PRECONCENTRATION OF INDOLIC COMPOUNDS AT A CARBON PASTE ELECTRODE AND INDIRECT DETERMINATION OF L-TRYPTOPHAN IN SERUM BY ADSORPTIVE STRIPPING VOLTAMMETRY

The pre-concentration of indole and other indolic compounds of biochemical and pharmaceutical interest at a carbon paste electrode (Nujol/graphite) has been studied. All the compounds examined can be determined, by direct voltammetric measurements in their solutions, in the concentration range 1-8 μM. Indole, methylated indoles, harmaline and serotonin were accumulated at the electrode by a combined adsorption/extraction process. By applying the medium-exchange procedure, the accumulated compounds can be determined in the same concentration range, after a 60-s pre-concentration period, enhancing, therefore, the selectivity of the voltammetric determination. A procedure for the indirect determination of L-tryptophan in serum has been developed, which is based on these findings. L-Tryptophan was cleaved to indole by tryptophanase, and indole was subsequently determined voltammetrically after a 2-min pre-concentration period at the carbon paste electrode. From 0.3 to 1.2 μg of L-tryptophan in a total of 75 μl of serum sample (20-80 μM) can be determined with an average error of ca. 0.03 μg, whereas the recovery of added L-tryptophan in serum samples is in the range 105-115%.