Stable isotopes are non-radioactive isotopic variants of elements that occur in the natural environment in highly consistent ratios. For example, nitrogen exists in two stable isotopes, the common 14nitrogen (99.63%) and the rare 15nitrogen (0.37%). Metabolic tracers, tagged with rare stable isotopes provide an ideal tool to quantitatively track metabolic processes because they are seamlessly utilized in biochemical reactions without cellular or organismal toxicity and their fate is quantifiable with high precision by mass spectrometry. Because they are innocuous, there is extensive precedent dating back nearly a century for application of stable isotope tracer methodology to human metabolic studies, including the most vulnerable populations, such as infants, pregnant women, and the critically ill.
A NanoSIMS ion microprobe directs a primary beam of ions (Cs+) through electrostatic lenses to sputter the surface of a sample, resulting in the emission of atoms and small atomic clusters from the sample, a fraction of which are ionized. The ionized fraction of so-called secondary ions – representative of the atomic composition of the sample surface – are collected by an immersion objective. A second set of ionic lenses shapes and transports the negatively charged secondary ions to the mass spectrometer, where individual detectors count up to 7 masses simultaneously. The isotope ratio is calculated by the simultaneous acquisition and measurement of ions of two different stable isotopic forms. Label incorporation in a domain of interest is therefore identifiable and measurable by the increase in isotope ratio above the highly consistent natural ratio.