Isotope ratio mass spectrometry

Isotope ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.

Introduction
The isotope ratio mass spectrometer (IRMS) allows the precise measurement of mixtures of stable isotopes. It is much more precise than a conventional spectrometer because measurements are repeated many times. The dual inlets of the instrument enable reliable repetition of measurements by supplying a continuous streams of reference and sample gases which are sequentially switched by a changeover valve. The IRMS's collector also features an array of Faraday cups (conductive, metal vessels which neutralise ions that hit them whilst themselves becoming charged), or "multicollector", which allows the simultaneous detection of multiple isotopes. Samples must be introduced as pure gases, achieved through combustion, gas chromatographic feeds or chemical trapping. By comparing the detected isotopic ratios to a measured standard, an accurate determination of the isotopic make up of the sample is obtained. For example, carbon isotope ratios are measured relative to the international standard for CO2 which is produced from a fossil belemnite found in the PeeDee formation, a limestone formed in the Cretaceous period in South Carolina, U.S.A. with a 13C:12C ratio of 0.0112372. Because the difference between the reference and samples is often very small, the carbon isotope ratios are expressed as parts per thousand relative to the standard.

Operation
A Nier mass spectrometer is an instrument for analysing the isotope ratios. It was designed by Alfred Nier. In the most general terms the instrument operates by ionizing the sample of interest and subjecting the resultant ions to a physical process that separates them according to their mass to charge ratio (m/z).

Instruments have been developed based on several techniques for mass separation and tuned to a wide range of applications. This article 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 Arctic snowfall (standard light Arctic 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.

Moving Wire IRMS (MW-irMS)
This technique permits the analysis of small samples. Samples consisting of as little as 1&mu;L are dried onto a nickel wire, which is passed through a furnace; the analysis occurs as the material combusts. The technique is typically used for Carbon-13 analysis, where samples containing as little as 1 nano-mole of Carbon can yield precise (within 1&permil;) results.

Applications
The variation outlined above has applications in hydrology since most samples will lie between these two extremes. Given a sample of water from an aquifer, and a sufficiently sensitive tool to measure the variation in the isotopic ratio of hydrogen in the sample, it is possible to infer the source, be it ocean water seeping into the aquifer or precipitation seeping into the aquifer, and even to estimate the proportions from each source.

Another application is in paleotemperature measurement for Paleoclimatology. For example one technique is based on the variation in isotopic fractionation of oxygen by biological systems with temperature.

Species of foraminifera incorporate oxygen as calcium carbonate in their shells. The ratio of the oxygen isotopes oxygen 16 and oxygen 18 incorporated into the calcium carbonate varies with temperature and the oxygen isotopic composition of the water. This oxygen remains "fixed" in the calcium carbonate when the forminifera dies, falls to the sea bed, and it's shell becomes part of the sediment. It is possible to select standard species of forminifera from sections through the sediment column, and by mapping the variation in oxygen isotopic ratio, deduce the temperature that the forminifera encountered during life if changes in the oxygen isotopic composition of the water can be constrained.

In forensic science, research suggests that the variation in certain isotope ratios in drugs derived from plant sources (cannabis, cocaine) can be used to determine the drug's continent of origin.