What Is an Isotope Ratio Mass Spectrometer?

An isotope ratio mass spectrometer (IRMS) is an instrument that measures the ratios of different isotopes of particular elements. All elements have isotopes that differ from one another only in the number of neutrons in the nucleus, giving them different atomic weights. The principle behind the isotope ratio mass spectrometer is to differentiate the isotopes on the basis of their different masses and determine the ratios between pairs of isotopes. This device can provide vital information about the age and origin of a sample of material. The isotope ratio mass spectrometer has applications in many areas, including geology, biology and forensic science.

The design of isotope ratio mass spectrometers can vary, but generally, they follow the same basic principles. There will be an inlet where the sample is introduced, leading to a combustion chamber where the material is converted to a gas, possibly with some means of separating different gases that may be produced. This stage also converts complex biological materials into the simple compounds required for analysis, such as carbon dioxide (CO₂), water (H₂O) and nitrogen (N₂). The resulting gas is fed into an ionization chamber where it is ionized by a beam of electrons. The ionized gas is then focused as a beam into a mass separation area, where an electromagnet is used to deflect the ions, such that different isotopes will be separated according to their masses.

After passing through the mass separation area, the ions reach collectors that generate electrical signals proportional to the number of ions detected. Ions of the lighter isotopes will have been deflected more by the magnetic field than the heavier ones, so the collectors will be positioned accordingly. Thus, the relative proportions of different isotopes can be calculated.

The samples must be prepared prior to being introduced into the isotope ratio mass spectrometer. In the case of biological substances, for example, the samples may be in the form of leaves, soil or other non-homogenous material. Solid material will generally be dried and ground into a fine powder. Liquid samples will either be dried or absorbed onto porous solid material. Before carrying out an isotope ratio analysis, calibration using materials of known element and isotope ratios will usually be performed.

The overall ratios of stable isotopes of any given element on the Earth were fixed at the time of the planet’s formation. Although different isotopes of an element have the same chemical properties, other factors such as mobility and volatility are influenced by the isotopes’ masses. Due to these differences, various geochemical and biochemical processes can concentrate or deplete particular isotopes relative to their background values, a phenomenon known as isotopic fractionation. For example, photosynthesis results in a small but significant depletion of the isotope carbon-13 relative to the atmosphere.

Differences in the ratios of isotopes of elements such as carbon, oxygen, nitrogen and others can provide important information about the origin and history of a sample. It is possible using an isotope ratio mass spectrometer to determine whether a material is of organic origin and even, in some cases, to pinpoint the geographical area where it originated. This can be of use in forensic science. For example, samples of illegal drugs can be traced back to their origins and soil samples taken from a suspect can be compared isotopically with those from a crime scene.

As temperature and precipitation can influence isotopic fractionation, isotope ratio mass spectrometry can be used to investigate the earth’s climate in past times. The rates of uptake and deposition of carbon and oxygen isotopes by shell-forming marine organisms vary according to the climate. Isotope ratios of fossilized remains of these organisms can thus be used to obtain information about the climatic conditions when they were alive.

In geology, radiometric dating is an important application for the isotope ratio mass spectrometer. The isotope ratios of certain metallic elements can be used to determine the age of a rock sample. When rock is formed, it will contain some radioactive isotopes. These decay into other isotopes, either of the same element or, more commonly, a different element, at a known rate. The ratio of the original — or “parent” — isotope to the decay product — or “daughter” — isotope can thus be used to determine the age of the rock.