PSR Discoveries - February 1998

Accelerator Mass Spectrometry
A. J. T. Jull and others

The rock being analyzed, in our case pieces of martian meteorites ALH84001 or EET79001, is crushed to a powder and placed in a small, glass tube. After the air is pumped out of the tube oxygen is put in. The organic material in the rock powder is burned in the oxygen at different temperatures. This combustion produces carbon dioxide (CO2) which is let into a mass spectrometer. This instrument measures the precise quantities of the carbon isotopes to determine the ratio of 13C to 12C. The gas is then recovered from the mass spectrometer using liquid nitrogen. The liquid nitrogen freezes out the gas into a glass ampoule, which is then transferred to another system. Here, it is reduced to graphite and the 14C content of the sample is determined by accelerator mass spectrometry (AMS.)

Note: The ratio 13C/12C is usually expressed in terms of delta13C, which is the deviation of 13C/12C compared to a known standard. The parameter delta13C is almost always expressed in parts per thousand, or per mil, abbreviated ‰.