Assessment of performance rates on the elemental comparison of small and irregular glass fragments using µ-XRF and LIBS

This study describes a systematic assessment of the performance rates when analyzing small and irregular glass fragments using micro-X-ray Fluorescence Spectrometry (µ-XRF) and Laser Induced Breakdown Spectroscopy (LIBS). One hundred glass fragments were collected from the inner and outer panes of a vehicle windshield to assess the false exclusion rates. Additionally, 100 glass fragments originating from different vehicle windshields were used to evaluate the discrimination capabilities. To compare the effects of fragment size on the performance rates, half of the collected fragments were small (longest length between 0.4 mm and < 1 mm, and thickness greater than 0.4 mm for LIBS and 0.1 mm for μ-XRF), and the other half were full-thickness fragments (2 mm and greater). The study shows that precision deteriorates for small/irregular fragments and comparison items must have a similar size, shape, and thickness to minimize error rates. Thus, comparisons between full-thickness and small/irregular fragments should be avoided, regardless of the analytical method. Although this general concept is well known for µ-XRF, this effect was not previously reported as a concern for LIBS. Moreover, this study provides new sampling and comparison recommendations when using modern silicon drift detectors (SDD) and reduced fragment size. Using a 3 s (3 %RSD) comparison interval reduces the false exclusion rates to < 12 % for µ-XRF, and to < 4 % for LIBS when using either a 3 s or 4 s (3 % RSD) criterion. At least 4 known fragments are recommended for full thickness fragments and 6 to 9 known fragments for the small/irregular comparisons.