... higher MS resolving power,so the instrument of choice has been the expensiveFourier transform mass spectrometer (FT MS) [3,5,23,24]. FT MS has the added advantage that itcan give MS ⁄ MS spectra ... cor-responding to, for example, a single substitution; MS ⁄ MS will then show the substituent positions of dif-ferent isomers. A problem for MS ⁄ MS of either thepeptides for the bottom-up approach ... importance, but thesedepend on the combination of separation methods, MS instrumentation, and computation employed.Top-down MS ⁄ MS of the selected molecular ionmass representing a specific protein...