A group of researchers from the LGC Chemical Metrology Laboratory in the United Kingdom and the University of Oviedo, Spain, have come up with a method to detect how the proportions of isotopes in a chemical element (atoms with an equal number of protons and electrons but different numbers of neutrons) vary throughout the length of a single hair. The mid-term objective is to be able to use these methods to track the geographical movements of people, including international crime suspects and victims.
In order to carry out this study, which is published this month in the journal Analytical and Bioanalytical Chemistry, the scientists focused on the most abundant sulphur isotopes in hair keratin – sulphur-32 (32S), which accounts for about 95%, and sulphur-34 (34S), which makes up around 4%. This proportion can change slightly in response to people's diets and if they travel from one country to another, and the technique is able to detect these small variations.
"The new method is based on combining a laser ablation system and multicollector inductively-coupled plasma mass spectrometry (abbreviated to LA-MC-ICP-MS)", Rebeca Santamaría-Fernández of LGC, lead author of the study, tells SINC. To summarise, the laser makes contact with the selected fraction of the hair, generating an aerosol, which later ionises within plasma, with the spectrometer providing the exact proportions of the sulphur isotopes.
"The advantage of this method compared with others is the high resolution resulting from use of the laser", points out Santamaría-Fernández. This advance has enabled the scientists to confirm that the sulphur variations in hair can be linked to peoples' geographical movements.
The traveller experiment
The researchers collected hair samples of more than 4cm in length donated by three volunteers. Two were permanent residents in the United Kingdom, while the third – dubbed "the traveller" – 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", Justo Giner, another of the study's authors, tells SINC. In addition, as hair grows an average of 1.25cm per month, the data obtained from a hair measuring between 4cm and 6cm 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 traveller's hair indeed showed significant variations in the sulphur 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, and they are already working with hair samples from 150 volunteers with different diets and geographical origins in order to move forward in this area. In addition, the researchers will also measure the isotopic variations of other elements apart from sulphur in their study, for example carbon and nitrogen.
The scientists are confident 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 centre (previously known as the Laboratory of the Government Chemist) is working with various national and international research groups, among them the University of Oviedo.
Reference literature: Santamaría-Fernandez R., Giner Martinez-Sierra J., Marchante Gayón JM, García Alonso J.I., Hearn R. "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, mayo de 2009.