Laser light to analyse lipstick in forensic labs

Traces of lipstick are often found at crimes scenes, with forensic examination helping link the lipstick to suspects, victims or witnesses. But current analysis techniques tend to require either human opinion or some destruction/contamination of the samples.

Forensic scientists at the University of Kent have used Raman spectrometry to identify 20 brands of lipstick smeared on everyday items - textile fibres, cigarette butts and tissues - without removing such items from their evidence bags. This means the samples remain isolated and avoid contamination, and “continuity of evidence … can be maintained”, explained Professor Michael Went of the university’s School of Physical Sciences.

Raman spectroscopy is a process involving light and vibrational energy of chemical bonds. When a material (eg, lipstick) scatters light, most of the light is scattered at its original wavelength but a very small proportion is scattered at altered wavelengths due to changes in vibrational energy of the material’s molecules. This light is collected using a microscope to give a Raman spectrum, which gives a characteristic vibrational fingerprint, which can be compared to spectra of lipsticks of various types and brands.

The scientists used three excitation wavelengths to conduct their study - 473, 633 and 784 nm. They said in their research paper, “Raman spectroscopy is an easy, rapid and non-destructive method which requires no sample preparation and can analyse samples contained in evidence bags. The unique vibrational spectra of molecules result in a high degree of confidence in identification.”

The researchers believe the technique is suitable for samples which have been “exposed to variable environmental conditions”, as well as evidence from older cases, as “the majority of the spectra of deposited lipstick samples remained unchanged over a period of up to two years”. And when intense fluorescent interference was encountered (in both the lipstick and textile fibres), the 633 nm laser had difficulty but the other methods were successful.

The study has been published in the Royal Society of Chemistry journal. Meanwhile, new research into applying the same method on other types of cosmetic evidence - such as foundation powders, eyeliners and skin creams - is currently underway.

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