R is for Reference Materials
Reference Materials, usually referred to as standards, are materials (powders, liquids, or gasses) of known isotopic value. All stable isotope labs globally use the same reference materials, so that we can all compare our data.
In stable isotope geochemistry, especially for light isotopes like carbon, hydrogen, oxygen, nitrogen, sulfur, and chlorine, it is not possible for us to actually directly count the exact number of heavy atoms and light atoms of an element to determine a ratio. Until such technology exists, we rely on reference materials with defined values, and compare with that. For example, Standard Mean Ocean Water (SMOW) is assigned the value of 0‰ for both δ2H and δ18O. All other water samples are compared with this.
It’s a little like temperature scales. In Celsius, zero degrees is assigned to the temperature at which freshwater freezes. One hundred degrees Celsius is the temperature at which freshwater boils. Only recently we have determined “Absolute Zero” which is the temperature at which atomic motion stops. This we call zero degrees Kelvin.
For light isotopes the ‘absolute zero’ (the actual ratio of heavy atoms to light atoms) is not known, so standards of assigned values (like freezing water = zero degrees Celsius) are necessary.
For water, SMOW is the reference material assigned a value of zero for both d18O and d2H. For carbonates and bioapatites, where there is carbon and oxygen, there is a different reference standard called PDB, which stands for Pee Dee Belemnite. The Pee Dee Belemnite is – or was – a fossil of an animal similar to a squid, which was assigned values of zero for δ13C and δ18O.
Both PDB and SMOW are set as zero for δ18O, but as they are different materials, zero PDB and zero SMOW are actually different, much like zero degrees Fahrenheit and zero degrees Celsus are not the same temperature.