Using reversed-phase high performance liquid chromatography, 25 amino acids as o-phthaldialdehyde derivatives were determined with a 30-min sample turnover. The derivatization procedure is rapid, performed in aqueous medium, and has few transfer steps. The precision is ±0.5% at the 80-pmol level. Recoveries of standards added to urine samples were >98%. Intercallbration with an amino acid analyzer showed an agreement of >95 %. The detection limit Is about 50 fmol.The overall retention is controlled by the organic modifier, but second-order effects are ascribed to Ion repulsion between anions In the eluent and the ionized carboxylate on the derivatives. Clinical, biochemical, and environmental applications are given. Amino acids and ammonia were determined In 25

A precolumn fluorimetric labeling method was tested for shipboard HPLC analysis of dissolved fret amino acids (DFAA) in natural waters. No sample preparation is required and all naturally occurring amino acids plus ammonium are sufficiently separated within 30 min, Striking diel trends were observed to a depth of at least 60 m, with maximal DFAA concentrations (200400 nM) in the late evening and minimal concentrations (3040 nM) in the early day. Concurrent with the evening concentration maxima were elevated relative abundances of basic amino acids (especially ornithinc and lysine) and ammonium. During the day very low levels of amino acids were found in the upper water. The magnitude of the night-to-day drop in total DFAA suggests a net heterotrophic uptake rate of up to 6 pg AA-liter-'ah-'. A pronounced amino acid maximum at the 0,-H!+ interface in the water column was presumably derived from microbial populations inhabiting this region. Pronounced, coincidental minima in amino acid and ammonium concentrations just above this interface may be related either to scavaging by precipitating metal oxyhydroxides or to enhanced heterotrophic utili-

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