Ionisation mass spectrometry (IMS)
About this technique
Ionisation mass spectrometry (IMS) measures isotopic ratios for a great range of applications in geochemistry, geochronology, cosmochemistry, archaeology, etc. It can make very precise measurements of isotope ratios of elements although different instruments take different approaches to ionisation and have their own advantages with respect to different types of elements and processing requirements for the experiment.
For those elements that can be ionised thermally, thermal ionisation mass spectrometry (TIMS) can be used. Samples, typically in the form of fine rock powders, rock chips or mineral separates are dissolved in ultra-pure acids and then separated using standard ion exchange columns for separation and purification of the elements of interest, before they can be measured on the mass spectrometer. The types of acids used for dissolution and the types of resins used for separation and purification depend on the types of materials and the elements of interest. All the chemical processes and TIMS measurements are carried out in HEPA-filtered ultra-clean laboratory environment. This ensures negligible contamination from air or reagents. The isolated elements are loaded onto a filament where they are vapourised by passing a current through the thin metal filament under vacuum. Levels of elements to be loaded on the filaments for TIMS measurements are very low, from nanogram levels (10-9 gram) for Sr, Nd, and Pb isotopes to picogram or femtogram (10-12–10-15 grams) for U and Th isotopes.
The primary application of TIMS is to measure the isotope ratios in order to either determine the age of the material such as rocks and minerals, or to trace the origin of, or evaluate the interaction between, geochemical systems and/or reservoirs (isotope geochemistry). For terrestrial systems, common applications in geochronology and tracer studies involve the following radiometric systems:
- U-Th-Pb
- Rb-Sr
- Sm-Nd
- Lu-Hf
- Re-Os
- U-series disequilibrium
When elements cannot be thermally ionised, inductively coupled plasma mass spectrometry (ICP-MS) can be used. It is highly sensitive and capable of the determination of a range of metals and several non-metals with a detection limit below one part trillion (10-12 g/g). It ionises and vapourises the elements using an inductively coupled plasma (ICP) at very high temperatures (6,000-8,000 K). Samples to be analysed are introduced into the ICP as a mist of liquid formed by passing the liquid sample into a nebuliser. As a droplet of nebulised sample enters the ICP, it evaporates and any solids that were dissolved in the liquid vaporise and then break down into atoms. At the temperatures prevailing in the plasma a significant proportion of the atoms of many chemical elements are ionised, each atom losing its most loosely bound electron to form a singly charged ion. The ions are separated on the basis of their mass-to-charge ratio and a detector receives an ion signal proportional to the concentration. The concentration of various elements in a sample can be determined through calibration with certified reference material such as single or multi-element reference standards. ICP-MS also lends itself to quantitative determinations through isotope dilution, a single point method based on an isotopically enriched standard.
The technique is capable of high-precision measurements of over 40 elements in the periodic table to precisions typically less than 5%, with some around 1-2% or less. An example of an application is the high-precision and high-throughput Sr/Ca and Mg/Ca analyses of coral skeletons (to less than 0.2% RSD) for past sea-surface temperature reconstruction at monthly to fortnightly resolution.
The application of quadrupole ICP-MS instruments to isotope ratio measurements is limited by the comparatively poor precision of the analytical data. On the other hand, elements with high ionisation potentials are difficult to analyse by TIMS at high precision and the application of TIMS necessitates careful attention to sample preparation and is associated with low sample throughput. Multi-collector (MC)-ICP-MS instruments are designed specifically to overcome the limitations of both the Q-ICP-MS and TIMS. To achieve this goal, they combine the Ar ICP-source of conventional ICP-MS instruments with the magnetic sector analyser and multiple-Faraday collector array of the TIMS. At the high temperatures that are attained in the Ar plasma (~ 6000-8000 K), elements with first ionisation potentials of < 10 eV are ionised to an extent of 75% or more. Thus, with a plasma source, virtually all elements of the periodic table are accessible to isotopic analysis. This stands in contrast to TIMS where thermal ionisation is readily achieved for only a limited number of elements with low ionisation potentials.
The samples for analysis are similar to those used for TIMS but rather than being loaded onto a filament ribbon for measurement, the final purified elements of interest will be dissolved in 2% distilled nitric acid for injection into the plasma through a nebuliser. Compared with TIMS, the MC-ICP-MS is far more efficient in terms of sample throughput and the sensitivity is much higher for many elements so sample sizes can be reduced dramatically or higher precision can be achieved. For instance, for U-series dating, 20-30 samples can be dated over the period of 24 hours, 5-10 times faster than TIMS. For Pb isotope analyses, the precisions can be increased by an order of magnitude.