Laser-Induced Breakdown Spectroscopy (LIBS) has become a very attractive spectroscopic technique for elemental analysis in forensic science. Its potential to provide fast conclusions without intensive sample preparation makes it a very serious tool for field analysis and in particular for soil analysis. Although LIBS cannot perform trace analysis with a level of precision comparable to mass spectrometry-based techniques, it is the only technique that could provide a fast screening of carbon and nitrogen species as well as anions and cations, at the surface, as well as in the depth of the soil of across an area. This type of screening can help to thoroughly sample soil specimen on-scene, even in-situ, for further analysis in the laboratory and reference it with quantitative background to specific areas of a scene for laboratory analysis. However, LIBS being a laser-ablation based technique, its results are usually matrix-dependent. And soil is perhaps the most complex medium from this viewpoint. LIBS results from soil show a dependence on moisture, temperature and texture without mentioning concentration of major elements. The goal of this proposal is to quantify the matrix effects (moisture, texture) on the LIBS elemental analysis of soil samples and correct for them for a reproducible elemental profile\ .
This project will fulfill this goal in three steps:
(i) The study of moisture effects. Water reduces the LIBS emission by wasting the laser energy in its evaporation instead of creating a strong emitting plasma. The effect of moisture content on the LIBS signal and its reproducibility will be studied for three types of grain size (sand-, silt- and clay-type). The study will first investigate the elemental profile of major elements and then minor elements. Validation with certified NIST samples will be performed.
(ii) The effect of the soil texture. Once the effect of moisture and temperature will be understood on pure soils, the project will evolve towards more realistic soils. Controlled mixtures of sand-, silt- and clay-type powders will be fabricated to simulate soil textures and extend the conclusions of the first step. Validation with certified soil NIST samples will be performed.
(iii) The spectral correction protocol. From the conclusions from the previous steps of the study, a spectral correction protocol will be established to be able to correct the LIBS emission spectra and retrieve the correct elemental signature by compensating for the eventual unknown physical state of the soil (moisture, type, grain size). This correction will be spectrum-based and include physical information from the plasma rather than uncontrolled multivariate analysis.
This project will be led by Dr. Matthieu Baudelet, with the co-direction of Dr. Martin and sponsor a student at the Townes Laser Institute. The experience of the PIs in LIBS and its use for forensic applications, soil analysis and matrix effects ensure the scientific feasibility of the proposed project.
This project will provide forensic field practitioners with an evaluation of and a protocol for LIBS as a screening method for soil sampling in various conditions of moisture, temperature and soil types. This study could become a milestone towards a quantitative field instrument for forensic soil analysts and provide the field with a better choice of samples for laboratory analysis.