LAMIS in the gas phase: A new approach for obtaining Ca elemental and isotopic information via CaF molecule formation

This work introduces a novel method for generating the calcium monofluoride (CaF) diatomic molecule by adding the molecule-forming reagent in the gaseous phase (a methyl fluoride-argon mixture), in order to perform laser-induced breakdown spectroscopy (LIBS) and laser ablation molecular isotopic spectrometry (LAMIS) measurements. By optimizing the instrumental parameters, CaF molecule formation was successfully achieved within the plasma plume, upon ablation of dried liquid samples. The isotopic shift for the X²Σ→A²Π (0,1) CaF vibronic transition at 583.0 nm was calculated to be 292.3 pm. The method proved capable of providing quantitative information for determining calcium concentrations in real samples, such as tap water and skimmed milk, using internal standardization (with Sr as internal standard; limit of detection, LOD, 20 mg L^-1) and isotope dilution (which can be applied from 400 mg L^-1on), respectively. Partial least squares regression (PLS) analysis was employed to enhance the quality of the isotopic data. The Ca concentration found in the tap water was 47 ± 16 mg L^-1 (reference flame atomic absorption spectrometry, FAAS, value: 59 ± 0.2 mg L^-1), and 1100 ± 140 mg L^-1 for the skimmed milk (reference FAAS value: 1240 ± 120 mg L^-1). No significant difference between LIBS and FAAS results could be established using a t-test at the 95% confidence level. Overall, this novel approach allows for the determination of calcium in terms of both the elemental concentration and the isotopic composition, thus broadening the applicability of LIBS (e.g., for tracer experiments, besides the already mentioned application of isotope dilution).