The Role of Surface Analysis in Forensic Science

The non-destructive chemical characterisation of small quantities of material that constitute physical evidence in criminal cases is often highly desirable, leaving the evidence intact for further testing or review at a later date. Many of the analytical methods which can be employed in forensic examinations result in the consumption/destruction of the sample during testing, which is problematic in cases where only small/trace amounts of evidential material are available.

Optical microscopy is one of the most important non-destructive techniques used in forensic examinations. Of course, it does not provide chemical information and is limited for the inspection and differentiation of very small samples/features and very thin residues/smears. Whilst a range of non-destructive spectroscopic techniques exist (e.g. uv/visible, infrared, Raman, fluorescence), they are not best suited to very small samples or very thin layers.
Surface analysis techniques such as XPS and ToFSIMS can play an important role in the forensic area with some key capabilities:

• Direct analysis of most types of solid material/surface as received, including clothing, soft furnishings, furniture, household wares, paper, glass, plastics and fibres/hair.
• Non-destructive analysis i.e. leaving the evidence intact.
• Analysis of very small sample areas (less than 0.5 mm with ease and down to 0.05 mm with the very latest instrumentation).
• Analysis of very thin smears/residues (down to ca. 0.000005 mm thick or even less) which may be invisible optically and are not detectable by other less surface sensitive techniques.

X-ray Photoelectron Spectroscopy (XPS)

Using soft X-rays, X-ray Photoelectron Spectroscopy (XPS) can probe the surface of a material and determine the elements present, their concentrations (in atomic %) and the chemical states of the elements found. This non-destructive quantitative testing method can detect elements down to 0.1%. The technique is illustrated in Figure 1 by the XPS analysis of cosmetic powders.
Figure 1 (top) shows the XPS survey spectrum for a talcum powder. Only magnesium, oxygen, silicon (in a silica/silicate form) and carbon were detected. This corresponds to a magnesium silicate, as expected for a talcum powder, with some organic residues on the surface. In addition to this qualitative analysis, the quantitative capability of XPS can be exploited to differentiate between different talcum powders. For example, three white talcum powders (as above, only exhibiting Mg/Si/O/C) can be differentiated on the basis of their different Mg:Si ratios of 1:1.26, 1:1.16, and 1:0.95.
Figure 1 (bottom) shows the XPS survey spectrum for a face powder. In addition to Mg/Si/O/C, fluorine, titanium, aluminium and potassium are present. This powder composition is clearly different to the talcum powders analysed, comprising a mixture of talc and mica(s) or just a mixture of micas (e.g. muscovite, phlogopite, fluorophlogopite).

Figure 1 : XPS survey spectra for two cosmetic powders

Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS)

Using a primary ion beam, ToFSIMS provides mass spectra from a material under study, which gives information re. elements, functional groups, polymer groups and molecules. Unlike XPS, the technique is not quantitative but it can provide some semi-quantitative information. As a mass spectrometry, however, it does provide more detailed structural information than XPS, particularly for organic and polymeric components. In static mode, using very low primary ion doses, samples can be analysed with, effectively, no damage. [Note : In the ToFSIMS experiment, the positive and negative secondary ions, emitted from the sample by primary ion bombardment, are mass analysed separately. Thus, both a positive ion spectrum and a negative ion spectrum are acquired for each sample analysis]

The powerful chemical fingerprinting capability of ToFSIMS is demonstrated in Figure 2, which shows partial mass range spectra obtained for two similarly-coloured (lilac/purple) nail polishes. There are many common peaks in the two sets of spectra, which show that both nail polishes contain nitrocellulose lacquers (e.g. see Figure 2a, both showing NOx signals). But, with so many secondary ion masses emitted from the samples (note, spectra were collected over the mass ranges 0 – 1000; only parts of these mass ranges are shown), differences in at least some part of the spectra are likely to be observed. This is illustrated in Figures 2b and 2c where, visually, the partial mass range spectral fingerprints are quite different (note also, in Figure 2a, the clearly higher fluorine content in Nail Polish 1, corresponding to the presence of a fluorocarbon component).

Figure 2a : ToFSIMS : negative ion range 5 - 80

Figure 2b : ToFSIMS : positive ion range 0 - 160

Figure 2c : ToFSIMS : positive ion range 400 - 800

The ability to differentiate between samples within the same class of material/product is also demonstrated in Figure 3, where two similarly-coloured (red) lipsticks show distinctive ToFSIMS spectral fingerprints. Skin lotions have also been analysed, with detailed information contained within the ToFSIMS spectra (e.g see Figure 4).

Figure 3 : ToFSIMS analysis of red lipsticks

Figure 4 : ToFSIMS analysis of skin lotion

Recent advances in ToFSIMS instrumentation now allow the acquisition of spectra with spatial information. This chemical imaging capability (e.g. see Figure 5, which shows the distribution of an organic residue (red) on the surface of a human hair) augments the spectral fingerprinting feature of ToFSIMS.

Figure 5 : Mapping an organic residue (shown in red)
on a human hair by ToFSIMS