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

Abstract

The utility of fast atom bombardment (FAB) ionization on a sector mass spectrometer, and of electrospray ionization (ESI) on a Fourier transform ion cyclotron 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 α-(1-naphthyl)ethylammonium (guest). Both use an achiral amine (benzylamine for the FAB experiments, cyclohexylamine for the ESI experiments) as an internal reference compound and involve competitive complexation of the achiral and chiral amines with the chiral host. The FAB experiments are shown to give stable, reproducible results, but exhibit a smaller degree of enantiodiscrimination than the ESI experiments. The ESI experiments, which involve measurement of apparent guest exchange equilibrium constants, show a linear relationship between apparent equilibrium constant and enantiomeric excess. The apparent equilibrium constant is shown to be a composition-weighted average of the equilibrium constants for the two pure enantiomers. Enantiomeric impurities as small as about 2% can currently be detected.