Level of confidence in elemental analysis by LIBS

Laser-Induced Breakdown Spectroscopy has become a very attractive spectroscopic technique for elemental analysis of forensic evidence. Its potential to provide fast conclusions on a large set of samples has been shown over the past ten years. Nevertheless, its use in a court is not commonly accepted. Despite the scientific foundation and recognition of the technique by the scientific community, the main reasons are that there are no operation standards in LIBS, neither are there known error rates. When the elemental information is needed from the LIBS spectrum, identification of elemental emission relies on the assignment of spectral lines based on wavelength tables. But these tables have been established by arc excitation that doesnt reproduce the excitation conditions of LIBS that add dynamic shift and broadening to the spectral line. Furthermore, the low spectral resolution of LIBS systems makes it possible that spectral interferences occur for the majority of the lines. Such interference makes the elemental profile uncertain. The goal of this proposal is to quantify the level of confidence in an elemental analysis by LIBS. This project will fulfill this goal in three steps: (i) The creation of the parameter representing the level of confidence. A LIBS spectrum can be considered as a distributed sequence of weighing distributions (the emission peaks with their own profile) over a discrete list of wavelength-ordered emitters. The statistical weight for each emitter line in the list is not equivalent and depends on the emission probability of the transition, the prior knowledge of the sample elemental composition, the temperature of the LIBS plasma. The quality of the spectra and the amount of spectral lines available for decision is also crucial in the confidence for assignment of lines. This fundamental design of the level of confidence is the crucial first step of this project. (ii) The experimental measurement of the level of confidence. Depending on the laser conditions, excitation conditions, detection and analysis conditions, the LIBS spectrum changes and as a consequence the level of confidence of the deduced elemental profile. This essential and extensive parametric study of the level of confidence for the forensic community will provide unique insights in the confidence they can have in LIBS systems for sample analysis. (iii) The confirmation of the procedure by blind testing. This blind test will be designed for confirming the validity of the basic method for future applications of LIBS with commercial systems. The blind testing and the random re-analysis will be managed by our team and will provide a complete final report of the evaluation of the level of confidence in elemental analysis by LIBS. This project will be conducted by Dr. Matthieu Baudelet and sponsor a student in the Townes Laser Institute (College of Optics and Photonics, UCF). The experience of the PI in LIBS and especially its use for quantitative analysis reinsures the scientific feasibility of the proposed project. This project will provide forensic experts with a complete quantitative evaluation of the input data either they just need to provide a qualitative list of elements present in a sample or they need to establish a comparison between samples. This analysis could become a standard for forensic use of LIBS and its use could be optimized by the spectrochemical industry for specific LIBS system design and even lower the price of the LIBS system, what will make the introduction of LIBS easier in forensic laboratories. ca/ncf