Isotope analysis

Isotope analysis is the identification of isotopic signature, the distribution of certain stable isotopes and chemical elements within chemical compounds. This can be applied to a food web to make it possible to draw direct inferences regarding diet, trophic level, and subsistence.

Oxygen isotopes
Oxygen Isotopes and their Relative Abundances:
 * 16O = 99.763%
 * 17O = 0.0375%
 * 18O = 0.1995%

Present in the ratios above, oxygen atoms of all isotopes are incorporated in to molecules, including water. All isotopes of oxygen have similar properties, but water that incorporates 16O isotopic oxygen evaporates preferentially to water with the 18O isotope.

In isotopic analysis, the absolute abundances of isotopic oxygen are not considered. Rather, the ratio of 18O to 16O in the sample is compared to the ratio in a standard (VSMOW – Vienna Standard Mean Ocean Water) using the equation:

The values are reported in permil units (permil = per mille = per thousand) using the symbol ‰. While the differences between samples and the standard may appear small, a difference of even 1 permil is significant.

Variation by latitude
As moist air masses are carried away from the equator by the prevailing weather patterns they lose the heavier, more easily condensed, 18O water leading to lower and lower isotopic oxygen ratios toward the poles. Consequently, the amount of 16O relative to 18O in the water vapour becomes less and less as it approaches the poles, losing 16O water in the form of rain and snow.

Variation occurring from the hydrological cycle
The ratios of isotopic oxygen are also differentially affected by global weather patterns and regional topography as moisture is transported. Areas of lower humidity cause the preferential loss of 18O water in the form of vapour and precipitation. Furthermore, evaporated 16O water returns preferentially to the atmospheric system as it evaporates and 18O remains in liquid form or is incorporated into the body water of plants and animals.

Tissues affected
Isotopic oxygen is incorporated into the body primarily through ingestion at which point it is used in the formation of, for archaeological purposes, bones and teeth. The oxygen is incorporated into the hydroxylcarbonic apatite of bone and tooth enamel.

Bone is continually remodelled throughout the lifetime of an individual. Although the rate of turnover of isotopic oxygen in hydroxyapatite is not fully known, it is assumed to be similar to that of collagen; approximately 10 years. Consequently, should an individual remain in a region for 10 years or longer, the isotopic oxygen ratios in the bone hydroxyapatite would reflect the oxygen ratios present in that region.

Teeth are not subject to continual remodelling and so their isotopic oxygen ratios remain constant from the time of formation. The isotopic oxygen ratios, then, of teeth represent the ratios of the region in which the individual was born and raised. Where deciduous teeth are present, it is also possible to determine the age at which a child was weaned. Breast milk production draws upon the body water of the mother, which has higher levels of 18O due to the preferential loss of 16O through sweat, urine, and expired water vapour.

While teeth are more resistant to chemical and physical changes over time, both are subject to post-depositional diagenesis. As such, isotopic analysis makes use of the more resistant phosphate groups, rather than the less abundant hydroxyl group or the more likely diagenetic carbonate groups present.

Anthropology
Bone recovered from archaeological sites can be analysed isotopically for information regarding diet and migration. Carbon and nitrogen isotope composition are used to reconstruct diet, and oxygen isotopes are used to determine geographic origin.

To obtain an accurate picture of palaeodiets, it is important to understand processes of diagenesis that may affect the original isotopic signal.

Archaeology/anthropology
Bone recovered from archaeological sites can be analysed isotopically for information regarding diet and migration. Tooth enamel and soil surrounding or clinging to the remains may also be used in isotopic analysis. Carbon and nitrogen isotope composition are used to reconstruct diet, and oxygen isotopes are used to determine geographic origin. Strontium isotopes in teeth and bone can be used to determine migration and human movement.

The isotopes are imbued into the fauna during its lifetime through eating, drinking and particles inhaled. This process ends with the organism's death, from this point on isotopes no longer accumulate in the body, but do undergo degradation. For best result the researcher would need to know the original leaves, or an estimation there of, of isotopes in the organism at the time of its death. To obtain an accurate picture of palaeodiets, it is important to understand processes of diagenesis that may affect the original isotopic signal. It is also important for the researcher to know the variations of isotopes with in individuals, between individuals, and over time.

Ecology
All biologically active elements exist in a number of different isotopic forms, of which two or more are stable. For example most carbon is present as 12C, with approximately 1% being 13C. The ratio of the two isotopes may be altered by biological and geophysical processes, and these differences can be utalised in a number of ways by ecologists. The main elements used in isotope ecology are carbon, nitrogen, oxygen, hydrogen and sulfur.

Forensics
A recent development in forensic science is the isotopic analysis of hair strands. Hair has a recognisable growth rate of 9-11mm per month or 15cm per year. Hair growth is primarily a function of diet, especially drinking water intake. The stable isotopic ratios of drinking water are a function of location, and the geology that the water percolates through. 87Sr, 88Sr and Oxygen isotope variations are different all of the World. These differences in isotopic ratio are then biologically 'set' in our hair as it grows and therefore it has become possible to identify recent geographic histories by the analysis of hair strands. For example, it could be possible to identify whether a terrorist suspect had recently been to the Middle-East from hair analysis. This hair analysis is a non-invasive method which is becoming very popular in cases that DNA or other traditional means are bringing no answers.

Isotope analysis can be used by forensic investigators to determine whether two or more samples of explosives are of a common origin. Most high explosives contain carbon, hydrogen, nitrogen and oxygen atoms and thus comparing their relative abundances of isotopes can reveal the existence of a common origin. Researchers have also shown that analysis of the 12C/13C ratios can locate the country of origin for a given explosive.

Stable isotopic analysis has also been used in the identification of drug trafficking routes. Isotopic abundances are different in morphine grown from poppies in South-east Asia versus poppies grown in South-West Asia. The same is applied to cocaine that is derived from Bolivia and that from Columbia.