Commissioning and performance characterisation of a high-resultion laser ablation ionisation mass spectrometer

De Koning, Coenraad Pieter (2023). Commissioning and performance characterisation of a high-resultion laser ablation ionisation mass spectrometer (Unpublished). (Dissertation, Universität Bern, Physikalisches Institut)

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Direct chemical analysis of solid material, especially of features with dimensions in the micrometre scale, is of significant interest in a wide variety of industrial and scientific fields. Capturing the chemical information of such features requires specialised instrumentation capable of conducting analysis with high spatial resolution, high sensitivity, and accurate quantification. This dissertation presents the performance characterisation of the Laser Mass Spectrometer – Gran Turismo (LMS–GT) instrument, a recently realised fs-Laser Ablation Ionisation Mass Spectrometer capable of achieving mass resolution exceeding 10’000 m/ΔmFWHM.
An evaluation of the element analysis performance by developing measurement procedures and data analysis routines tailored to the characteristics of the LMS–GT was carried out. Applying these protocols to analyse metallic standard reference materials (SRMs) showed that the LMS–GT is capable of quantitative element analysis with high sensitivity. The achieved mass accuracy facilitated confident identification of monatomic, polyatomic, and multiply charged species, which led to accurate quantification of these species.
The use of UV laser radiation was found to be essential when analysing dielectric materials. Identification and quantification of a wide range of trace element isotopes with abundances down to single-digit ppb atomic fraction was realised, while IR radiation only yielded detection of abundances at the percent level. This denotes an improvement in detection limits of two orders of magnitude over traditional LIMS instrumentation with UV radiation.
The next step in the performance evaluation involved isotope ratio analysis, entailing the investigation of six different isotope systems spanning the full mass range (i.e., from lithium to lead). The development of a robust data analysis procedure tailored to data produced by LMS–GT is documented in this thesis. The developed procedure yielded precisions below the per mill level, which is a substantial improvement over commercially available LIMS instrumentation. After correcting for observed instrument mass fractionation, lead isotope ratios were derived with an accuracy below per mill, yielding uncertainties of ±10 Myrs in the context of the Pb-Pb dating system.
Several technical developments on the hardware of the LMS–GT were realised during this project, all aimed at improving its performance characteristics. The implementation of a mass-selective beam blanking device let to a shift down by almost two orders of magnitude in the investigated concentration range, facilitating an improvement of detection limits of the same factor.
In summary, the results presented in this thesis show the tremendous potential of the LMS–GT. The combination of performance metrics specific to the instrument make it a valuable addition to the existing suite of quantitative microscale analytical techniques. Improving the sensitivity, e.g., through post-ionisation of the ablation plume, has the potential to push detection limits to parts-per-trillion, which would allow the LMS–GT to occupy an analytical niche beyond what competing techniques can achieve.

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