Visible diode laser-induced fluorescence detection of phenylacetic acid in plasma derivatized with Nile blue and using precolumn phase transfer catalysis.

This study reports the application of Nile blue (NB), a farred oxazine label, as a precolumn derivatization reagent for the measurement of free levels of phenylacetic acid (PAA) in plasma. The measurement of PAA in psychiatric populations is important because it provides a marker for 2-phenylethylamine (PEA), which has been implicated in the pathogenesis of schizophrenia and major depression. PAA was derivatized with NB through an amide linkage in the presence of 2-chloro-1-methylpyridinium iodide (carboxylic acid activator, CMP) and triethylamine (base catalyst, TEA), respectively. The formation of the NB-PAA derivative was confirmed using normal phase and reversed phase thin-layer chromatography, reversed phase liquid chromatography, and electrospray mass spectrometry. The formation of the NB-PAA derivative was optimized using a sequential single factor approach. The optimal conditions for the formation and chromatographic separation of the derivative were determined to be 8.0 nmol/mL NB, 390 nmol/mL CMP, 2 mumol/mL TEA, a reaction time of 45 min, and a reaction temperature of 25 degrees C. This derivatization scheme was performed in a phase transfer catalysis mode that enabled the simultaneous extraction, preconcentration, and derivatization of the analyte in a single step. The limit of derivatization of PAA was determined to be 1.0 x 10(-9) M in phosphate-buffered saline, a PAA-free matrix. This derivatization was limited not by the kinetics of the reaction but by the chromatographic separation of the derivative from a side reaction product. The method was used to estimate endogenous free levels of PAA in human plasma samples. The levels of PAA in four sources of plasma were determined to be within 30-70 ng/mL using the method of standard addition and reflected levels that have been reported in the literature. The limit of detection of the derivative was determined to be 7.33 x 10(-11) M using a laboratory-constructed HPLC-VDLIF detector.