A mass spectrometry method based on ion/molecule reactions for determining enantiomeric excess in mixtures of amino acids is illustrated. Protonated complexes of amino acids with beta-cyclodextrin are produced in the gas phase using electrospray ionization. Under a background pressure of N-propylamine, the amino acid is replaced by the alkylamine. The rates of the guest exchange reaction vary with the chirality of the amino acid. A calibration curve employing varying mixtures of D- and L-amino acids is produced and the fractions of the D isomer in the test mixtures are determined. The quality of the curve is compared to the chiral selectivity of the reaction. In general, the best calibration curves are obtained in systems with the greatest selectivity.