Sulfur in a single hair--can it expose the guilty without nailing the innocent?

Researchers from the LGC National Measurement Institute's Chemical Metrology Laboratory (Teddington, England) and the University of Oviedo (Oviedo, Spain) have developed a laser-based method to detect how the proportions of isotopes in a chemical element vary throughout the length of a single human hair. One of their objectives is to use the method to track the geographical movements of people, including international crime suspects and victims.¹

To carry out this study, the scientists focused on the most abundant sulfur isotopes in hair keratin: sulfur-32 (32S), which accounts for about 95%, and sulfur-34 (34S), which makes up around 4%. This proportion can change slightly in response to people's diets, as well as when they travel from one country to another, and the technique can detect these small variations.

The new method is based on combining a laser-ablation system with multicollector inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS), says Rebeca Santamaría-Fernández of LGC. The laser ablates the selected fraction of the hair, generating an aerosol that then ionizes within plasma; the spectrometer provides the exact proportions of the sulfur isotopes.

"The advantage of this method compared with others is the high resolution resulting from use of the laser," says Santamaría-Fernández. This has enabled the scientists to confirm that the sulfur variations in hair can be linked to peoples' geographical movements.

The traveler experiment

The researchers collected hair samples of more than 4 cm in length donated by three volunteers. Two were permanent residents of the United Kingdom, while the third--dubbed "the traveler"--had spent the past six months in Croatia, Austria, the United Kingdom, and Australia.

"We are what we eat, and the small variations in the 34S/32S relationship reflect changes to our diet, which can in turn be related to movements from one country to another," says Justo Giner, another of the study's authors. In addition, as hair grows an average of 1.25 cm per month, the data obtained from a hair measuring between 4 cm and 6 cm can provide information about its owner's activities in the months leading up to the sample being taken.

The results of the experiment revealed that the traveler's hair indeed showed significant variations in the sulfur isotopes, while changes in the hairs of the two people living in the United Kingdom were minimal, and similar in both samples.

The authors believe they have overcome "the first hurdle"--developing an effective method to measure longitudinal isotope variations in hair, with the potential to relate these changes to geographical movements. The next objective is to demonstrate the global significance of these variations; they are already working with hair samples from 150 volunteers with different diets and geographical origins. The researchers will also measure the isotopic variations of other elements, such as carbon and nitrogen.

The scientists believe they will be able to create databases that will one day make it possible to link the relationship between a specific isotope in hair keratin and a country or region, which would be of great help to the police in tracking down international criminals.

"Although we still cannot say that a certain isotopic variation in a person's hair shows that he or she has been in a particular country, the method can help to break down the alibis of some terrorists who claim not to have moved over recent months," says Santamaría-Fernández.

Various British security forces, such as the London Metropolitan Police, have already expressed their interest in this project. The LGC center (previously known as the Laboratory of the Government Chemist) is working with various national and international research groups, among them the University of Oviedo.

One of many questions

However, as with any brand-new forensic technique that has the potential to point an incriminating finger, this approach should be thoroughly investigated, all the while being viewed with a critical eye. At this point, one could imagine many questions that yet need to be answered. Here's just one:

What if the suspect simply likes to eat imported foods?

REFERENCE

1. R. Santamaría-Fernandez et al., "Measurement of longitudinal sulfur isotopic variations by laser ablation MC-ICP-MS in single human hair strands," Analytical and Bioanalytical Chemistry 394 (1):225-233, May 2009.