Isotope Geochemistry

Isotopic analysis can be performed on minerals, rocks, soil, tissue, gases, and liquids – just about anything. In view of the broad possibilities, we will only cover a few generalities here. In most instances, the element to be analyzed must be first isolated and purified before it can undergo isotopic analysis in a mass spectrometer. Purification is necessary to avoid isobaric interferences and well as to enhance ionization efficiency, This can be done in a number of ways: • Dissolving the sample and then chemically purifying it (a process that often involves chromatography) • Reacting the sample with a reagent to produce a gas. For example, oxygen isotopic analysis of silicates involves reacting the sample with a fluorine compound such as bromine pentafluoride to produce CO2 gas; in the case of carbonates, the process is similar but the reagent is phosphoric acid. Purification is then achieved either cryogenically, or more commonly, using a gas chromatograph. For organic compounds, pyrolysis (thermal decomposition) may preceed such reactions. • Heating or fusing a sample in vacuum to release a gas. This is the usual method for noble gases. Individual gas components are separated cryogenically. • Noble gases contained in vesicles and inclusions can be released by crushing in vacuum. • If the sample is a liquid, the element of interest can be extracted chromatographically from the solution. With instruments in which the analyte is a gas or a liquid, a chromatograph can be used to select a specific compound, for example the alkenones discussed in Chapter 10. This technique is termed compound-specific isotopic analysis. Micro-analytical techniques in which solids can be directly analyzed have become increasingly important in the last several decades. The first of these is secondary ion mass spectrometry (SIMS), in which an ion beam (generally Cs or O) is fired at at a polished surface to produce secondary ions that are then swept into the mass spectrometer. An alternative approach is laser ablation in which a finely focused laser is fired at a polished surface ablating a small pit. The ‘debris’ consists of very fine particles that are swept up by a carrier gas, generally argon and carried into the mass spectrometer. A variation of this technique is laser fluorination, in which a laser is focused on a small spot of the sample in the presence of fluorine gas, producing a reaction that releases the element of interest (for example, oxygen or argon).