The Technical Details: Determining Delta Values When we talk
about the isotopic ratio in a sample, we talk about the delta
value. Let's look at how a delta value is actually calculated: -
The first step in figuring out the δ13C for a sample is to find
the ratio of 13C to 12C within the sample. Next compare (by
dividing) this ratio to the ratio of 13C to 12C in a standard. -
There is a specific standard, with a known, unchanging ratio of
13C to 12C that all laboratories use in their comparison. For
the stable carbon isotopes, this standard is a limestone (called
Pee Dee Belemnite—or PDB) from South Carolina. Although PDB is
no longer run as the standard, other carbonates (with a known,
unchanging 13C to 12C ratio) are used and compared on the PDB
scale. The carbonate standard is reacted with an acid to create
gaseous CO2, so that the sample and standard are both in the
same phase. - Often the sample and standard may have very
similar ratios of the two stable isotopes, which will give you a
value very close to