Truly "exact" mass:: Elemental composition can be determined uniquely from molecular mass measurement at ∼0.1 mDa accuracy for molecules up to ∼500 Da

At sufficiently high mass resolution, not all molecular masses are possible, even for arbitrary elemental composition. Here, we consider all possible lelemental compositions, CcHhNnOoSs, c unlimited, It unlimited, 0 < n < 5, 0 < o < 10, 0 < s < 3, for even-electron ions (M + H)(+) and (M - H)(-), of molecular mass, 300-500 Da. Masses are then sorted into "bins" of width, 5, 1, 0.5, and 0.1 mDa, with inclusion (or not) of up to two C-13's. The number of different elemental compositions per bin varies from 0 to similar to 20 for 5 mDa bins, dropping to 0-2 for 0.1 mDa bins (including one C-13) and 0-1 for 0.1 mDa bins (not including C-13). Thus, for molecules up to similar to 500 Da in mass, mass resolution and accuracy of similar to 0.1 mDa generally suffice to yield a unique elemental composition for molecules in even the most complex natural mixtures (namely, petroleum crude oil), and higher mass resolution and accuracy are not necessary. The required resolution and accuracy are now becoming available with high-field (9.4-14.5 T) Fourier transform ion cyclotron resonance mass spectrometry. (C) 2006 Elsevier B.V. All rights reserved.