gases extracted from a sample of the Millbillillie meteorite. At optimised instrument settings, mass spectrometric measurements can be conducted with a mass resolution m/∆m of up to approximately 1200 (16 O and CH4 can be resolved), with a dynamic range of approximately 6 orders of magnitude and a mass calibration accuracy of approximately 100ppm. The high detection sensitivity of the system allows the detection of gas species at partial pressures down to the low 10-16 mbar level (corresponding to <10 particles/cm3 at standard temperature and pressure, including an ion transmission of approximately 80%). Measurements using standard gases demonstrated that the isotope ratios for a given element can be measured with an accuracy at the per mill level (relative to terrestrial values). Measurements of Ar extracted from the meteorite Millbillillie gave a 36 Ar/38 Ar ratio of approximately 1.6, which is in good agreement with literature values.