Identification of Organic Pigments in Automotive Coatings Using Laser Desorption Mass Spectrometry*

Abstract

When one looks at an automotive coating, one sees color due to pigments. Modern organic pigments, with high molar absorptivities, may be only minor components of the mixture. Laser desorption mass spectrometry (LDMS) has been shown to be a useful tool for the analysis of colorants such as pen ink dyes. Here, LDMS is used to determine its utility for the identification of pigments, in simple media and in more complex paints. Small paint chips can be introduced into the LDMS instrument, and when an ultraviolet laser is focused on a portion of a chip, ions representative of the pigment(s) are selectively formed. Some pigments such as quinacridones and copper phthalocyanine are very stable and are desorbed and ionized intact. In contrast, benzimidazolones, which contain some single-skeletal bonds, form fragment ions. This method proves to be sensitive and convenient, as no sample preparation is required. The presence of inorganic pigments in addition to modern organic pigments can be determined, and pigments can be directly identified in actual automotive paint chip samples.