Abstract
Application of refractive index (RI) measurement and analysis of trace impurities with synchrotron radiation X-ray fluorescence spectrometry (SR-XRF) was investigated for the forensic discrimination of headlight glasses. Glass fragments smaller than 1 mm were collected from 17 kinds of different headlight samples for this experiment. It was found that glass fragments taken from the surface of the headlight was not appropriate for the forensic comparison and should be excluded from the RI measurement, because some of them showed inappropriate RI values different from those of the fragments from the bulk. Discrimination of headlights was carried out by comparing the range of average RI value ±2SD among 136 possible pairs of the 17 samples. The comparison of RI could distinguish 118 pairs and but not the other 18 pairs. As a result of SR-XRF analysis for the 17 sample headlights, 8 elements (Zn, As, Sr, Zr, Sb, Ba, Ce and Hf) were selected for the comparison, because these elements were considered to be effective for the discrimination. The X-ray intensities of these elements were normalized using the element which provided the highest X-ray intensity in each X-ray measurement and was followed by cluster analysis. In the dendrogram obtained, 11 of 17 pairs of data measured with 2 fragments taken from the same sample on different experimental days were classified into the nearest cluster. This result indicates that the analysis of 8 elements by SR-XRF could give useful information for the discrimination of headlight glasses. The number of indistinguishable pairs was reduced to 4 from 18 by the addition of analytical results using SR-XRF to the results of RI measurement, which demonstrates that the combination of these two methods could achieve significant improvement in the discrimination capability of headlight glasses.
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