Mass Spectrometry in Mineralogy
A mass spectrometer separates atoms and molecules by their mass, allowing the relative abundance of different elements - and different isotopes of the same element - to be measured with extraordinary precision. In mineralogy, the technique is most widely applied to isotope ratio measurements, which underpin radiometric dating and a broad range of stable isotope geochemical studies.
Instrument Geometry and Operating Principle
The operating principle of a mass spectrometer begins with converting the sample into a gas of charged ions. The sample is vaporized and introduced into an evacuated ionization chamber. There, electrons streaming from a hot filament to an anode collide with the gaseous atoms and knock electrons out of them, leaving positively charged ions. A voltage maintained between two plates (A and B) accelerates these ions through a narrow slit and into a curved analyzer tube surrounded by a strong magnet. [1]
Inside the analyzer tube, the magnetic field deflects the paths of the ions. The amount of deflection depends on the mass of each ion: light ions are deflected more than heavy ions, so ions of different atomic mass follow different curved paths and strike the detector at different positions. By precisely adjusting the strength of the magnetic field and the accelerating voltage, the instrument can be tuned so that ions of a specific atomic mass arrive at the detector. The detector senses the arriving ions as a tiny electric current, and the strength of that current is proportional to the abundance of the element or isotope being measured. [1]
Applications in Mineralogy
Because mass spectrometers measure mass directly, they are uniquely suited to resolving different isotopes of the same element - two atoms of iron that differ by one or two neutrons, for example, have identical chemical properties but measurably different masses. This makes mass spectrometry the dominant tool for measuring isotope ratios in minerals, and it is therefore central to radiometric dating of geological materials and to stable isotope studies of temperature, fluid source, and geochemical cycling. [1]
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References & Citations
- 1.Introduction to Mineralogy Nesse, W. D.
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