Microscale analysis of amino acids using gas chromatography–mass spectrometry after methyl chloroformate derivatization

To conduct studies of stable isotope incorporation and dilution in growing plants, a rapid microscale method for determination of amino acid profiles from minute amounts of plant samples was developed. The method involves solid-phase ion exchange followed by derivatization and analysis by gas chromatography–mass spectrometry (GC–MS). The procedure allowed the eluent to be derivatized directly with methyl chloroformate without sample lyophilization or other evaporation procedures. Sample extraction and derivatization required only ca. 30 min and quantification of the 19 amino acids eluted from the cation exchange solid-phase extraction step from a single cotyledon (0.4 mg fresh weight) or three etiolated 7-day-old Arabidopsis seedlings (0.1 mg fresh weight) was easily accomplished in the selected ion monitoring mode. This method was especially useful for monitoring mass isotopic distribution of amino acids as illustrated by Arabidopsis seedlings that had been labeled with deuterium oxide and 15N salts. Sample preparation was facile, rapid, economical, and the method is easily modified for integration into robotic systems for analysis with large numbers of samples.