Molecular formulas of identical exact mass

Precise high resolution mass spectrometers can even provide masses with a precision of 6 decimal places. Even such accuracy is not sufficient to determine a molecular formula unambiguously. The first pair of molecular formulas of identical exact mass in occur at mass 129.954034. This and other such pairs are as follows  

While one could object that compounds corresponding to these molecular formulas do not exist, adding identical numbers of C and H atoms results in further pairs of molecular formulas of identical exact mass. Thus, the first pair of valid molecular formulas of identical exact mass obtained by adding 5 C atoms and 10 H atoms to the first example above is C5 H14 F2 SSi2 and C5 H11CIO2 P2 at 200.032284 Da, and these are valid molecular formulas. Kind and Fiehn [159] performed an extensive exploration of molecular formulas in the range of 0-500 Da and also came to the conclusion that mass accuracy 

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