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How do mass spectrometers measure isotopes in rocks and minerals?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

In geochronology, mass spectrometers measure isotopes by identifying and quantifying atoms based on their mass. The process generally involves creating a beam of charged atoms (ions) from a geologic sample, which is then passed through a strong magnetic field within a vacuum.1

This field separates the beam into multiple paths, each containing atoms of a specific mass. Because these atoms carry a unit electric charge, their quantity can be measured by collecting the beams in a Faraday cup, where the resulting electrical current is recorded.1

To ensure accuracy, geologists often use isotope dilution, which involves adding a known amount of an artificially enriched isotope, or "spike," to the sample. By measuring the ratio of natural to enriched isotopes, researchers can determine the precise number of atoms present, even if some material is lost during preparation.12

Modern advancements have expanded these capabilities:
* Ion Microprobes (e.g., SHRIMP): Allow for the analysis of mineral grains in place by using a focused ion beam to blast atoms from a tiny surface spot, avoiding the need for chemical dissolution.2
* Laser Ablation (LA-ICP-MS): Uses a laser to ablate material, which is then analyzed by an inductively coupled plasma mass spectrometer, enabling rapid dating of numerous individual grains.2