Isotope-ratio Mass Spectrometry - Gas Source ('stable Isotope') Mass Spectrometry

Gas Source ('stable Isotope') Mass Spectrometry

Instruments have been developed based on several techniques for mass separation and tuned to a wide range of applications. The article below describes one of these application areas, instruments adapted specifically to measure the relative abundance of masses up to around mass number 66.

This field is of interest because the relative variation in mass between isotopes in this range is large enough to give rise to variation in chemical, physical and biological behaviour. This leads to measurable effects on the isotopic composition of samples characteristic of their biological or physical history.

As a specific example, the hydrogen isotope deuterium (heavy hydrogen) is almost double the mass of the common hydrogen isotope. Water molecules containing the common hydrogen isotope (and the common oxygen isotope, mass 16) have a mass of 18. Water incorporating a deuterium atom has a mass of 19, over 5% heavier. The energy to vaporise the heavy water molecule is higher than that to vaporize the normal water so isotope fractionation occurs during the process of evaporation.

Thus a sample of sea water (Vienna Standard Mean Ocean Water, or VSMOW) will exhibit a quite detectable isotopic-ratio difference when compared to Antarctic snowfall (Standard Light Antarctic Precipitation, or SLAP).

It is critical that the sample be processed before entering the mass spectrometer so that only a single chemical species enters at a given time. Generally, samples are combusted or pyrolyzed and the desired species (usually hydrogen gas H₂, nitrogen (N₂), carbon dioxide, or sulfur dioxide) is purified by means of traps, filters, catalysts and/or chromatography.

The two most common types of IRMS instruments are continuous flow and dual inlet. In dual inlet IRMS, purified gas obtained from a sample is alternated rapidly with a standard gas (of known isotopic composition) by means of a system of valves, so that a number of comparison measurements are made of both gases. In continuous flow IRMS, sample preparation occurs immediately before introduction to the IRMS, and the purified gas produced from the sample is measured just once. The standard gas may be measured before and after the sample or after a series of sample measurements. While continuous-flow IRMS instruments can achieve higher sample throughput and are more convenient to use than dual inlet instruments, the yielded data is of approximately 10-fold lower precision.