Background
Stable isotope analysis is based on measuring natural variation in the chemical elements present in biological samples.  Many common elements, such as hydrogen, oxygen and carbon occur in different forms, known as isotopes.  The presence or relative abundance of different isotopes allows isotope profiles to be generated for individual samples, which can then be compared to each other, or to reference data.

For any single element, the proportion of one isotope to another is known as the isotope ratio.  Isotope ratios vary in the environment due to a number of physical, geological and biological factors.  These factors may correspond to different geographic localities and therefore isotope ratios can be used to infer the geographical origin of a sample. 

In addition to measuring variation between isotope ratios, it is also possible to measure differences within an isotope.  For example, in the units of stable isotope measurement, the minor hydrogen isotope (2H) can assume about 700 different values while the minor carbon isotope (13C) can assume about 110 different values.  Analysing organic material such as animal hair and bird feathers for their precise isotopic composition with regard to hydrogen (H), carbon (C), nitrogen (N) oxygen (O) and sulphur (S) will theoretically produce a very specific profile.

Isotopes are measured using mass spectrometry techniques, allowing quantitative comparisons to be made between test samples and reference samples.  With individual samples, it is possible to differentiate between them on the basis of exclusion, if they do not have matching isotope profiles.  If sufficient isotope data is available from multiple samples, it is possible to identify which region or population a sample belongs to, through the use of multivariate statistical analysis.

Stable isotopes as forensic tools
Stable isotope analysis was first applied as a forensic technique for the analysis of foods, in order to identify illegal labelling and illegal trade.  Analytical methods traditionally applied in forensic science laboratories establish a degree of identity between one substance and another by identifying its constituent elements, functional groups, and by elucidating its chemical structure.  For example, when comparing two samples of sugar, all of the these measures would correspond and it would be concluded that they are chemically indistinguishable, as they are indeed both sugar.  However, although the two samples of sugar are chemically indistinguishable they may not be the same isotopically if they do not share the same origin or are derived from a different source.  The two main sources of sugar are sugar cane and sugar beet.  With the help of stable isotope fingerprinting it is perfectly straight forward to determine if a sugar sample is either cane sugar or beet sugar.  In addition, it is even possible to say where approximately in the world the sugar cane or sugar beet was grown and cultivated.  This type of technique has been widely applied in the food industry to examine wines, spirits, high-quality single seed vegetable oils, natural flavourings and honey in order to determine / verify authenticity or to detect fraudulent labeling and misrepresentation.

These principles have also been applied over recent years to behavioural and ecological studies examining the feeding and migration patterns of animals and birds and stable isotopes are now being used as a tool in wildlife forensics.  For example, isotope analysis of feathers may be used as means of determining the origin and movement of traded birds. The heavy isotope content of water varies widely and systematically across the globe, providing a marker that is incorporated through diet into the bird’s feathers.  As a result, these isotopes are potentially ideal tracers of geographic origin.  This isotope method has excellent potential where strong variation or difference of isotopes exist between two potential points of geographic origin, e.g. bred and reared in captivity versus illegally trapped in the wild.

The use of stable isotopes in wildlife forensics is in its infancy, but the technique offers the potential for producing powerful enforcement data, complimentary to that provided by genetic analysis.
 
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