Isotope abundance ratio measurements using femtosecond laser ablation ionization mass spectrometry

Tulej, Marek; Neubeck, Anna; Riedo, Andreas; Lukmanov, Rustam; Riedo, Valentine; Ligterink, Niels F.W.; Ivarsson, Magnus; Bach, Wolfgang; de Koning, Coenraad; Wurz, Peter (2020). Isotope abundance ratio measurements using femtosecond laser ablation ionization mass spectrometry. Journal of mass spectrometry, 55(12), e4660. Wiley 10.1002/jms.4660

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Accurate isotope ratio measurements are of high importance in various scientificfields, ranging from radio isotope geochronology of solids to studies of element iso-topes fractionated by living organisms. Instrument limitations, such as unresolved iso-baric inferences in the mass spectra, or cosampling of the material of interesttogether with the matrix material may reduce the quality of isotope measurements.Here, we describe a method for accurate isotope ratio measurements using our laserablation ionization time-of-flight mass spectrometer (LIMS) that is designed for in situplanetary research. The method is based on chemical depth profiling that allows foridentifying micrometer scale inclusions embedded in surrounding rocks with differentcomposition inside the bulk of the sample. The data used for precise isotope mea-surements are improved using a spectrum cleaning procedure that ensures removalof low quality spectra. Furthermore, correlation of isotopes of an element is used toidentify and reject the data points that, for example, do not belong to the species ofinterest. The measurements were conducted using IR femtosecond laser irradiationfocused on the sample surface to a spot size of ~12 μm. Material removal was con-ducted for a predefined number of laser shots, and time-of-flight mass spectra wererecorded for each of the ablated layers. Measurements were conducted on NISTSRM 986 Ni isotope standard, trevorite mineral, and micrometer-sized inclusionsembedded in aragonite. Our measurements demonstrate that element isotope ratioscan be measured with accuracies and precision at the permille level, exemplified bythe analysis of B, Mg, and Ni element isotopes. The method applied will be used forin situ investigation of samples on planetary surfaces, for accurate quantification ofelement fractionation induced by, for example, past or present life or by geochemicalprocesses.

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