To maintain compatibility with MolecularWeight,
MonoisotopicMass, and IsotopicDistribution
the elemental formula can contain only C,H,N,O,S,P,Br,Cl,F,I,Si, and
Sn (I and Sn are default elements in MolecularWeight only). Elements not in this set will be ignored and a warning will be
generated. The function can handle repeated elements, but not element
multiplication indicated by parenthesis.
Known issue: Lower case letters after numbers, such as the “o” in
“C12oBr5H5” will not be caught with a warning and the resulting
list will contain the incorrect number of elements.