Scanning Electron Microscope (SEM) analysis of paint
Dr. Emma Passmore

The SEM is a powerful microscope that uses electrons instead of light to image materials at very high magnifications.

Method
Using a scalpel two flecks of paint were removed from the object; a white fleck from the side of the car above one of the wheels, and a red fleck from the rear seat. Both flecks were less than 2 mm long and approximately 0.5 mm wide. Each fleck was then mounted onto an aluminium SEM stub, onto an adhesive carbon pad. Both samples were then coated with a thin (approx. 30 μm) layer of evaporated carbon. This process ensured that the samples would conduct electrons when placed in the SEM.

Both samples were imaged in the SEM using two types of imaging; secondary electron (SE) mode and back-scattered electron (BSE) mode. Secondary electrons are generated when the electron beam interacts with the surface of the sample, causing electrons to be ejected from the sample surface. These electrons are then detected, building up an intuitive 3D image of the sample, with obvious surface topography. Back-scattered electrons are generated when the electron beam is scattered from the sample surface. Elements that have high atomic number (i.e. large, heavy elements, such as lead) cause a large amount of electron scattering, and consequently appear bright on the image. The BSE image mode is therefore useful for looking at the sample compositionally – variations in the contrast on the surface sample correspond to variations in chemical composition.

Both samples were also analysed semi-quantitatively using an Energy Dispersive X-ray detection system. When the electrons from the electron beam interact with elements in the sample the elements give off x-rays, and these x-rays have energies that are characteristic to that particular element. By measuring the energy value of x-rays given off by the sample it is therefore possible to determine which elements are present in the sample. By measuring the intensity of the x-rays given off it is possible to roughly determine the relative proportions of the elements.

Results
Quite speculative!
White paint – dominated by Ti and O.
Red paint – may be a mixture of two types of paint. One is dominated by Pb, C and O and appears bright white in BSE mode. The other may contain the same elements, plus Cr.

What next?
Ideally, someone familiar with the chemical constituents of paint pigments could interpret these results further and perhaps determine the type of paint used.