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 ≈ 0.7‰ 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‰ per amu for Mg isotope measurements performed using the Neptune) to large and significantly non-mass-dependent (i.e., up to 6‰ mass dependent/3‰ 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.