current as of 2-March-06
A sample being ablated by infrared laser radiation. The invisible radiation passes through a zinc selenide (ZnSe) window above the sample that is transparent to infrared wavelength light.
Fossil rodent molar with four ablation pits. Field of view is approximately 2.5 mm.
Smaller samples The traditional way to sample tooth enamel for isotopic analysis is to grind on a portion of a tooth with an abrasive drill bit, and collect the resulting powder. The analytical methods used to analyze the powder, including phosphoric acid digestion (carbon and oxygen isotopes) and phosphate reduction techniques (oxygen isotopes only), are relatively accurate and precise, but require a large amount of sample material. Laser ablation GC/IRMS (gas chromatography/isotope ratio mass spectrometry) is a method capable of producing carbon and oxygen isotope data with minimal damage to the tooth.
The method Sharp and Cerling (1996) and Cerling and Sharp (1996) described the method for this particular brand of laser ablation mass spectrometry. In short, a pulse of infrared laser radiation strikes the sample surface, creating a short-lived plasma from which condenses carbon dioxide. This gas is collected in a cryogenic trap, and then directed through a gas chromatograph and on into the mass spectrometer for isotopic analysis. The sample chamber and GC column are coupled using a 6-way valve. A PowerPoint cartoon animation of the 6-way valve extraction method is given here (376 KB).
Refinement and applications The method developed by Sharp and Cerling last decade showed promise for in-situ analysis, but was limited in a practical sense by leaving relatively wide and deep ablation pits. I have worked on refining the methodology so that animals with very thin tooth enamel (e.g. rodents), and precious teeth (e.g. hominids) can be analyzed without penetrating through the enamel or leaving excessive damage. This refined methodology (Passey and Cerling 2006) has allowed the study of seasonal variation in hominid diets (Sponheimer et al. 2006). It has also been applied to isotope fractionation and turnover studies in arid-adapted woodrats (Dave Podlesak et al., in prep), isotope paleoecology of Pliocene rodents in North America (David Fox et al., in prep), and dietary changes recorded in developing teeth of rabbits (Passey et al., in prep). We anticipate many new and exciting applications in small mammal and primate systems.