Analysis of enantiomeric excess using mass spectrometry: fast atom bombardment/sector and electrospray ionization Fourier transform mass spectrometric approaches

The utility of fast atom bombardment (FAB) ionization on a sector mass spec trometer, and of electrospray ionization (ESI) on a Fourier transform ion c yclotron resonance mass spectrometer, for enantiomeric excess measurements was explored. Both methods involved the same host-guest system: (R,R)- or ( S,S)-dimethyldiketopyridino-18-crown-6 (host) and alpha-(1-naphthyl)ethylam monium (guest). Both use an achiral amine (benzylamine for the FAB experime nts, cyclohexylamine for the ESI experiments) as an internal reference comp ound and involve competitive complexation of the achiral and chiral amines with the chiral host. The FAB experiments are shown to give stable, reprodu cible results, but exhibit a smaller degree of enantiodiscrimination than t he ESI experiments. The ESI experiments, which involve measurement of appar ent guest exchange equilibrium constants, show a linear relationship betwee n apparent equilibrium constant and enantiomeric excess. The apparent equil ibrium constant is shown to be a composition-weighted average of the equili brium constants for the two pure enantiomers. Enantiomeric impurities as sm all as about 2% can currently be detected. (Int J Mass Spectrom 185/186/187 (1999) 977-988) (C) 1999 Elsevier Science B.V.