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Telltale hairs
Feb 28th 2008
From The Economist print edition

You can tell where someone has been from his hair
从一个人的头发能知道他去过哪儿

POLICE now have a newtest to help catch criminals and verify alibis. By analysing the chemical compositionof human hair, researchers can determine the source of the water someone has been drinking in recent months. And that can indicate where he has been.

警方现在掌握了一种抓捕罪犯和核实不在场证明的最新技术。通过分析人发中的化学成分，研究员能测定出此人近几个月的饮用水源，进而判断出他的踪迹。

The technique depends upon studying isotopes. These are naturally occurring variants of elements,which share the same chemical properties but have different weights because their nuclei contain different numbers of neutrons. James Ehleringer and Thure Cerling, at the University of Utah in Salt Lake City, and their colleagues collected human hair from barbers' shops in 65 American towns. They report inthe Proceedings of the National Academy of Sciences that isotopes in hair closely match the hydrogen- and oxygen-isotope ratios found in the local water.

这项技术基于对同位素的研究。同位素是存在于元素的变体，天然存在，同位素与元素具有相同的化学性质，但由于所含的中子数不同而重量不同。盐湖城犹他大学的James Ehleringer、Thure Cerling及其同僚从美国６５个城镇的理发店中收集毛发。他们在"美国国家科学院院刊"上发表报告，说头发中的氢和氧的同位素比例与在当地水源中的相当匹配。

This research has obvious implications for law enforcement and is being used by Salt Lake county sheriffs to help identify a murdered woman. Isotope analysis of her hair revealed that she spent the years before she died in America's western mountains, which helps detectives concentrate their search. The new method may also help anthropologists understand more about the history and migration of ancient humans at sites where bits of hair are preserved.

这项研究与法律实施显然有所关联，并且被盐湖城的县行政司法长官用来识别一名被害女性。分析其头发中的同位素，显示了她在被害前曾在美国西部的山区生活过几年，这有助于警探集中调查。这项新技术或许也能助益人类学家，通过分析遗址中毛发中留存着的信息，进一步了解古人类的历史和迁徙。

The technique works because when water evaporates at sea, a combination of hydrogen and oxygen isotopes rise to form a cloud, which then releases rain when it moves inland.Heavier isotopes tend to fall sooner. So those living close to the sea, or in corridors suffering frequent storms, will tend to absorb more of the heavy isotopes in their hair.

这项技术很有效，因为当水从海里蒸发，氢和氧的同位素组合升空形成云，接着飘至陆地上空降雨。越重的同位素沉降的越快。所以那些住在海边、或者在暴风雨频发的狭长地带的人的头发会含有更多的重同位素。

For that reason,however, it is not perfect. In America, storms can come from the Pacific or the Atlantic. This creates regions with geographically similar isotope values. So,if a murder took place in Los Angeles and a suspect claimed to have been in San Diego all the time, hair analysis could not help, because the cities havesimilar isotope signatures. But if the suspect were in Las Vegas, which has adifferent signature, he could find himself nicked.

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