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Forensic Glass Classification according to ASTM E2926

Micro-XRF is the perfect pre-screening method. It does not require sample preparation, leaves the sample undamaged and it quickly reveals spatially resolved information on the elemental composition of the sample. These benefits make micro-XRF an ideal technique for forensic applications, and it is nowadays a well-established technique in this field.

For the Forensic Comparison of Glass, there is an ASTM test method for the use of micro-XRF (ASTM E2926-17). The comparison is based on measured element ratios, so it does not rely on a full quantification of the glass. The ASTM test method refers to the certified reference material NIST 1831 soda-lime glass. The polycapillary optic of the M4 TORNADO (PLUS) maintains a very high photon flux, even though the X-ray beam is focused onto a spot with < 20 µm diameter. With the high signal throughput capabilities of the SDD(s) up to 100000 counts per second can be used for the glass comparison of very small glass fragments. This advance in technology allows much better Signal-to-Noise Ratios and detection limits than stated in the ASTM E2926 document.

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NIST 1831 soda-lime sheet glass. This certified reference material comes as a 3 mm thick glass plate. It is “infinitely thick” for the XRF analysis of the certified elements, as can be seen in the chart: for Zr the information depth in glass is 800 µm (calculated using https://xrfcheck.bruker.com/InfoDepth).
Overlay of 20 spectra recorded with an M4 TORNADO, 10 spectra were measured for 600 s, the other 10 spectra had an integration time of 1200 s. The detection limits for the elements are based on the signal-to-noise ratio, which are significantly better than reported in the ASTM E2926 document.
The ratios of the detected fluorescence are used for classifying glasses. Due to the significantly higher detected count rates (even with only one SDD) and the improved spectral resolution of nowadays detector technology the stability of the ratios is better than needed for compliance with the ASTM E2926 test method.