||Chiral recognition of the amino acids, leucine (Mr = 132) and isoleucine (Mr = 132) were achieved by chemical ionization (CI) reaction mass spectrometry. A 1:1 mixture of the D- and 15N-L-isomers of the amino acids were used. A chiral compound, (S)-2- methyl-1-butanol, was employed as the CI reagent gas, which reacted stereoselectively with the D- and L-isomers of the amino acid in the ion source. The diastereomeric and mass-shifted pair of dehydrated adduct ions were selected and their peak intensities were compared under the same experimental condition. For leucine, the abundance ratio of characteristic ion 1 (m/z 202) to characteristic ion 2 (m/z 203) averaged from four consecutive analyses was 3.763. For isoleucine, the abundance ratio of characteristic ion 3 (m/z 202) to characteristic ion 4 (m/z 203) averaged from four successive analyses was 1.805. In the second part of my research, the quantitative determination of enantiomeric purity by chemical ionization mass spectrometry was demonstrated for the first time. Using a similar methodology as for Part I, the abundance ratios of m/z 202 to m/z 203 for a series of standards with -20%, -l0%, 0%, 10% and 30% e.e. of D-leucine in D,L-leucine were determined. Accordingly, a linear calibration curve (experimental peak intensity at m/z 202 to m/z 203 vs. % e.e. of D-leucine in D,L-leucine) could be constructed. (r=0.985) The % e.e. of D-leucine in D,L-leucine for an "unknown" (14%) was found to be 14.89% from the calibration curve and the relative error was 6.43%. In the final part of my research, a different approach in the chiral differentiation of leucine was presented. The chiral discrimination of leucine enantiomers by tandem mass spectrometry was shown. Under a low-energy collision condition, the characteristic ions formed from D- and 15N-L-leucine with the protonated (S)-2-methyl-1-butanol ions underwent an unimolecular dissociation, producing daughter ions of m/z 132 and 133 respectively. By comparing the percentage of the daughter ions of m/z 132 (P132) and m/z 133 (P133), the D- and L-isomers of leucine could be distinguished. The experimental ratios of P132 to P133 of a 1:l mixture of D- and 15N-L-leucine for two consecutive CID-MS experiments were 0.758 and 0.762, corresponding to an average value of 0.760. The result revealed that the diastereomeric ion formed from D-leucine was more stable than that formed from the L-isomer.