The same applies to the analysis of isotopic ratios of carbon, nitrogen, oxygen and hydrogen (δ13C, δ15N, δ18O and δ2H). Their analysis will provide information on the diets of ancient populations and the geographical or botanical origins of agri-food products.
Stable isotope analysis with IRMS
The technique dedicated to the study of stable isotopes is IRMS, isotope ratio mass spectrometry. We measure the ratios of stable isotopes such as 13C/12C, 15N/14N, 2H/1H and 18O/16O. Measurements are compared with international references, considered as "zero points": for carbon, this is the Vienna Standardized Peedee Belemnite (VPDB), for nitrogen it's air, and for hydrogen and oxygen it's the Vienna Standardized Mean Ocean Water (VSMOW). CIRAM laboratories use a vario ISOTOPE select elemental analyzer (EA) and an IRMS Isoprime precisION from ELEMENTAR.
Distortions in plant isotope ratios
It is the different types of photosynthesis that are responsible for distortions in plant isotope ratios. For example, woody trees, barley, rice and beans (C3 plants) have a d13C carbon isotope ratio of less than -20 ‰, while maize, sorghum and sugar cane (C4 plants) have a d13C of between -10 and -20 ‰. It's the presence of nitrogen in proteins that will provide information on their origin: terrestrial or marine. By coupling the study of carbon and nitrogen isotopes, we can define the diet of an individual or animal: carnivorous, herbivorous or omnivorous; and the origin of its diet: terrestrial or marine.
Industrial applications of stable isotope analysis
In industry, stable isotope analysis is used for product authentication in general, and in particular to verify provenance, identify adulteration or define the original raw material for biofuel production, for example.
Stable carbon isotopes will be used to identify honey adulteration with corn-derived fructose syrup. Corn, a C4 plant, has a d13C of between -20 and -10‰, when authentic honey has a δ13C of less than -23.5 ‰. A d13C measurement greater than -21‰ will correspond to adulteration. Carbon isotope ratios will furthermore identify a "true" cider vinegar with a δ13C around -25‰, when a cider vinegar adulterated with biosynthetic acetic acid will have a δ13C around -10‰.
Adulteration of fruit juice or wine can be detected by studying stable oxygen isotopes. The δ18O value of the adulterated product will be negative, while that of the pure products will be positive.
Stable hydrogen isotopes will be used to differentiate between two C3 plants: orange and beet, for example. An orange juice adulterated with beet sugar will have a δ2H around -50‰, while authentic orange juice will have a δ2H between -40 and -13‰.
Stable isotope analysis is a very important source of information on ancient dietary habits, botanical origins and the authentication of raw materials. But these are qualitative techniques only. In most cases, it will be essential to couple these studies with carbon-14 dating or radiocarbon analysis, in order to obtain chronological information or quantify adulteration.
CIRAM, an isotope analysis and carbon-14 dating laboratory since 2005, can help you solve your archaeological and industrial problems.
