February 3, 2009
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
With a monoisotopic mass identified from a mass spectrum, the next step is to fit a molecular formula (or elemental composition) to the unknown. For organic compounds, the starting elements of choice are Carbon, Hydrogen, Oxygen, Nitrogen and sometimes Sulfur. Information from starting material and known derivatives can also help when deciding what elements and valences to work with.
The mass of an unknown organic compound is 950.298 g/mol accurate to 0.002 Da. If the elements C(0-50), H(0-100), O(0-10) and N(0-10)—atoms ranges are listed within the brackets—are used, then no molecular formula fits this mass. For this case, expanding the elemental list to include sulfur will produce a list of 54 candidate MFs. The top 9 MFs, based on lowest error, are displayed below.
No. Formula Monoisotopic Mass Error
1 C42H78OS11 950.2980 0.0000
2 C27H82N8OS13 950.2981 -0.0001
3 C28H88NO6S13 950.2981 -0.0001
4 C20H84N7O9S12 950.2979 0.0001
5 C41H58N8O8S5 950.2981 -0.0001
6 C34H74N6O4S10 950.2979 0.0001
7 C21H96N3O3S16 950.2983 -0.0003
8 C49H62N2O5S6 950.2983 -0.0003
9 C35H72N3O10S8 950.2983 -0.0003