The analysis of stable carbon and nitrogen isotopes can be used for research into past environments, as proxies for climate and habitat, as well as for modern environmental studies. They can also be used with modern material with diverse applications such as identifying anthropogenic impacts on freshwater ecosystems, see E Keaveney.
The nitrogen stable isotope (δ15N) analysis also identifies the trophic level of an individual. The consumer collagen usually displays δ15N ratios, which are about 3-5‰ greater than its diet.
This fractionation effect results in a ca. 3-5% increase in δ15N as an individual ascends each step of a food-chain. For example, the δ15N values of most modern plants are between 0 and 5‰. Therefore, the nitrogen isotopic values of populations that rely on terrestrial animal protein in their diet, would average from around 9‰. The nitrogen levels of a consumer will increase when relying more heavily on aquatic resources. The mixture of both terrestrial and marine (fish and mammals) food resources in the diet would typically reveal δ15N ratios of around 10‰; the highest δ15N ratios are usually expected from populations using aquatic sources as a main staple in the diet, because the food-chains in aquatic ecosystems are more extended than those of terrestrial ones. It should be noted that a number of non-dietary factors may influence the carbon and nitrogen isotopic ratios of individuals.
These include climatic and canopy effects for δ13C ratios and climatic, physiological, crop manuring and nursing effects for δ15N ratios.
The gases are purified so that only elemental CO2 and N2 continue to a gas chromatographic (GC) separation column. magnetic/electrostatic deflection system separates the charged ions.
A ‘repeller’ plate inside the machine carries a low positive potential and accelerates the positive sample gas ions via a series of focusing plates and slits to the next part of the system. The ions will follow different paths according to their mass-to-charge ratio (m/z) and therefore gas species of different masses can be separated before being measured on the collector.
The ion collector measures the current flowing in a selected beam. This detector is a sensitive charge collector (e.g. Faraday cup). The ions strike the detector. Electrons ‘jump’ from the metal to neutralise the positive charge leading to an electron shuffle and hence a current which can be amplified and recorded. This is passed to earth via a large resistor where the voltage developed is measured.
This voltage is directly related to the number of ions being received at the detector. Masses 28, 29 and 30 are recorded for N2; and masses 44, 45 and 46 for CO2.