Ultrasonic-assisted derivatization reaction of amino acids prior to their determination in urine by using single-drop microextraction in conjunction with gas chromatography

A derivatization–extraction method that avoids tedious preconcentration steps is established in order to determine amino acids accurately at nanogram levels. The method involves conversion of the analytes of concern to N(O,S)-ethoxycarbonyl amino acid ethyl esters and subsequent extraction by single-drop microextraction (SDME) followed by GC analysis. The reaction proceeds smoothly and rapidly under ultrasonication which removes the bubbles from the bulk solution. Precision is acceptable and 12 non-hydrolyzed amino acids can be determined in urine in this manner. As long as the extraction conditions are consistently applied, quantitative analysis can be performed accurately. The limits of detection were satisfactory in the range 0.010–0.025 μg/ml for GC–FID and 0.26–68 ng/ml for GC–MS(SIM) with 1 ml sample volume.