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Sheep Horns Reveal Diet

Photonics Spectra
May 2007
Laser ablation provides way to search for toxins in the dead tissues of living creatures.

Hank Hogan

Jörg Feldmann, a professor of analytical chemistry at the University of Aberdeen in the UK, could someday look at your fingernails in much the same way he has already examined the horns of seaweed-eating sheep. He and his colleagues at the university and at the Scottish Crop Research Institute in Dundee have demonstrated that trace elements can be monitored in sheep horns by using chemical analysis.

TWSheep.jpg

Researchers used laser ablation on the horns of North Ronaldsay sheep to test the levels of iodine and arsenic in the animals’ diet. Solid arrows indicate where the scientists scanned along the growth axis of each horn; the dotted lines indicate where they scanned along cross sections. Horn material varies at the cortex (A) and center (B) of each horn. Reused with permission of the American Chemical Society.

A key aspect of the group’s technique is laser ablation, which was used to release minute samples for testing. “The laser was extremely useful in the context that we could do high-resolution monitoring, since the horn was growing very fast, and so we could deduce exposure almost on a daily basis,” Feldmann said.

Keratin-rich tissues such as hair, fingernails and horns grow steadily and incorporate traces of at least some elements. However, once formed, they are metabolically inactive and thus can act as dietary archives. Extracting that information, though, requires analyzing sections of the sample, and the smaller the segment, the finer the time determination. In their work, the researchers wanted to do microscale monitoring, which is not possible with conventional milling techniques but possible using laser ablation.

For their proof of concept, the investigators selected North Ronaldsay sheep. Living on the Orkney Islands in Scotland, these animals eat large amounts of iodine- and arsenic-containing seaweed for most of their adult life. Shortly before giving birth, ewes are moved inland, where they feed on grass. Lambs spend their first five months living on grass and on their mother’s milk. After that, they move to the beach, where they eat the only food available — seaweed.

The researchers used a laser ablation system from Cetac Technologies of Omaha, Neb., that employs an Nd:YAG laser operating at 266 nm, and they focused the beam down to a 200-μm spot size. Once ablated, the material was fed into an inductively coupled plasma mass spectrometer for analysis. The investigators complemented this method with arsenic speciation and stable carbon and nitrogen isotope analysis.

After examining several sheep horns, the scientists found that arsenic could act as a marker but that iodine could not. The former came only from seaweed, and so the start of seaweed consumption could be seen. However, the latter was present in the ewe’s milk, which masked the effect of the onset of eating seaweed. “Milk eating would result also in the incorporation of iodine at the same level,” Feldmann said.

He would like to have a laser ablation system that can produce an even smaller crater size. That would enable a finer time resolution of trace element exposure in the living.

Feldmann also is involved in archaeological research centered on the lives of pre-Columbian Chileans, and dietary monitoring could be helpful in the project. “We would trace their movement due to the exposure to different amounts of trace elements, especially arsenic,” he said.

Environmental Science & Technology, ASAP article, March 13, 2007, doi: 10.1021/es062241y.


GLOSSARY
laser ablation
The removal of material from a surface by high intensity pulsed or CW laser radiation emission.
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