Homogeneous liquid-liquid extraction method for the selective spectrofluorimetric determination of trace amounts of tryptophan

Twenty-one amino acids were derivatized with fluorescamine (FLA) under basic conditions (pH 9) and the extraction of the amino acid-FLA derivatives was investigated using a homogeneous liquid-liquid extraction with perfluorooctanoic acid (HPFOA) based on phase separation under strongly acidic conditions. Under the optimum concentration conditions for the reagents (PFOA]T = 3 x 10(-3) mol dm-3, acetone]T = 3 vol.%, HCl]T = 1.8 mol dm-3), the concentration factor was approximately 1000-fold (i.e., 30 microliters of the sedimented liquid phase was produced from 33 ml of the homogeneous aqueous solution). The percentage extraction (E) was determined for the 21 amino acid-FLA derivatives; the value for the tryptophan (Trp)-FLA derivative was 80.9%, whereas the other derivatives were not almost extracted (E < 0.4%). The Trp-FLA derivative was selective for the extraction using the homogeneous liquid-liquid extraction method with HPFOA. After the sedimented liquid phase containing Trp-FLA has been placed on a polytetrafluoroethylene filter-paper, the fluorescence intensity was determined using a spectrofluorimeter with filter-paper as the solid-sample holder. The calibration graph of Trp was linear over the range 1.0 x 10(-8)-1.5 x 10(-6) mol dm-3. The relative standard deviation for the central value of the calibration graph was 4.5% (five determinations) and the detection limit (S/N = 3) was 8.9 x 10(-9) mol dm-3. When the proposed method was applied to the highly sensitive spectrofluorimetric determination of Trp in animalin-L syrup, the results were satisfactory.