Isotope-ratio Mass Spectrometry - Introduction

Introduction

The isotope-ratio mass spectrometer (IRMS) allows the precise measurement of mixtures of stable isotopes. This technique has two different applications in the earth and environmental sciences. The analysis of 'stable isotopes' is normally concerned with measuring isotopic variations arising from mass-dependent isotopic fractionation in natural systems. On the other hand, radiogenic isotope analysis involves measuring the abundances of decay-products of natural radioactivity, and is used in most long-lived radiometric dating methods.

Most instruments used for precise determination of isotope ratios are of the magnetic sector type. This type of analyzer is superior to the quadrupole type in this field of research for two reasons. First, it can be set up for multiple-collector analysis, and second, it gives high-quality 'peak shapes'. Both of these considerations are important for isotope-ratio analysis at very high precision and accuracy.

The sector-type instrument designed by Alfred Nier was such an advance in mass spectrometer design that this type of instrument is often called the 'Nier type'. In the most general terms the instrument operates by ionizing the sample of interest, accelerating it over a potential in the kilo-volt range, and separating the resulting stream of ions according to their mass-to-charge ratio (m/z). Beams with lighter ions bend at a smaller radius than beams with heavier ions. The current of each ion beam is then measured using a 'Faraday' detector or multiplier detector.

Many radiogenic isotope measurements are made by ionization of a solid source, whereas stable isotope measurements of light elements (e.g. H, C, O) are usually made in an instrument with a gas source. In the latter case, dual gas inlets enable reliable repetition of measurements by supplying continuous streams of the 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. (The CO₂ standard is produced from a fossil belemnite found in the Pee Dee formation, which is a limestone formed in the Cretaceous period in South Carolina, U.S.A.; its 13C:12C ratio is 0.0112372.) Oxygen isotope ratios are measured relative the standard, V-SMOW (Vienna Standard Mean Ocean Water).