It was found that in the fast atom bombardment mass spectra of some asymmetric secondary alcohols and amines, wben a pair of enantiomers, such as (2R,3R)-and (2S,2S)-2,3-diacetoxysuccinic anhydride and (2R,3R)-and (2S,3S)-2,3dibenzoyloxysuccinic anhydride, were used as reagents, the relative abundaaces of characteristic ions formed by the stereoselective reaction between a sample and a reagent of different configurations were much higber than tbose of ions formed by a sample and a reagent of the same configuration. The absolute configurations of the sample molecule may be predicted by examination of the mass spectra of tbe sample measured with reagents of R and S configurations. This approach proved to be a convenient way to determiw the absolute configuration of organic molecules at the micromole level by fast atom bombardment mass spectrometry, and it has advantages over the chemical ionization method reported previously for the analysis of polar and involatile compounds I N T R O D U C T I O N EXPERIMENTAL ( +)-(2R,3R)-2,3-Diacetoxysuccinic anhydride (m.p. 133-134 "C, [a];5 = + 97.6, c = 0.89 in dry CHCI,) and (-)-(2S,3S)-2,3-diacetoxysuccinic anhydride (m.p. 133-134 "C, [a];5 = -97.4, c = 0.86 in dry CHCl,),( +)-(2R, 3R)-dibenzoyloxysuccinic anhydride (m.p. 188-1 89 "C, [XI;' = + 152.8, c = 1.09 in dry acetone) and (-)-(2S, 3S)-2,3-dibenzoyloxysuccinic anhydride (m.p. 1 88-189 "C, ([r];' = -152.4, c = 1.07 in dry acetone) were prepared in this lab~ratory.~.' (R)-( + )-z-Phenylethylamine* ([a];6 = + 39.25) and (S)-( -)-aphenylethylamine ([a]i6 = -39.13),(R)-(+)-phenylmethyl~arbinol~ ([a];5 = + 42.84) and (S)-( -)phenylmethylcarbinol'O ([a];' = -41.7), (R)-( -))-lactic acid" ([r]h5 = -2.2, c = 1.5 in water) and (S)-(+)lactic acid ([r];' = + 3.7, c = 5.1 in water), (6s)-( +)-6hydroxytropan-3-one" (m.p. 127-128 "C, [a]ho = + 17.4, c = 0.91 in ethanol) and (6R)-( -)-6phydroxytropan-3-one (m.p. 124.5-126 "C, [cx];' = -16.8, c = 1.10 in ethanol), (R)-( -)-mandelic acid (m.p.