Tandem mass spectrometric accurate mass performance of time-of-flight and Fourier transform ion cyclotron resonance mass spectrometry: a case study with pyridine derivatives

The interpretation of mass spectra is a key process during compound identif ication, and the combination of tandem mass spectrometry (MS/MS) with high- accuracy mass measurements may deliver crucial information on the identity of a compound. Obtaining accurate mass data of fragment ions in MS/MS revea ls the particular problem of mass calibration when a lockmass, which is fre quently used to obtain accurate masses in MS, is absent. An alternative tec hnique is to recalibrate the MS/MS spectrum using a reference MS/MS spectru m acquired under the same conditions. We have tested and validated this app roach using a hybrid quadrupole/orthogonal acceleration reflectron-type tim e-of-flight (TOF) mass spectrometer. The results were compared with those o btained under similar conditions on a Fourier transform ion cyclotron reson ance (FT-ICR) instrument. We found that the mass accuracy observed with suc h an 'external' recalibration on the TOF instrument in MS/MS is identical t o what can be obtained on a similar instrument operating in one-dimensional MS mode using the lockmass technique. However, mass accuracy in both cases is one order of magnitude inferior to that obtained using FTMS, and also i nferior to that observed using sector field MS when operated at comparable resolution. Nevertheless, for small (< 200 Da) molecules, this mass accurac y was still sufficient to have the 'true' elemental composition identified as the first hit in about 70% of all cases. It was possible to elucidate th e fragmentation mechanism of eight azaheterocycles containing a pyridine mo iety, where the accurate mass data from the TOF instrument allowed distinct ion between two alternative fragmentation pathways. Copyright (C) 2001 John Wiley & Sons, Ltd.