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Elemental analysis of glass was conducted by 16 forensic science laboratories, providing a direct comparison between three analytical methods [micro-x-ray fluorescence spectroscopy (μ-XRF), solution analysis using inductively coupled plasma mass spectrometry (ICP-MS), and laser ablation inductively coupled plasma mass spectrometry]. Interlaboratory studies using glass standard reference materials and other glass samples were designed to (a) evaluate the analytical performance between different laboratories using the same method, (b) evaluate the analytical performance of the different methods, (c) evaluate the capabilities of the methods to correctly associate glass that originated from the same source and to correctly discriminate glass samples that do not share the same source, and (d) standardize the methods of analysis and interpretation of results. Reference materials NIST 612, NIST 1831, FGS 1, and FGS 2 were employed to cross-validate these sensitive techniques and to optimize and standardize the analytical protocols. The resulting figures of merit for the ICP-MS methods include repeatability better than 5% RSD, reproducibility between laboratories better than 10% RSD, bias better than 10%, and limits of detection between 0.03 and 9 μg g(-1) for the majority of the elements monitored. The figures of merit for the μ-XRF methods include repeatability better than 11% RSD, reproducibility between laboratories after normalization of the data better than 16% RSD, and limits of detection between 5.8 and 7,400 μg g(-1). The results from this study also compare the analytical performance of different forensic science laboratories conducting elemental analysis of glass evidence fragments using the three analytical methods.

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Micro X-ray fluorescence (m-XRF) spectrometry using an energy dispersive X-ray (EDS) detector is capable of detecting certain major, minor, and trace elements that permit potential discrimination of glass fragments in forensic cases on the basis of differences in elemental composition. Often, elements used for discrimination are present at concentrations near the detection limit of the EDS system, and the decision whether to utilize these minor peaks in a comparative analysis has generally been left to the discretion of the examiner. The use of signal-to-noise ratios (SNRs) of spectral peaks provides additional objectivity in peak identification/label decisions and in the selection of elements in semiquantitative ratio comparisons. In addition, the use of SNRs enables calculations of limits of detection and limits of quantitation and the monitoring of instrument performance, and facilitates performance comparisons of different m-XRF configurations. This paper demonstrates a practical method for applying the concepts of SNR, limits of detection, and limits of quantitation to m-XRF generated EDS-based spectra, discusses the implications of such determinations, addresses spectral features that must be considered when making the calculations, and illustrates the application of these concepts to the example of forensic examination and comparison of glass samples.

The value of source-type classification for small fragments of glass encountered in trace evidence casework is restressed. The incorporation of classification techniques into the classical refractive index/density comparison scheme is described. The techniques employed are applications of those developed by the British forensic science community over the past 5 years, targeted at differentiating the 2 most common end-use types of soda-lime-silicate glasses encountered in casework—sheet glass and container glass. The results of method verification studies on 30 window glass specimens for tempered/nontempered classification and on 140 window and container glass specimens for sheet/container classification are reported. Although some limitations were revealed with the domestic samples used, the overall success of the approach was established.