Comparative analysis of automotive paints by laser induced breakdown spectroscopy and nonparametric permutation tests |
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Authors: | Erin McIntee Emilie Viglino Caitlin Rinke Stephanie Kumor Liqiang Ni Michael E. Sigman |
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Affiliation: | 1. Department of Chemistry and National Center for Forensic Science, University of Central Florida, Orlando, FL 32816, United States;2. Department of Statistics and Actuarial Sciences, University of Central Florida, Orlando, FL 32816, United States |
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Abstract: | Laser-induced breakdown spectroscopy (LIBS) has been investigated for the discrimination of automobile paint samples. Paint samples from automobiles of different makes, models, and years were collected and separated into sets based on the color, presence or absence of effect pigments and the number of paint layers. Twelve LIBS spectra were obtained for each paint sample, each an average of a five single shot “drill down” spectra from consecutive laser ablations in the same spot on the sample. Analyses by a nonparametric permutation test and a parametric Wald test were performed to determine the extent of discrimination within each set of paint samples. The discrimination power and Type I error were assessed for each data analysis method. Conversion of the spectral intensity to a log-scale (base 10) resulted in a higher overall discrimination power while observing the same significance level. Working on the log-scale, the nonparametric permutation tests gave an overall 89.83% discrimination power with a size of Type I error being 4.44% at the nominal significance level of 5%. White paint samples, as a group, were the most difficult to differentiate with the power being only 86.56% followed by 95.83% for black paint samples. Parametric analysis of the data set produced lower discrimination (85.17%) with 3.33% Type I errors, which is not recommended for both theoretical and practical considerations. The nonparametric testing method is applicable across many analytical comparisons, with the specific application described here being the pairwise comparison of automotive paint samples. |
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Keywords: | LIBS Forensic analysis Automotive paints Nonparametric permutation test |
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