A flow-through radioactivity detector was used for the high-performance liquid chromatographic determination of amino acids and other nitrogenous substances labeled with 13N, a short-lived (t1/2 9.96 min) positron-emitting radionuclide. 13N-Labeled compounds were analyzed using cation, anion and amino columns, or as the o-phthaldialdehyde derivative on an ODS column. Use of column-switching valves and a high-performance liquid chromatographic system with a quaternary eluting capability permits two to three 20-min analyses of labeled samples from a single 13N experiment to be carried out on different columns using a binary or a single mobile phase. Radioactivity in liver metabolites was quantified using an on-line flow-through monitor with data processing capability for integrating peaks and correcting for radioactivity decay. As an example, 1 min following an L-[13N]glutamate injection via the hepatic portal vein, 77% of the label in the liver was in a metabolized form; at least ten labeled products were formed.
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