The Neptune is a sophisticated ‘double-focussing’ multi-collector ICP-MS. As with the other ICP’s, the ions are generated in a high temperature plasma formed from Ar gas- samples can be introduced into the plasma both by solution aspiration (tiny liquid droplets) or by laser ablation (tiny particles). The ions are then accelerated through an electrostatic analyser (essentially an energy filter) and a magnet, where the ions are filtered according to their mass/charge ratio, before reaching the detector array.
The AAC currently operates a Finnegan Neptune, high-resolution MC-ICP-MS. It is equipped with 2 different types of detectors. It has 9 moveable Faraday cups, which can simultaneously collect 9 different masses over a static mass range of 12% (e.g. 7Li to 6Li). This measurable mass range is extended to 17% (e.g. 202Hg to 238U) by an array for 4 ion counters that are attached to one of the Faraday cups, allowing the detection of 13 masses. The ion counters are useful for the measurement of very low signal intensities.
The instrument used with either solution-based samples or analysis of solids via laser ablation.
The main function of the Neptune involves the precise measurement of isotope ratios, which can be ascertained to the seventh decimal place. It is capable of measuring the isotope composition of all species in the periodic table, excepting C, N and O. The isotope information has two uses, these being in geochronology and in isotope tracing. For example, the ratio between 206Pb and 238U in the minerals zircon and monazite is a direct measure of the crystallisation age of igneous rocks. Isotope tracing involves using particular isotope ratios as a ‘fingerprint’ of certain Earth system processes. For example, the 87Sr/86Sr ratio of seawater can trace inputs of suspended particulates from rivers draining continental areas, whereas Pb isotope ratios have been used to monitor industrial contaminants in environmental systems, such as Pb from petrol consumption.
Laser ablation material is typically mounted on glass slides or in epoxy resin mounts. These are then cut and polished prior to ablation.
Where solution-based analysis is required great care must be taken to prevent sample contamination and such work typically requires access to clean lab environments.
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