The Analysis of New Plate-Like Pigments in Automotive Coatings
The knowledge of which pigments are present in a paint can be very helpful in assessing the evidential value of matching paint samples. As different paint suppliers often use different pigment mixtures for obtaining the same color shade, the identification of the pigment composition can enhance the discriminating power. The knowledge of which pigments are present can also be of paramount importance in hit-and-run cases. If their dates of introduction and/or composition are known, their identification can provide a guide to the make, model, and year of a vehicle.
In the last few years, some schemes have been developed for identifying "one-dimensional" organic and inorganic pigments. The protonation technique using trifluoroacetic acid must be mentioned.1 Applying this technique many organic pigments can be solubilized after which they can be separated by thin layer chromatography (TLC) and identified by microspectrophotometry (MSP). IR-spectroscopy2-5, Raman-spectroscopy6,7 and MSP8, when used on thin sections, can also be applied successfully. Microscopy (incident brightfield at a magnification of 200-500x) proved to be the easiest way to establish the presence of interference pigments and to measure their dimensions.9