The amino acids we have the most experience with are GABA, glutamate and arginine, though other amino acids can be measured. The method used is based on a derivatization reported by Jacobs (1 ) using o-phthalaldehyde in the presence of sulphite. The method was used by Pearson et al. (2 ), Smith and Sharp (3 ), and was further developed by Rowley et al. (4 ).