Trace elemental analysis of automotive paints by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS)

Paints and coatings are frequently encountered as types of materials that are submitted to forensic science laboratories as a result of trace evidence transfers. The aim of this study was to develop a method to complement the commonly used techniques in a forensic laboratory in order to better characterize these samples for forensic purposes. A laser ablation method has been used to simultaneously sample several layers directly prior to introduction into an inductively coupled plasma-mass spectrometer for the detection and quantification of the trace metals present in the layer(s). Time-resolved analysis plots displaying the elemental response and quantification of selected metals are compared to associate/discriminate paint samples. Matrix-matched standards were successfully incorporated into the analysis scheme for quantification of lead in the solid paint samples. Preparation of new matrix-matched standards for quantification of additional elements developed for this study are also presented. A sample set of eighteen (18) survey automotive paint samples have been analyzed with the developed method in order to determine the utility of LA-ICP-MS for trace element analysis of paints.     

Examination of Car Paint Samples Using Visible Microspectrometry for Forensic Purposes

Visible (Vis) microspectrometry (MSP) allows objective measurement of color as opposed to the subjective results of visual color comparison. In this paper, the usefulness of this method for comparison of small car paint traces of the same color and the same or different chroma was studied for forensic purposes. Sixteen samples of solid and metallic bright and dark red paints taken from different cars were examined. Each sample was measured in the reflectance mode using a light beam falling perpendicular to the top surface of the sample and on a cross-section. In addition to the MSP measurements, the Raman spectra were produced in order to detect the pigment composition of the samples. On the basis of the obtained results it was stated that discrimination of car paints of the same color is conditioned not only by the kind, number, and amount of the pigments, but rather by the morphology of the paint layer, that is, by the size, shape, orientation, and distribution of the pigment grains in a paint resin. A criterion allowing two samples of the same chroma to be distinguished was established.     

Comparative Analysis of Car Paint Traces in Terms of Color by VIS Microspectrometry for Forensic Needs

The usefulness of VIS microspectrometry for comparison of very small car paint traces was studied. Eighteen samples of solid and metallic paints taken from new and repainted cars were examined. Each sample was measured in both transmittance and reflectance mode (on top surface and on cross-section). Using the CIELAB units the repeatability of the color measurement was assessed. A criterion allowing two parts of the same sample and two samples of the same color to be distinguished was established. The method was proved to be an useful instrumental tool–complementary to other analytical methods (e.g., IR spectrometry)–in examination of car paint traces for forensic purposes.     

Forensic discrimination of automotive paint samples using pyrolysis-gas chromatography-mass spectrometry with multivariate statistics

Analytical pyrolysis-gas chromatography (Py-GC) has been a standard method for the forensic analysis of automotive paint for a number of decades. Automotive paints are often identified by visual comparison of pyrograms for peak presence and intensities; however, such analyses can be subjective and time consuming. A preliminary investigation based on Py-GC-mass spectrometric analysis of 100 automobile paint samples of five different colors is presented. Designed experiments are employed to select pyrolysis conditions for adequate discrimination. Pattern recognition techniques including principal component analysis and canonical variates analysis are used to visualize clustering of pyrograms to validate comparisons between different automotive paint pyrograms. These methods have the potential to ease the interpretation task for data sets involving a large number of comparisons.     

Detecting art forgeries using LA-ICP-MS incorporating the in situ application of laser-based collection technology

The nature of art lends itself to forgery as a skilled and determined forger can mimic the techniques and styles of an artist to a level where even an expert can be duped. The authentication of paintings is a subjective process, but modern techniques may provide the means to provenance artist pigments based on elemental composition. This study applies laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to the analysis of artist paints from different manufacturers to identify variation between the elemental association patterns of these materials. The technique facilitates comparison of the paints used by an artist with produced works of art to assist provenancing initiatives of questioned materials. The effects of the trace element profiles of the backing substrate and binder on analytical data were also identified. By applying the technique to paint scraped from real paintings, a limited database was created to allow comparison to be made with some of Australian artist, Kathleen O’Connor's artworks and assist in determining production chronology. Data from this study were able to facilitate comparison of blue paints from two different paintings and confirm their co-provenance consequently determining the relative production date of a separate painting of previously unknown age.Preliminary trials of a prototype collection device designed to reduce damage and allow for in situ sampling of artworks were also undertaken. The device, which allows direct laser-based sampling of a complete painting, was tested using a Francis Ryan painting. The prototype allows for the collection of debris directly generated by LA-ICP-MS of a predefined area of a painting prior to subsequent analysis using direct LA-ICP-MS. This collection method significantly minimizes the amount of damage produced by conventional sampling methods. Analyses of the debris collected, using the prototype, were found to be comparable to the scrapings of equivalent paint analysed using direct LA-ICP-MS analysis.     

The Analysis of Automotive Paints by Microscopic Spectrophotometries and Pyrolysis-Gas Chromatography

The automotive paints could be generally differentiated by color,layer sequence and chemistry of the paint layers comprising each of the topcoat and the primer system.     

Characterization of Paint by Fourier-Transform Infrared Spectroscopy,Raman Microscopy,and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy

The identification of automotive coatings has become important for forensic scientists to trace suspects. Popular automotive coatings include acrylic, amino, alkyd, nitro, and polyurethane paints. Various pigments and additives are added to the coatings, which may create difficulty in characterization of paint evidence. In this study, Fourier-transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy were used to characterize an eleven-layer paint sample. Most layers were only a few micrometers thick. Infrared spectroscopy has several advantages in identifying resins and additives, while Raman is more effective in characterizing additives and inorganic pigments. Scanning electron microscopy-energy dispersive X-ray spectroscopy provides elemental analysis information. The results reveal that the combination of these techniques provides more accurate paint identification than using them separately.     

Comparative analysis of automotive paints by laser induced breakdown spectroscopy and nonparametric permutation tests

Laser-induced breakdown spectroscopy (LIBS) has been investigated for the discrimination of automobile paint samples. Paint samples from automobiles of different makes, models, and years were collected and separated into sets based on the color, presence or absence of effect pigments and the number of paint layers. Twelve LIBS spectra were obtained for each paint sample, each an average of a five single shot “drill down” spectra from consecutive laser ablations in the same spot on the sample. Analyses by a nonparametric permutation test and a parametric Wald test were performed to determine the extent of discrimination within each set of paint samples. The discrimination power and Type I error were assessed for each data analysis method. Conversion of the spectral intensity to a log-scale (base 10) resulted in a higher overall discrimination power while observing the same significance level. Working on the log-scale, the nonparametric permutation tests gave an overall 89.83% discrimination power with a size of Type I error being 4.44% at the nominal significance level of 5%. White paint samples, as a group, were the most difficult to differentiate with the power being only 86.56% followed by 95.83% for black paint samples. Parametric analysis of the data set produced lower discrimination (85.17%) with 3.33% Type I errors, which is not recommended for both theoretical and practical considerations. The nonparametric testing method is applicable across many analytical comparisons, with the specific application described here being the pairwise comparison of automotive paint samples.     

100% electrostatic application of metallic basecoats

Paint Application by electrostatic high rotation atomizers is the most efficient way of applying spray paint to car bodies in automotive OEM paint shops. Only the 2^nd coat of metallic base coats usually is applied by pneumatic atomizers. The reason for this is due to color/design and process stability questions, both resulting from differences in atomisation and droplet deposition mechanisms of the two paint atomizing processes. The results in an applied metallic film are explained. A path towards a process is outlined, considering not only the spray process itself, but the whole process chain incl. : - design - the repair processes - the manufacturers of plastic parts for the body - the paint manufacturers in terms of paint reformulation and test equipment The success of this process depends on the careful evaluation of the paint shop targets and the consideration of the total process chain.     

Enamels in stained glass windows: Preparation, chemical composition, microstructure and causes of deterioration

Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject to severe degradation while others from the same period survived the ravages of time. A series of dark blue, green–blue and purple enamel glass paints from the same region (Northwestern Europe) and from the same period (16–early 20th centuries) has been studied by means of a combination of microscopic X-ray fluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim of better understanding the causes of the degradation. The chemical composition of the enamels diverges from the average chemical composition of window glass. Some of the compositions appear to be unstable, for example those with a high concentration of K₂O and a low content of CaO and PbO. In other cases, the deterioration of the paint layers was caused by the less than optimal vitrification of the enamel during the firing process. Recipes and chemical compositions indicate that glassmakers of the 16–17th century had full control over the color of the enamel glass paints they made. They mainly used three types of coloring agents, based on Co (dark blue), Mn (purple) and Cu (light-blue or green–blue) as coloring elements. Blue–purple enamel paints were obtained by mixing two different coloring agents. The coloring agent for red–purple enamel, introduced during the 19th century, was colloidal gold embedded in grains of lead glass.     

Determination of elements in paints and paint scrapings by inductively coupled plasma atomic emission spectrometry using microwave assisted digestion

Summary Inductively coupled plasma atomic emission spectrometry (ICPAES) is applied to the determination of elements in paints and paint scrapings to evaluate potential occupational and environmental exposure to metals in paints. A microwave assisted digestion procedure of these materials using nitric and hydrofluoric acid mixtures under controlled pressure has been developed. The accuracy and repeatability of the recommended procedure for lead are documented and evaluated by analysing NIST SRM 1579 Powdered Lead Based Paint. Neutron activation analysis was applied as a reference method for other elements. A variety of industrial and domestic paints of different chemical composition were analysed. The advantages and limitations of microwave digestion of paints are discussed.     

13C-Isotope ratio mass spectrometry as a potential tool for the forensic analysis of white architectural paint: a preliminary study

Paints have a dual role in society, to protect materials from environmental agents such as ultraviolet light, moisture and oxygen, and to make painted materials look more attractive. Variability in paint samples is often due to binder and pigment type within the sample. The most common resin used in decorative paints is drying oil alkyd resin, which incorporates soybean oil and vinyl acrylic based latexes. Traditional analytical methods used by forensic scientists may be able to say whether two paint samples are indistinguishable but cannot conclusively say that they both originate from the same source. To find out if isotopic composition can provide an added dimension of information, 28 different white architectural paints were analysed for (13)C abundance using isotope ratio mass spectrometry. In addition, variations in application, drying time and thickness were also investigated to assess the discriminatory power of (13)C data from white paints with an unknown history. Preliminary results indicate that this method could aid screening of paint samples.     

Molecular weight evaluation of poly(dimethylsiloxane) on solid surfaces using silver deposition/TOF-SIMS.

Molecular ions include information about end groups, functional groups and molecular weight. A method for the direct detection of these in the high mass range (m/z > 1000) from poly(dimethylsiloxane) (PDMS) on a solid surface was investigated. It was found that a TOF-SIMS analysis of silver-deposited surfaces (silver deposition/TOF-SIMS) is useful for this purpose. Using the silver-deposition/TOF-SIMS method, silver-cationized quasi-molecular ions were clearly detected from PDMS on solid surfaces, and their structure and molecular weight were evaluated. In addition, their images were observed without the interference of deposited silver. By applying to the analysis of paint defects etc., it was confirmed that this technique is useful to analyze actual industrial materials. Silver-cationized ions were detected not only from PDMS, but also from other organic materials, such as lubricant additives and oils on solid surfaces. Therefore, the silver deposition/TOF-SIMS method was proved to be useful for the analysis of ultrathin substances on solid surfaces.     

Differentiation and evaluation of evidence value of styrene acrylic urethane topcoat car paints analysed by pyrolysis-gas chromatography

Pyrolysis (Py)-GC/MS was applied to differentiate between automobile paint samples. The method was used for analysis of 36 samples of styrene acrylic urethane clearcoats that were indistinguishable on the basis of their infrared spectra and elemental composition. Differences observed in the obtained pyrograms of the compared paint samples were relatively small. Therefore, statistical analysis of the obtained results was performed. The likelihood ratio test suitable for multivariate data analysis supported by analysis of data structure by graphical model was used. This approach allowed not only distinguishing the samples compared, but also allowed the evaluation of the evidential value of such observations, which is very important from a forensic point of view.     

A study of ochres from an Australian aboriginal bark painting using thermal methods

The potential of thermogravimetric analysis (TG) as a tool for the characterisation of ochre paint used in indigenous Australian bark paintings has been investigated. TG has been combined with differential scanning calorimetry (DSC) and mass spectrometry (MS) to identify and quantify the main inorganic and organic components present in the paints. The results obtained were supported by comparison with infrared spectra and XRD data obtained for the same specimens. The potential of thermal methods for the characterisation ochres has been demonstrated, with subtle differences between small samples being able to be identified.     

Classification of protein binders in artist's paints by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry: an evaluation of principal component analysis (PCA) and soft independent modelling of class analogy (SIMCA)

Proteomics techniques are increasingly applied for the identification of protein binders in historical paints. The complex nature of paint samples, with different kinds of pigments mixed into, and degradation by long term exposure to light, humidity and temperature variations, requires solid analysis and interpretation methods. In this study matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectra of tryptic-digested paint replicas are subjected to principal component analysis (PCA) and soft independent modelling of class analogy (SIMCA) in order to distinguish proteinaceous binders based on animal glues, egg white, egg yolk and milk casein from each other. The most meaningful peptide peaks for a given protein class will be determined, and if possible, annotated with their corresponding amino acid sequence. The methodology was subsequently applied on egg temperas, as well as on animal glues from different species. In the latter small differences in the MALDI-TOF mass spectra can allow the determination of a mammal or sturgeon origin of the glue. Finally, paint samples from the 16(th) century altarpiece of St Margaret of Antioch (Mlynica, Slovakia) were analysed. Several expected peaks are either present in lower abundance or completely missing in these natural aged paints, due to degradation of the paints. In spite of this mammalian glue was identified in the St Margaret samples.     

Surface-enhanced Raman spectroscopy analysis of house paint and wallpaper samples from an 18th century historic property

Conservation efforts for heritage buildings require a substantial knowledge of the chemical makeup of materials that were used throughout the lifetime of the property. In particular, conservators are often concerned with the identification of colorants used in both interior and exterior wall treatments (paint, wallpaper, etc.) in order to gain perspective into how the building may have appeared during a certain time period in its existence. Ideally, such an analysis requires a technique that provides molecular level information as to the identity of the colorant as well as other sample components (binders, fillers, etc.), which is useful for dating purposes. In addition, the technique should be easily applied to paint layer samples which can be extremely thin and fragile. Herein we report the surface-enhanced Raman spectroscopy (SERS) analysis of paint and wallpaper samples taken from exterior and interior surfaces of a historic building. Several pigments were identified in the samples, which ranged from early inorganic pigments (lead white, barium sulfate, calcium carbonate, anhydrous chromium(III) oxide) which have been used in house paints for centuries, to a more modern pigment (phthalocyanine blue), developed in the middle of the 20th century. This analysis highlights the usefulness of SERS in such a conservation effort, and demonstrates for the first time pigment identification in house paints and wallpaper using SERS, which has far-reaching implications not only in the field of conservation, but also in forensics, industrial process control, and environmental health and safety.     

Comparative Study of the Quantity of Volatile Organic Compounds in Water-Based Paint and Solvent-Based Applied Polyurethane

Summary: The concern about the environmental impacts generated in the production of goods and services has increased last decades. The industry has used paints and varnishes in the manufacturing process have been pressed to improve air pollution prevention. Thus, the present work aims to identify the quantitative differences of VOC's and analyze the effect of VOC's in the burning rate, in a solvent and water-based paint, applied in the manufacture of automotive steering wheels. The results has showed that the solvent-based paint contains nine times more VOC's in your formulation in relation to water-based paint, when compared liquid and volatile organic compounds present in the solvent-based paint increase the speed of combustion of the polyurethanes samples. These data indicate that a discussion on the subject must be initiated in order to reduce these emissions that can harm society.     

微波消解-ICP-OES法测定汽车涂料中8种重金属

建立微波消解-电感耦合等离子体发射光谱(ICP-OES)测定汽车涂料中Pb,Cr,Se,Ba,Sb,As,Cd,Hg含量的方法。以HNO_3-H_2O_2(体积比为4∶1)混合酸消解样品,各元素分析谱线:Pb 220.353 nm,Cr 267.716 nm,Se196.090 nm,Ba 233.527 nm,Sb 217.581 nm,As 189.042 nm,Cd 228.802 nm,Hg 184.950 nm。8种元素测定结果的相对标准偏差为2.02%~12.94%(n=6);对白色、蓝色、红色汽车漆样品进行加标回收试验,Pb,Cr,Se,Ba,As,Cd,Hg,的加标回收率为81.26%~99.79%,Sb的回收率为62.43%~87.61%。该方法快速、简便,精密度、准确度较高,可用于汽车涂料中重金属含量的监控。     

Comparing the Color of Forensic Traces

Microspectrometry in the visible range is routinely applied in comparative examination of criminalistic traces such as paint chips, single fibers, and inks on questioned documents. It makes it possible to analyze a very small amount of sample and provides objective information about its color as well as helping to distinguish samples of similar colors. Measurements of Vis spectra are performed both in transmittance and reflectance mode, depending on the kind of traces. Application of the CIELab color system enables us to define color precisely and make use of color databases.