57 - Stable Isotopes

Abstract

The use of stable isotopes as tracers dates to the early 1930s and the groundbreaking work of Schonheimer. These early studies utilized a mass spectrometer to quantitate abundance of stable isotope tracers in biological samples. The same basic analytical approach is used today, with few conceptual differences. Although the isotope ratio of any number of gases can be measured, most instruments designed for use in metabolic studies are set up to conveniently analyze H2, N2, or CO2 gas. The inlet system to the mass spectrometer is usually a vacuum-line/gas-handling system with holding cells for both a reference gas and the sample gas. The pressures of the reference and sample gases can be equilibrated to eliminate pressure variation as a potential source of error. Capillary “leak” lines are attached to the gas-holding cells and lead to a pair of switching valves that admit the sample and the reference alternately to the ion source. The gas molecules stream from the holding chamber via the molecular “leaks” into the mass spectrometer, which is under high vacuum. This “dual-inlet” system allows for the almost simultaneous analysis of a sample and a reference standard.