SIRA is an acronym for “Stable Isotope Ratio Analysis” (now you understand the company name…). If you’re talking about carbon isotopes, it is sometimes referred to it as “SCIRA” (Stable Carbon Isotope Ratio Analysis). What we do is measure the ratio of a rare isotope (eg. carbon-13) to a common isotope (eg. carbon-12) in a sample. The ratio can vary by several parts per thousand (or permille) depending on its source and history as a result of reaction kinetics and bonding energies.
The carbon in the atmosphere (as carbon dioxide) has ratio of about -7 on our scale and is taken up by plants and used to produce sugars, starches, proteins and other compounds. Those compounds in the so-called C4 plants such as corn, sorghum and sugarcane have carbon ratios of about -10. Most of the flowering plants (the C3 plants) have a different carbon fixation pathway and usually have carbon ratios of about -25. We can measure the carbon isotope ratio to an accuracy of about 0.1 part per thousand. Therefore, we can tell if corn, cane or sorghum has been added to a C3 plant product with a high degree of certainty.
Other elements are also useful for isotope ratio study as they undergo changes in the atmosphere, biosphere and geosphere. Oxygen, hydrogen, nitrogen and sulfur are of great interest to scientists trying to understand the processes they observe.