The atoms hydrogen, oxygen, and carbon co-exist naturally in specific proportions with their stable isotopes, 2H (or D), 18O and 13C respectively, in different proportions as shown in the figure 2 below.
The amount and distribution of the different isotopes in a molecule is influenced by:  
* Environmental (climatic and geographical) conditions - for natural products 
* Chemical or biochemical processes such as the photosynthetic metabolism in plants
This phenomenon is known as natural isotopic fractionation (see Figure 3). The resulting isotopic fingerprint can provide information on the origin - botanical, synthetic, geographical - of the molecule or product.