Characterization and forensic analysis of soil samples using laser-induced breakdown spectroscopy (LIBS) |
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Authors: | Sarah C Jantzi José R Almirall |
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Institution: | (1) Department of Chemistry and Biochemistry and International Forensic Research Institute, Florida International University, 11200 SW 8th Street, Miami, FL 33199, USA; |
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Abstract: | A method for the quantitative elemental analysis of surface soil samples using laser-induced breakdown spectroscopy (LIBS)
was developed and applied to the analysis of bulk soil samples for discrimination between specimens. The use of a 266 nm laser
for LIBS analysis is reported for the first time in forensic soil analysis. Optimization of the LIBS method is discussed,
and the results compared favorably to a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method previously
developed. Precision for both methods was <10% for most elements. LIBS limits of detection were <33 ppm and bias <40% for
most elements. In a proof of principle study, the LIBS method successfully discriminated samples from two different sites
in Dade County, FL. Analysis of variance, Tukey’s post hoc test and Student’s t test resulted in 100% discrimination with no type I or type II errors. Principal components analysis (PCA) resulted in clear
groupings of the two sites. A correct classification rate of 99.4% was obtained with linear discriminant analysis using leave-one-out
validation. Similar results were obtained when the same samples were analyzed by LA-ICP-MS, showing that LIBS can provide
similar information to LA-ICP-MS. In a forensic sampling/spatial heterogeneity study, the variation between sites, between
sub-plots, between samples and within samples was examined on three similar Dade sites. The closer the sampling locations,
the closer the grouping on a PCA plot and the higher the misclassification rate. These results underscore the importance of
careful sampling for geographic site characterization. |
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