Identification Of The Automotive Paint

In traffic accident cases paint is one of most often seen and very important material evidences which can prove the existence of the accident. Paint analysis is significant in identifying the car brand, car type and finally the specific car that caused the accident. In traffic accident cases the analysis of the information obtained from the paint fragments at the scene of the crime is often used to decide the direction for case detection, and its comparison with the analysis of paint fragments of the suspicious car can be used for homogeneity identification. Therefore, paint analysis plays an important role in forensic practice.Nowadays with the cars increasing dramatically, the number of the traffic accident cases goes up rapidly, and so is the number of cases in which car paint detection is necessary. The paint sample detecting technique keeps developing with the amelioration of the paint synthetic techinique. Meanwhile the continuing improvement of paint technology, the higher requirement on automobile emission by the environment legislation and the increasing mergers and the acquisitions among paint manufacturing factories render that the distinction in respect of paint thickness and color, etc. is small either among different paint products made by different factories or among the same kind of paint products made by the different sectors of the same factory. Using traditional instrumental analysis techniques such as Optical Microscope, Thin-Layer Chromatography, Infra-Red Spectrometry, Pyrolysis Gas Chromatography and Scanning Electron Microscope, etc sometimes can not satisfy fully the requirements of forensic practice because of their destructive effect on the paint samples, spectral interference and detecting sensitivity, etc. So a new technique more appropriate for the paint detection under current circumstances shall be found to make up for the lack of the existing analysis techniques.Viewing the technological bottle-neck of the existing analysis methods and considering the characteristic of car paint composition currently, this research initiatively establishes the paint sample identification analysis technique system by using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) which has broad application, high sensitivity, high precision, high analysis speed and can provide precise isotope information, combined with the brand-new Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR). By using this new analysis technique system this research detected the inorganic and organic components of each layer of the paint sample, studied and established the corresponding evaluation technique and index. The main contents of this research are as follows:1. Studying and establishing the sample extraction and pre-processing guidelinesThe problems about the sample extraction and processing occurred in previous cases were gathered and analyzed. The guidelines for paint sample extraction at the scene of traffic accident cases were established, which include the general principles, methods for paint sample collection as well as the attentions needed to be paid under special circumstances.As for the pre-processing of paint sample, the paint sample contaminated by dirt or grease can be cleaned using ethanol and deionized water, and the contamination caused by rubbing or collision of cars can be separated by such physical separation methods as using metal blades under the microscope.2. Establishing the ATR-FT-IR qualitative analysis method to analyze the paint sampleThe problem of analyzing and determining the organic components of paint sample was solved. The problems of instrument noises, baseline drift and light scattering were resolved by optimizing the instrument system parameters in order to reduce the systematic error and attain the optimal analysis sensitivity and precision. A methodology study on this method was made.3. Establishing the LA-ICP-MS analysis method to analyze the paint sampleThe problem of analyzing and determining the inorganic components of the paint sample was resolved. Firstly the analysis parameters of ICP─MS were optimized using NIST612 glass standard reference materials to ensure the sensitivity and precision of ICP─MS in the mass range. Then a paint sample was chosen at random, three elements of this paint sample, i.e.55Mn,137Ba and 208Pb were taken as the study objects. The changes of the sensitivity and precision of each element with the change of the parameters of laser ablation instrument were evaluated in order to determine the optimal laser ablation parameters. What is more, the homogeneity of the paint sample and the precision of the analysis method were evaluated and it turned out that the relative standard deviation of the element ratio was less than 10%. The distributions of metal element of all the paint samples were determined in accordance with the threshold of an element that the response value of the element to be determined is 10 times more than the response value of that element in the carrier gas. 4. Exploring the evaluation index for homogeneity identification of car paint38 kinds of car paint most often seen in the market taken as the studying object, the evaluation index for homogeneity identification of car paint were established.(1) The organic components of different paint samples were determined by detecting the intensity, number and functional group information of absorption peak of each layer of the paint fragments in the red region using ATR-FTIR technique. (2) The inorganic components of each layer of paint were determined by the qualitative and quantitative analysis using LA-ICP-MS technique. The metal element types of the paint samples were determined in accordance with their threshold values. (3) After the detection results of each layer of the paint samples were attained, the element types and the organic components among the paint samples shall be compared firstly. If they are different, these samples can be distinguished directly. If they are the same, these samples can be distinguished by the relative ratio±20% of element response values. If the paint samples have the same element types, organic components and relative ratio of element response values, they shall be regarded as having high similarity and match with each other.In summary, this research initiatively establishes the paint sample analysis and evaluation technique system using LA-ICP-MS and ATR-FTIR, which includes the paint sample extraction and pre-processing guidelines, paint sample analysis technique using ATR-FTIR and LA-ICP-MS as well as the evaluation index for paint homogeneity identification. The paint sample extraction and pre-processing guidelines are able to ensure the determinacy, sufficiency, reliability of the material evidences and the evidentiary effect; the LA-ICP-MS technique has such characteristics and advantages as in situ, real time, rapid analysis, high sensitivity, good spatial resolution (<10μm), ability to detect and determine multi elements at the same time and to provide the isotope ratio information, etc that it can satisfy the requirements for the evidence preservation and effect reliability in judicial forensic science; the evaluation index and technique for paint homogeneity identification has such characteristics as systematism and multi index including the paint composition layers, the organic and inorganic components, element types and the relative content, etc that it can ensure the scientificity and reliability of the determining result.After completing this research, we, together with other 84 forensic laboratories from different countries in the world, participated in the Proficiency Testing on the Paint Detecting in Trace Evidence organized by CTS (Collaborative Testing Services). There were four paint samples provided in this testing, No.1 was collected from the suspicious car, No.2,3 and 4 were collected from different scenes. The testing required the participants to identify the relationship between No.2,3,4 paint samples and No.1 paint sample respectively. We used LA-ICP-MS and ATR-FTIR to analyze and detect the four paint samples. The detecting result showed that:(1) the four paint samples were all three-layer car paint, and the color and thickness of each layer were the same; (2) the organic component and the element composition of the basecoat layer and clearcoat layer of the four paint samples were all the same; (3) the element type and the infra-red spectrum of primer layer of No.3 paint sample were different from those of No.1,2 and 4 paint samples; (4) the element type and the infra-red spectrum of primer layer of No.1,2 and 4 paint samples were all the same, and the ratio among the elements were not different (the differences among the ratios were all within±20%). So we concluded that No.1,2 and 4 paint samples were from the same source. Our detecting result turned out to be the same as that announced by CTS.The application practice shows that the new technique system established in this research can satisfy fully the requirements of the paint evidence detection in forensic science. And it can also be used for reference or as an example for the detection of other material evidences in forensic science.