Matrix effects in the analysis of Mg and Si isotope ratios in natural and synthetic glasses by laser ablation-multicollector ICPMS: A comparison of single- and double-focusing mass spectrometers

We report composition-dependent matrix effects in the in-situ measurement of Mg and Si isotope ratios by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in natural and synthetic silicate glasses. These were determined using two 193 nm wavelength excimer laser ablation-multicollector ICPMS systems, one employing a double-focusing mass spectrometer (ThermoFinnigan Neptune) and one employing a single-focusing mass spectrometer with a hexapole/collision cell (GV Instruments IsoProbe). Observed matrix effects in isotopic measurements by LA-ICPMS range from subtle (i.e., less than approximate to 0.7 parts per thousand per amu for Si isotope measurements in natural and synthetic glasses in all instrumental configurations) to significant but mass-dependent (i.e., less than 1.2 parts per thousand per amu for Mg isotope measurements performed using the Neptune) to large and significantly non-mass-dependent (i.e., up to 6 parts per thousand mass dependent/3 parts per thousand non-mass dependent for Mg isotope measurements of natural glasses made with the IsoProbe). Composition-dependent differences in the magnitude and direction of within-run isotopic fractionation (particularly for Mg isotope measurements) suggest that isotopic fractionation at the site of ablation is the main source of matrix effects in measurements employing the double-focusing mass spectrometer. However, the large and significantly non-mass dependent (but systematic and reproducible) matrix effects affecting LA-ICPMS Mg isotope data obtained using the single-focusing IsoProbe appear to be largely due to non-linear scattering processes occurring in the hexapole/collision cell as Mg becomes increasingly diluted by matrix elements. (C) 2010 Elsevier B.V. All rights reserved.