Mass spectrometry comprises a range of different sample introduction and ionisation techniques, but all (TIMS, PIMS, N-TIMS, ICP-MS, etc.) effectively analyse the sample in the same way. Many applications of mass spectrometry are oriented towards measurement of isotopic ratios, but mass spectrometric methods also offer some of the highest precision concentration determinations for a wide range of elements. Therefore this section will review what a mass spectrometer is and how it works, before touching upon quantitative analysis of elemental abundances in addition to precise isotopic analysis.

Principles of mass spectrometry

Gas source mass spectrometers are generally used for lighter-mass stable isotope studies and Ar isotope geochronology, whereas solid and plasma source mass spectrometry are used for other isotopic and trace element analysis.

Regardless of sample preparation, introduction and ionisation (see below), the actual measurement procedure of all mass spectrometers revolves around the same basic method. In essence, this comprises ionisation of the sample, and then accelerating the ions to a known velocity in a high vacuum. The ions are then deflected by virtue of their mass through a strong magnetic field. Hence ions of a single atomic mass will form a beam which can be measured in terms of its intensity – the more ions present, the more intense the beam of a given atomic mass. The only variation on the magnetic mass discrimination just described involves the use of a quadrupole; however, ‘magnetic sector’ instruments still provide the greatest mass resolution and hence precision on isotopic ratios. Both methods are able to measure the amount of a given element present and also the relative amounts (ratios) of isotopes present.

Ion extraction

Regardless of how the sample is ionised (see below), ion extraction measurement systems are typically at very high vacuums (3×10−7 to 8×10−8 mbar). Acceleration of the cloud of ions is achieved by a series of plates (the source collimator stack). The first (extractor) plate is held at very high potential – usually 10 000 V (10 kV). The potential on the plates progressively decreases, until the last is at 0 V, resulting in a beam of ions which are all travelling at exactly the same velocity. Slightly altering the voltages on the subsequent plates allows the ion beam to be focused. The beam of ions then passes through into the flight tube (Figure 3.1), and on to the magnet for mass discrimination.