Pyrolysis-gas chromatography/mass spectrometry analysis as a useful tool in forensic examination of automotive paint traces

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Abstract

Pyrolysis (Py)-GC/MS was applied in comparative analysis of polymer binder of 150 automobile clearcoat samples. It was found that binders of identical type and similar infrared spectra can be, in most cases, effectively differentiated, sometimes based only on the presence of peaks of very low intensity originating from minor paint components. The qualitative procedure of pyrograms comparison was developed. The results obtained, enable one to draw the conclusion that Py-GC/MS appears as valuable, very informative analytical technique of car paint samples examination. Providing identification of paint pyrolysis products, it enables discrimination of samples on the basis of their composition. However, in some cases, paint samples were still indistinguishable on the basis of their pyrograms.

Introduction

Paint chips are commonly found at the scene of car accidents. They can be collected from the road or the victim's clothing and compared with the paint from suspected car in order to state if they could have originated from the same paint coat. The criminalistic examination of car paint samples includes the comparison of their chemical content. The complex chemical composition of paints as well as their small amount available for examination makes their analysis difficult. Usually, applied microscopic and spectrometric methods (especially FT-IR technique) enable, in many cases, to distinguish between the paints [1]. However, if the paint samples belong to the same class and contain similar polymer binder and main pigments/fillers, the further individualisation required the application of more sensitive and discriminating analytical methods, e.g. pyrolysis (Py)-GC/MS [1], [2].

During pyrolysis, polymeric binder may degrade via a number of mechanisms like random scission, monomer reversion or the side group elimination and as a result, smaller (simpler) compounds which can be easily identified are formed. Additionally, minor components of paint samples could be visible as separate peaks in the pyrograms obtained. Py-GC is used primarily for the comparative analysis of the organic components of paint. The pyrolysis patterns obtained from two or more samples are compared visually noting the presence or absence of peaks, their retention times and relative intensities. Coupled with MS and subsequent library search, the technique is used to identify pyrolysis products [3], [4], [5].

Py-GC/MS can be applied to classify, characterise and compare coating materials and polymers by determination of chemical components such as binders, additives, solvents, plastisizers and residual monomers. It is also possible to find subtle structural or compositional variations within a similar matrix. Challinor reported its application for examination of different samples of forensic interest like paints, motor vehicle rubbers, fibres, adhesives and polyurethane foams [6], [7]. Some examiners tried to use the method for automobile paint classification and identification [8], [9], [10], [11]. Automotive paint binder type can be identified on microgram-sized samples of topcoat. Information about the binder composition of some samples can be extended if paints are derivatised during pyrolysis [2], [14]. Burns and Doolan [12], [13] have recently applied the method to split the examined paints samples into several groups characterised by different break down products obtained by pyrolysis suggesting that it might be useful for criminalistic identification of paints. However, the authors took into consideration only high and clearly recognisable peaks in pyrograms, omitting smaller ones. So, the pyrograms obtained were simple and easy for interpretation, but without information about some minor components of paints.

As noted by some authors, this method of analysis may offer improved discrimination of chemically similar paints [8], [12]. Py-GC/MS, however, is a destructive technique and its applicability depends on the paint type and amount of sample. It is best applied to individual paint layers.

The aim of the study was evaluation of the Py-GC/MS application in discrimination between paint samples of very similar composition for forensic purposes. Three groups of car paints of indistinguishable infrared spectra within each group were analysed and components of polymer binder were identified. The procedure of comparison of similar pyrogram patterns was proposed. The obtained results became a starting point for developing a statistical approach to discrimination of the samples examined.

Section snippets

Materials and methods

Samples were pyrolysed by means of CDS Pyroprobe 2000 (Analytix, UK) and then transferred to the RTx-35MS chromatographic capillary column (30 m × 0.25 mm, 0.25 μm), where compounds were separated [15]. As a GC/MS system, TurboMass Gold (Perkin-Elmer Instruments) was used. The GC pyrogram was: 40 °C held for 2.5 min; ramped 10.5 °C min−1 to 320 °C; 320 °C maintained for 5 min. The stationary phase of GC column consisted of 35% diphenylpolysiloxane and 65% dimethylpolysiloxane. Carrier gas was helium,

Results

Collection of 150 acrylic clearcoat samples taken from different cars were analysed by means of Py-GC/MS. In most cases, differences in obtained pyrograms were significant and clearly visible. However, there were several samples which differ only slightly, and evaluation of the significance of the observed differences was difficult. So, the procedure was elaborated to face the problem of differentiation of paints characterised by the same set of major components where Py-GC/MS analysis gave

Conclusions

The results obtained, draw to the conclusion that Py-GC/MS appears as valuable, very informative analytical technique, being complementary to the FT-IR technique in the field of investigation of car paint samples for forensic purposes. In the case when paint samples belong to the same group characterised by the same polymer binder (i.e. when they are indistinguishable on the basis of their IR spectra), the application of Py-GC/MS can lead to their complete differentiation. Firstly, samples of

Acknowledgement

The research was financially supported by the State Committee for Scientific Research, Poland, within the project no. 0 T00C 013 26.

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