Accurate quantitative elemental analysis of solid samples has highly important applications in various scientific and industrial fields, ranging from planetary sciences, to material sciences, to the semiconductor industry. Aspects typically of interest range from elements present at the minor (per mille to ppm) and trace (ppm and below) level (e.g. the presence of toxic heavy metals in consumer products), to distribution of major, minor and trace elements throughout the material (e.g., the spatial distribution of different metallic species in alloys), to locally enhanced concentrations for elements in chemically heterogeneous samples (e.g. microfossils embedded in geological host material). Accurate identification of all these aspects relies, to a certain extent, on the applied measurement techniques offering spatially resolved chemical analysis.

With the emergence of stable fs laser systems, laser ablation has proven to be a highly suitable technique for direct probing of solid samples with high spatial resolution at the micrometer level and below. The widespread use of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Laser Induced Breakdown Spectroscopy (LIBS) attests to this. However, both techniques have their advantages and drawbacks. One major advantage of LIBS is the capability to do standoff measurements several meters away from the target. However, it is usually described as being semi-quantitative and shows limited detection sensitivities for certain elements.¹ LA-ICP-MS, on the other hand, is highly sensitive (detection limits at the ppm level and below) and quantitative, but suffers from non-stoichiometric processes, leading to elemental fractionation.² This complicates analysis by requiring matrix-matched standards for accurate quantitative analysis. Laser Ablation Ionization Mass Spectrometry (LIMS) partially addresses the issue of element fractionation, by eliminating several known sources of elemental fractionation (e.g. transport of aerosol particles to the ICP, and vaporization, atomization, and ionization within the ICP), and can thus be expected to rely less on use of matrix-matched standards for accurate analysis. However, LIMS analysis is often complicated by the presence of isobaric interferences in the mass spectra, which cannot be resolved due to relatively low mass resolution of the mass analyzer systems applied in combination with LIMS so far.

Recently we designed and constructed a novel laboratory-scale LIMS system at the University of Bern, the Laser Mass Spectrometer “Gran Turismo” (LMS GT) instrument. The system was designed and developed specifically to address the issue of isobaric interferences. This LIMS system comprises a femtosecond laser system (775 nm, ~190 fs, 1 kHz) as ablation and ionization source and a double-reflectron time-of-flight mass spectrometer with a total flightpath length of ~4 m. The instrument has previously been shown to be capable of analysis with high lateral resolution (micrometer range), low limits of detection (ppm range and below), and high mass resolution (m/Δm exceeding 10 000).³ In this contribution, the capabilities of the LMS GT to conduct accurate quantitative analysis based on a large study conducted on several NIST SRM steel standards will be discussed. The discussion will cover different aspects, including the importance of high dynamic range in acquisition, adaptations to our analysis software to accommodate the data produced by LMS GT, the influence of the achieved mass resolution on mass determination accuracy and quantification accuracy, and the observed element fractionation. Combined, these aspects will allow making an in-depth assessment on the capabilities of our LMS GT, and its considerable potential for spatially resolved chemical analysis of solids.

[1] D. Hahn, N. Omenetto, Applied Spectroscopy, 2012, 66, 347-419.
[2] J. Koch, G. Detlef, in Encyclopedia of Spectroscopy and Spectrometry,  2016, 526-532
[3] R. Wiesendanger, V. Grimaudo, M. Tulej, A. Riedo, R. Lukmanov, N. Ligterink, R. Faush, H. Shae, P. Wurz, Journal of Analytical Atomic Spetrometry, 2019, 34, 2061-2073