microscope examination of paint chip

Example cross section of a paint chip

TECHNOLOGY FOR THE ANALYSIS OF TRACE EVIDENCE

 

Automotive Paint Chips

 

Automotive paint, as trace evidence, is commonly encountered in forensic casework.

Colour is one of the most important comparative characteristics of paint and examining paint for differences in colour is one of the first steps taken in a forensic paint comparison.

The human eye is very sensitive to differences in colour and is therefore used as a screening tool in the initial stages of paint examinations.

However, it can be difficult to reliably compare colours when only small fragments are available.

Furthermore, some paints can appear visually similar but have different pigment compositions.

 

Application Examples

A UV-Vis microspectrophotometer is able to provide an objective means for colour measurement at the microscopic level and potentially distinguish paints that the human eye cannot.

Here we show how the μ-spec has the potential for discriminating automotive paint samples, a technique that is accepted by SWGMAT (Scientific Working Group for Materials Analysis) and standardised by ASTM International.

In this application note, 45 automotive paint samples were analysed using the μ-spec, focusing around 400-750nm in the electromagnetic spectrum. 

 

Method

The paint samples were embedded in light-activated resin and microtomed to produce nominal 3μm cross-sections.

The paint sections were mounted in glycerine on glass slides.

Spectra of the base coat were collected in transmission mode using a 40x objective with 100 watt tungsten halogen illumination.

Paint is heterogeneous at the microscopic level. Therefore, to observe the range in variation present in each paint sample, up to 10 spectra from different areas were collected where sample size allowed.

 

Results and Discussion

A total of 45 paint chips were analysed (see appendix for details).

There were 29 paint chips that comprised a range of colours namely reds, greens, blues and purples. It was found that the 29 coloured paints could be distinguished on the basis of their spectral curve shape, including paints that were visually similar in colour.

 

 Red Paint 1
 paint sample1 Paint spectral curve1 
 Red Paint 2
 paint sample2  Paint spectral curve2
   

The remaining 16 samples were considered achromatic paints and included blacks, greys and silvers.

Achromatic paints were included as the results of a study (Kopchick and Bommarito) suggest that paint analysis schemes should incorporate visible MSP for apparently achromatic black and gray/silver paint samples. This is due to the presence of secondary pigments in some paints giving rise to spectral information.

The achromatic paints were analysed using the μ-spec and the results support previous published findings. The table below summarises these results.

Sample set Total number of samples Total yielding MSP spectra Associated wavlengths (nm)
Black 4 4 430, 450, 590, 695
Grey/Silver 12 8 Throughout visible range
2 410-460

All 4 of the black paint samples produced useful spectral information albeit that some were limited.

Close inspection of one of the black paint samples revealed the presence of blue pigments.

Similar results were obtained for the grey/silver paint samples. Again, secondary pigments were observed giving rise to useful spectral information. Out of the 12 grey/silver samples, only 2 produced a featureless curve expected of achromatic paints.

 

 Blue Pigment
 paint sample3  Paint spectral curve3
 

Conclusion

The μ-spec has the ability to discriminate automotive paint samples, including paints that are visually similar in colour and apparently achromatic paints. It allows for the objective measurement of colour of very small paint samples and is nondestructive.

Common methods for the forensic examination of paint chips involve physical examination, microspectrophotometry, FTIR, SEM-EDS. Here we show how LIBS can be used effectively in the forensic examination of paint.

 

Method

LIBS spectra of small sections (< 1mm) of paint from various sources were recorded. 10 one shot spectra of each sample were averaged.

All LIBS spectra were acquired using the ECCO laser induced breakdown spectrometer. This consisted of an air-cooled actively Q-switched flashlamp pumped Nd:YAG laser delivering 60 milliJoule, 7 nanosecond pulses of 1064 nm of laser radiation at a repetition rate of 0.5 Hz. The spectrometer range was 225 – 600 nm. The spectrometer contained 3 CMOS sensors. The gate delay between the laser firing and the CMOS sensors shutters opening was 1 microsecond.

All LIBS spectra were recorded in an Argon atmosphere, with a flow rate of approximately 6 litres/minute.

Spectra were compared qualitatively for the presence or absence of elements, such as Cr, Ba, Ti, Ca, Pb and Sr.

 

Results

Below are the spectra from 4 different coloured paints, including white, grey, red, blue and green.

paint3

Figure 1: Blue paints, showing difference in the proportion of Calcium, Titanium and Barium

paint2

Figure 2: Grey paints, showing difference in the proportion of Calcium, Titanium and Barium

paint4

Figure 3: Green paints, showing difference in the proportion of Chromium, Barium and Calcium.

paint5

Figure 4: Red paints, showing difference in the proportion of Chromium, Calcium, Strontium and Lead.

T

THE FOLLOWING PRODUCTS CAN BE USED FOR THE ANALYSIS OF PAINT CHIPS
ECCO for Laser Induced Breakdown Spectroscopy

ffTA multi-examination system for the analysis of trace evidence