What applications do stable isotopes have?
Stable isotopes surround the world we live and breath in. It is of no surprise then that the applications for stable isotopes are both broad and varied. From the geologist determining the age of a rock structure, to the nutritionist assessing our health, to the chemist measuring the purity of our foods, the scientific applications are far reaching. Rather than attempt to explain all the applications, we will examine three fields of science with three different applications. In these applications you will encounter two concentrations of stable isotopes, those at natural abundance and those at 'tracer' levels. Stable isotope tracers are designed and manufactured materials with elevated concentrations of heavy isotope, as an example nitrogen-15 of air, at natural abundance, is 0.366 atom % while a manufactured 'tracer' such as ammonium nitrate for agronomy research can be as high as 99% nitrogen-15. Tracers can be used to follow natural cycles, such as fertiliser use and photosynthesis.
Adulteration of Honey with High Fructose Corn Syrup- The addition of inexpensive sweeteners to honey to increase bulk and lower production costs is known to be practised by unscrupulous producers. As honey is a low yield and high input product, it demands a high market price. In many developed countries the market demand for honey cannot be met by domestic production, and is heavily dependent on import. Corn syrup is a relatively inexpensive sweetener, which can be added to honey to increase yields. Although overall taste may not be affected, addition of honey to corn syrup is prohibited by law in many countries. The use of carbon stable isotopes for the detection of the addition of high fructose corn syrup to honey has been routinely used since 1978.
In terrestrial plant tissue, the principle source of variation in carbon-13 is derived from the different photosynthetic pathways for carbon dioxide fixation. Plants incorporate carbon dioxide photosynthetically by three difference mechanisms: the Calvin cycle (C-3) pathway, the Hatch-Slack (C-4) pathway and the Crassulacean acid metabolism (CAM) pathway. The C-3 pathway results in a large change in the carbon isotope proportions relative to atmospheric carbon dioxide. The C-4 pathway produces a much smaller change. In the case of honey, bees that feed on single or multi-floral sources produce honey that is directly related to the carbon-13 content of the source. Most bees produce honey from plant nectar derived from the Calvin (C-3) photosynthetic cycle and produce a relatively uniform carbon-13 value near -25 per mil*, using the PDB scale. High fructose corn syrup is produced from plants that use the Hatch-Slack (C-4) photosynthetic cycle and is 'heavier' in carbon-13 with values ranging from -10 per mil to -16 per mil. Because of the difference in carbon-13 between adulterants and pure honey, measurement of carbon stable isotope ratios can indicate if honey has been adulterated.
*For a definition of the per mil scale, go to the glossary pages. Go there…