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Fitting a Molecular Formula to a Mass

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


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