chemical formula in unimod standard

[trishorts]

CHEMICAL FORMULA Chemical formulas of modifications may be specified following the mandatory key “formula.” Formulas must use Unimod symbols (http://www.unimod.org/masses.html) and follow the Unimod composition rules (http://www.unimod.org/fields.html). The formula is displayed and entered as atoms, optionally followed by a number in parentheses. The number may be negative and, if there is no number, 1 is assumed. The atom order is not important. C, F, H, etc. are symbols for elements, not one letter codes for amino acids. Isotopes of atoms are specified by the integer preceding the atomic symbol (e.g. 13C).

[zrolfs]

This should be functional, but I'm struggling with the syntax. If I want to show heavy carbon in the formula "C6H12N2O", neither "13C6H12N2O" nor "(13C)6H12N2O" works.

[trishorts]

C{13}