GAS-PHASE ACID-BASE CHEMISTRY AND ITS EFFECTS ON MASS ISOTOPOMER ABUNDANCE MEASUREMENTS OF BIOMOLECULAR IONS

Various parameters which affect mass isotopomer abundance measurements of derivatives of palmitic acid ionized by electron ionization (EI) a nd electron-capture negative chemical ionization (ECNCI) were tested o n a sector-field double-focusing mass spectrometer. Results on methyl palmitate ionized by EI are as follows: (i) sample size had a signific ant effect on mass isotopomer abundance ratios (MIARs); (ii) the elect ron multiplier gain of the detector also had an effect on MIARs; and ( iii) ion scattering by ion-neutral collisions in the mass analyzer did not appear to have any significant effect on MIARs (under standard an alysis conditions). However, 'reagent' gas pressure (methane) had a si gnificant effect on MIARs of pentafluorylbenzyl palmitate ionized by E CNCI. It was concluded that there are two compensatory effects which a lter MIARs of methyl palmitate ionized by EI: (i) gas-phase acid-base chemistry in the source (specifically, proton transfer between fragmen t cations and neutral molecules); and (ii) detector non-linearities, s pecifically, underestimation of less abundant isotopomers due to their signal disproportionately falling within the signal-to-noise ratio le vel of the electron multiplier, Gas-phase chemistry is the dominant ca use of inaccuracy in MIAR measurements for large sample sizes, while d etector non-linearity is the dominant cause of inaccuracy in MIAR meas urements at small sample sizes. However, in a narrow intermediate rang e of sample size, these two effects balance each other and result in M IARs which are 'acceptable' when compared with the known MIAR values. It is emphasized that these two effects are present regardless of the type of mass analyzer used, e.g. quadrupole, sector-field. Improvement s in the accuracy of MIAR measurements will require developments in ma ss spectrometry aimed at eliminating each of the contributory effects. (C) 1998 John Wiley & Sons, Ltd.