Direct Analysis of Underivatized Amino Acids in Plant Extracts by LC-MS-MS

In this chapter, we describe a method for quantification of 20 proteinogenic amino acids as well as 13 (15)N-labeled amino acids by liquid chromatography-mass spectrometry without the need for derivatization and use of organic solvents. Analysis of the underivatized amino acids is performed by liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS-MS) in the positive ESI mode. Separation is achieved on a strong cation exchange (SCX) column (Luna 5 μm SCX 100 Å) with 30 mM ammonium acetate in water (A) and 5% acetic acid in water (B). Quantification is accomplished by use of d(5)-phenylalanine as internal standard achieving limits of detection of 0.1-3.0 μM. The method was successfully applied for the determination of proteinogenic and (15)N-labeled amino acids in plant extracts.