Optimization and evaluation of spectral comparisons of electrical tape backings by X-ray fluorescence

Abstract

This study describes the process of parameter optimization of an XRF method for casework use. The work expands upon previous discrimination studies by broadening the total sample set of characterized tapes and evaluating the use of spectral overlay, spectral contrast angle, and Quadratic Discriminant Analysis (QDA) for the comparison of XRF spectra. The expanded sample set consisted of 114 samples, 94 from different sources, of which 90 were previously analyzed, and 20 from the same roll. XRF provided superior discrimination to SEM-EDS on the expanded dataset and a more comprehensive elemental characterization (15 elements by XRF vs. 8 by SEM-EDS). While previous SEM-EDS analysis of the 90 electrical tapes resulted in 15 distinct groups and a discrimination power of 87.3%, current XRF analysis resulted in 61 distinct groups with further subgroups providing a discrimination power of 96.7%. Duplicate controls and tape fragments from the same roll were also analyzed to assess inter-day, intra-day, and intra-roll variability (n = 20). Parameter optimization included comparison of atmospheric conditions, collection times, and instrumental filters. A study of the effects of adhesive and backing thickness on spectrum collection revealed key implications to the method that required modification to the sample support material. Figures of merit and the performance of different methods for comparing and contrasting spectra are reported. The optimization of this method was part of an assessment to incorporate XRF to a forensic laboratory protocol for rapid, highly informative elemental analysis of electrical tape backings and to expand examiners’ casework capabilities.