A-amino-n-butanoic acid

Quantification of the separated amino acids is usually performed by using external calibration or the internal standard method. Due to the large differences in chemical structure exhibited by the various amino acids, there is not a single ideal standard for the overall amino acid profile. Nevertheless, a suitable internal standard must be stable to hydrolysis and offer chromatographic resolution. The most popular choices comprise norleucine, norvaline, and a-amino-n-butanoic acid (AABA) [196]. 

Since no single internal standard can possibly mimic the chemistry of all the amino acids (for overall profile), the choice of internal standard has been based primarily on two criteria. The first is chemical stability. The internal standard must not be labile under the conditions employed. The second is that the internal standard must offer chromatographic resolution. This is not easy, since the overall profile produces a chromatogram that is already very cluttered. A review of the literature reveals that three internal standards are the overwhelming popular choices norleucine, norvaline, and a-amino-n-butanoic acid (AABA). It should be noted that norleucine and norva-line are very hydrophobic amino acids, whereas AABA is relatively hydrophilic. How these standards might behave during sample preparation steps (e.g., filtration) as a function of their hy-drophobicity should be taken into consideration. 

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