Effects of sample pretreatment and particle size on the determination of nitrogen in soil by portable LIBS and potential use on robotic-borne remote Martian and agricultural soil analysis systems

Field determination of nitrogen in soil is of interest for both terrestrial and Martian applications. Improved management of soil nitrogen levels on Earth could benefit global food production, whilst the determination of soil nitrogen on Mars is required to assess the planet's future habitability. In this study, a mobile laser induced breakdown spectroscopy (LIBS) system with a 1064 nm Nd:YAG laser delivering 25 mJ per pulse was used to assess the effects of sample pretreatment on the measurement of nitrogen in soil. Although pelletisation was preferred, simply milling the sample to <100 mm particle size – which may be more feasible on a remote rover-based analytical platform – improved the spectra obtained. Ablation craters formed in targets prepared from different particle size fractions of the same commercially-available topsoil showed a clear trend in morphology, with smaller particles yielding more uniform craters with fewer fractures. The LIBS emission intensity at 746.83 nm followed a similar trend to results obtained for total nitrogen content in the soil particle size fractions by microanalysis (Perkin Elmer CHN Elemental Analyser) and was well-correlated (R2 = 0.94) with soil nitrate determined by ion chromatography (Dionex DX-100). Although correlations were less good when analysing field soil samples collected from central Scotland (R2 = 0.82 for comparison between LIBS and microanalysis) the study nevertheless demonstrates the potential of portable LIBS for measurement of soil nitrogen content.