From hit-and-runs to vehicular homicides to automobile crashes, crime has a way of finding itself on the open roads. When it does, paint evidence can be one of the most crucial pieces of evidence that forensic analysts collect to solve these crimes. Such evidence can connect a car to a crime scene or victim in the absence of a witness or video footage.
Paint evidence is made up of four layers; electrocoat primer, primer surfacer, basecoat, and a clear coat (Figure 1). Electrocoat primer is the first layer and helps protect the car from erosion. The second layer, the primer surface, conceals imperfections in the surface of the car. The basecoat layers the desired color and finish to the vehicle. The final layer, clear coat, protects the bottom three layers from UV degradation and weathering.
Current techniques for the analysis of paint chip evidence include infrared spectroscopy (IR), which can analyze a paint chip rapidly and non-destructively, and pyrolysis gas chromatography/mass spectrometry (py-GC/MS). py-GC/MS is a form of GC/MS where the sample is first pyrolyzed or heated until it decomposes before being separated by gas chromatography and identified by mass spectrometry. It is particularly useful when two paint samples have similar binder systems and require further analysis with a more sensitive technique. While this technique is useful for its ability to differentiate between paint chips that IR is unable to, it is time consuming.
In an effort to develop a quicker method that yields the same quality results, Candice Bridge and her colleagues at the University of Central Florida investigated the use of direct analysis in real-time−time-of-flight mass spectrometry (DART-TOFMS) for the analysis of paint chips (Figure 2). This technique produces a high-resolution information on the molecular composition from a sample in any physical state without the need for lengthy separation via chromatography. This method has already been applied to drug, ink, and explosives analysis. In this study, the researchers used DART-TOFMS to analyze paint chips and compare its abilities to that of a py-GC/MS method used in forensic paint case. The analyst particularly focused on clear coat analysis, which is present in all paint chip as it is the top layer.
The investigators behind this study first acquired black paint chips with unknown make, model, year, generic paint clear coat formulation type, presence of metallic and pearlescent pigments in the basecoat, and the Vehicle Identification Number (VIN). They then utilized three different analytical methods to analyze the paint samples. The used the traditional method, py-GC/MS, a regular DART-TOFMS method, and a thermal desorption/py-DART-TOFMS method that heats the solid sample according to an increasing temperature gradient during analysis with DART-TOFMS.
After collecting the mass spectra of various paint chips from the three methods, Bridge and her colleagues used statistical analysis called Principal Component Analysis (PCA) to identify the different types of paint chips. In PCA, samples cluster together on a score plot based on certain characteristics, termed their “principal” component. The samples can be separated from another based on how they cluster, and the variability within a sample group can be assessed.
When comparing the mass spectra and PCA results from the py-GC/MS and DART-TOFMS methods (Figure 3), the researchers found that the two provided comparable information between samples. Interestingly, for some components, these two techniques provided different information. For example, paint binder melamine was identified using DART-TOFMS, but not py-GC/MS. Conversely, paint additive camphene was identified using py-GC/MS but not in the DART-TOFMS. These results suggest that these techniques may provide complement information about a paint sample. As for the thermal desorption/py-DART-TOFMS technique, they found that the technique separated compounds that were not identifiable with the other two techniques. Furthermore, this technique produced less variation between replicate data from the same sample and exhibited high reproducibility as shown in the thermal desorption plot in Figure 4.
These results from Bridge’s study reveals the real potential for DART-TOFMS and thermal desorption/py-DART-TOFMS as paint analysis techniques when IR won’t cut it. DART-TOFMS provided comparable, and in certain instances, complementary data to the standard technique py-GC/MS. Unlike py-GC/MS, which can take hours to obtained useful information, this method is completed in mere minutes. The combo method thermal desorption/py-DART-TOFMS was even able to characterize compounds the other two methods could not, while only taking roughly seven minutes to complete. The fast throughput associated with these two DART techniques can significantly impact extensive evidence backlogs that hamper the investigation process. For this reason, the researchers behind this study are working on a larger validation study to get this method one step closer to being used in the field.
|Title||DART-MS: A New Analytical Technique for Forensic Paint Analysis|
|Authors||Mark Marić, James Marano, Robert B. Cody, and Candice Bridge|
|Citation||Marić, M; Marano, J; Cody, R. B; Bridge, C; DART-MS: A New Analytical Technique for Forensic Paint Analysis. Anal Chem. 2018, 90, 11, 6877-6884. doi: 10.1021/acs.analchem.8b01067|