Isotope Ratio Mass Spectrometer

A mass spectrometer is an instrument which separates charged molecules by mass. Various types of methods can be employed to perform this.

An isotope ratio mass spectrometer (IRMS) works on this principle, but unlike other conventional mass spectrometers it has been specifically designed to measure the proportions of particular isotopes.

An IRMS will be much more precise, but much less sensitive than other mass spectrometers. The mass spectrometers used for isotopic analysis generally comprise three basic sections; an ion source, a mass analyser and an ion collection assembly.

In the simplified illustration (next column), gaseous molecules are introduced into the ionisation chamber where interaction with a focused electron beam causes electrons to be stripped from the molecules, resulting in the formation of positive ions.


The ions are then accelerated out of the chamber, down a flight tube which is placed between the poles of an electromagnet. Here, they are separated according to their mass-to-charge ratio (m/z).

The ions are typically collected by a simple collector array consisting of three Faraday cup collectors. In order to carry out IRMS analysis, only pure gases, e.g. N2, CO2, or pure gas contained within a carrier gas, can be analysed.

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