Assessment of the utility of X-ray Fluorescence for the chemical characterization and comparison of black electrical tape backings

Abstract

Elemental analysis of electrical tapes is typically conducted by Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), although Laser Ablation-Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) recently showed to enhance the sensitivity of the determinations. In this study, the utility of X-ray Fluorescence (XRF) is compared to previously published SEM-EDS and LA-ICP-MS data. Three XRF systems were used to evaluate a range of configurations commonly available at crime laboratories. A set of 40 electrical tape backings known to originate from difference sources was used to assess the inter-roll variability, discrimination and classification capabilities of the method. The discrimination for this tape set increased from 78.8% achieved by SEM-EDS to 81.5–91.0% by XRF, depending on the instrumental configuration. In comparison, LA-ICP-MS achieved 84.6% discrimination on these tapes. The overall characterization, classification, and discrimination capabilities for this set improved as follows: SEM-EDS < iXRF < small spot size benchtop XRF with SDD, LA-ICP-MS < large spot size XRF. A set of 20 pieces of tapes collected from the same roll were analyzed for intra-roll variability. Duplicate control same-source samples were used to evaluate inter-day and intra-day instrument variability. No false exclusions were observed in the data set, demonstrating the within-sample variability and instrumental variability are relatively lower than the inter-sample variability. One concern of the method is the penetration depth of the X-ray beam beyond the target backing, requiring careful sample preparation to avoid interference from the adhesive or sample holder. XRF is a viable analytical tool for the forensic examination of electrical tapes, with advantages of speed of analysis, minimal destruction of the tape, and high informing power.