The N-perfluoroacyl amino acid alkyl esters are probably the most convenient derivatives, allowing the determination of all common protein amino acids. The order of elution of a hydroxylated amino acid relative to its nonhydroxylated analog depends on the polarity of the stationary phase. The preparation of tert-butyldirnethylsilyl derivatives is a simple, one-step, and rapid process. The derivatives are particularly useful for gas chromatographic/mass spectrometric analysis involving the use of stable isotopes and selected ion monitoring due to the frequently prominent ion at 57 units less than the molecular ion. The separation of amino acid derivatives by gas chromatography is usually performed with packed columns, but more recently capillary columns have gained favor. Serum and tissue extracts proved much cleaner to analyze by gas chromatography than the more complex urine samples. The analysis of human saliva using enantiomer labeling gave an accuracy claimed to be equal or superior to that of other methods.