Assessing the automotive paint evidence in Singapore via population and discrimination studies

Various studies on differentiation of the automotive paints have been published in support of the evidential value of automotive paint comparisons [1] , [2] , [3] , [4] , [5] , and were based on the automotive populations in the regions outside of Singapore. However, the frequency information of the automotive paint characteristics in the previous studies may not be directly relevant to the situation in Singapore. To obtain the local frequency information of the automotive paint characteristics, a population study based on the automotive distribution in Singapore was necessary to apprehend the variations in the distribution of vehicle makes and colours, the visual and microscopical features, and the chemical compositions of the paint samples. This was facilitated with the data obtained from the local transport authority, the surveys conducted on random vehicle samples on the road, and the analysis of the existing collection of automotive paint samples taken from the laboratory’s vehicle paint database that closely resembles the vehicle population of Singapore. A discrimination study involving comparative analysis of 256 automotive paint samples from six colour groups was performed to evaluate the discriminating power of the laboratory's standard paint examination protocol: visual and microscopical examinations, Fourier Transform Infrared (FT-IR) Spectroscopy and Scanning Electron Microscopy with Energy Dispersive X-ray (SEM/EDX) Spectroscopy. Visual and microscopical comparisons of the physical characteristics in these paint samples yielded 54 indistinguishable pairs. When further subjected to FT-IR analysis, 51 pairs were distinguished in terms of their chemical compositions. The elemental compositions of the remaining three indistinguishable pairs were compared using SEM/EDX, and one pair remained indistinguishable. A combination of these analytical techniques yielded a discriminating power (DP) of 0.99981. The DP in this study is in support of the DP values reported in the published studies [1] , [2] , [3] , [4] , [5] .