Wetenschap - 21 juni 2012

The uses of nano-evidence

From hairs and nails to palm oil, all organic matter betrays its own origins through the atomic composition of the elements it contains. This is a very useful aid in rooting out fraud in the food industry. And identifying the culprits.

Towards the end of 2006, a severely wounded Vietnamese man was dropped off at a Welsh hospital. He soon died of his wounds and the police faced a puzzle: who was this anonymous victim? All attempts to trace him came to nothing, at which point the police force knocked at the door of the Forensic Laboratory for Stable Isotopes in Dundee. Using a 15 cm-long hair of the man's, these specialists traced his recent movements: three months in England preceded by stays in Germany and Eastern Europe. A well-known people-smuggling route: the detectives knew enough to set to work.
This achievement is based on one simple fact: you are not just what you eat but where you eat it. This is due to the natural variation in nature's building blocks, the elements. There are 'heavy' and 'light' versions of many elements, depending on the number of neutrons in the nucleus of the atom. Once you know how these variants, or isotopes, are distributed around the globe, you can create a map that enables you to determine the origin of specific isotopes.
Six years ago, scientist Jason West dubbed these maps Isoscapes, short for isotopic landscapes.
These isoscapes are hot news at the moment, and not just in the laboratory. Police forces, food authorities and ecologists are all enthusiastic about them too. They could be used by a food watchdog, for example, to establish whether a product really lives up to the claims made on the packaging. Does that orange juice really come from Greece? Does that pricy olive oil really come from that photogenic Tuscan village? Is there really no added sugar in that honey? In order to combat lucrative forms of fraud, the EU launched the TRACE project, with the help of Wageningen researchers. The project traced more than 600 European mineral waters using isoscapes.

Fingerprint
To put together an isoscape, scientists make use of the vast variety of isotopes in existence. 'It is not easy to make that kind of isoscape', says Grishja van de Veer, a researcher at Rikilt who works on TRACE among other projects. 'You first look for a naturally occurring variation associated with a combination of different isotopes in particular proportions.' The ratios of heavy and light hydrogen and oxygen are examples. When the average temperature drops somewhere, there are relatively fewer heavy isotopes. This is because of the water cycle: water evaporates most at the equator, and is transported towards the poles. During every rain shower along the way, water made up of heavier isotopes is more likely to fall to the earth. The proportion of heavier isotopes gets steadily smaller.
It is possible to make a rough isoscape of Europe using the proportions of oxygen and hydrogen isotopes. But the map is still too rough to be used to track a specific mineral water to its source. To do that you need other isotope ratios. When you lay one map over the other, large areas with the same ratios are suddenly subdivided into much smaller, unique patches. And that makes it a lot easier to find one picturesque Tuscan village.
Other isoscapes can be made using geological knowledge, for example. The isotope ratios of heavy elements such as strontium and lead vary greatly between types of rock and depending on their age. There are a few advantages to these heavy elements, explains Gareth Davies, professor of Petrology at Amsterdam VU university. They come in more varieties and you can determine their origin with greater precision. What is more, lead has not one but three ratios, as it is not a stable isotope and decays to form other substances.
Besides geographical data, researchers also make use of the fact that organisms prefer the light variant of an element, explains Caroline Plugge, assistant professor of Microbiology. 'In the natural world, one percent of all the carbon is the heavy carbon-13, and the rest is carbon-12. So if a bacterium absorbs carbon, the ratio shifts away from the original one. This shift varies per process. This sort of fingerprint gives away the bacteria's activity.'

Snapshots
There are various reasons for the rapid rise of isoscapes, says Gareth Davies. To start with, analytical techniques have improved vastly. They are now faster, can be put to a wide range of uses, require less material and are more user-friendly. Add to that the fact that technological development continually adds to geographical databases about isotope ratios. Without these databases, there would be no isoscapes and no answers to research questions. 'Maybe you want to find out whether a bird migrates to Utah', says Davies. 'But then you need data about Utah, of course.'
As in the UK, Davies uses his expertise to help the Amsterdam police's cold case team trace the origins of unidentified corpses. To do this he looks at hairs and nails for signs of their recent movements. He also looks at teeth, which can tell him something about where the person grew up, and bones, which can reveal where they lived as adults. 'At the moment they are just snapshots of a life', says Davies. 'We want to make this really quantitative. That is why we are researching how quickly signals appear in the body, something I am using my own body for. This year I have been to India and to Botswana.'

Silent witnesses
Research on isotope ratios in Wageningen is geared less towards the scope for use by the forces of law and order, and more towards supporting the food authorities. They, too, are very interested in the possibility of being able to determine the precise origins of organic material. Van der Veer developed a method of tracing palm oil to its origins in Brazil, West Africa or South-East Asia using one isotope ratio. This is useful information, as no sustainable palm oil is produced in Africa. But Rikilt would prefer to be able to locate the oil much more precisely.  According to Grisja van der Veer, you need more isotope ratios in order to do that. Wageningen is working on it with the business world. The increasing amounts of data available, improved software and statistics will make isoscapes more and more detailed and of increasing use for answering questions.
Meanwhile, isoscapes were a great help to the police in Wales. The clues about people-smuggling led them to a Vietnamese gang, and three men were eventually arrested. They had attacked the victim when it turned out he could not pay. Little did they know that there were silent witnesses on his head that would betray them.

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