A resonance ionization time of flight mass spectrometer with a cryogenic sample concentrator for isotopic analysis of krypton from extraterrestrial samples

Abstract

RIMSKI – Resonance Ionization Mass Spectrometer for Krypton Isotopes – is a newly developed ultra sensitive instrument with a cryogenic sample concentrator and a laser ion source designed for isotopic analysis of extraterrestrial material. Laser heating is used for extraction of krypton from samples into the mass spectrometer volume. Atoms continuously condense on a cold spot in the ion source, and are repeatedly released by laser heating. They are ionized in the evaporation plume using three tunable dye lasers and one frequency tripled Q-switched Nd:YAG laser. Two resonant transitions are employed (at 116.5 nm, produced by four-wave frequency mixing in Xe, and 558.1 nm) and are followed by single photon ionization at 1064 nm. Multiple analyses of air aliquots and <5 mg samples of meteorites, both containing ∼10⁶ total krypton atoms, show precision ∼1% for major isotope ratios. They are reproducible to this level through a day's analyses, allowing calibration for linear and non-linear mass discrimination by sample-standard bracketing. Cosmogenic ⁸¹Kr can be reliably detected at the few thousand atom level, corresponding to determination of cosmic ray exposure (CRE) ages from meteorite samples of a few milligrams. Measured CRE ages of noncumulate eucrites are: Pasamonte, 7.9 ± 1.3 My; Sioux County, 20.9 ± 4.0 My; Bereba, 23.7 ± 2.6 My; Stannern, 33.3 ± 2.5 My. These are in good agreement with literature values.