Quantitation of blood and plasma amino acids using isotope dilution electron impact gas chromatography/mass spectrometry with U-13C amino acids as internal standards

A method to quantitate blood and plasma amino acids by isotope dilution gas chromatography/mass spectrometry (GC/mS) is described. Samples were spiked with U-(13)C amino acids as internal standards and the tert-butyldimethylsilyl derivatives (tBDMS) separated by capillary column gas chromatography. Linear regression curves, generated for individual amino acids, gave correlation coefficients of 0.9999. The reproducibility of the method was assessed from the analysis of 10 replicate blood and plasma samples. For most amino acids a coefficient of variance (CV) of </=1% was obtained with the exception of aspartate which gave a value of 1.8%. This was probably due to the low concentration of this amino acid in the samples analysed. Recovery of amino acids added to plasma was between 96 and 103%. The use of electron impact ionization (EI) allows the method to be used in laboratories where only the more basic GC/mS is available and reduces the time spent on instrument maintenance. The method should prove useful in areas of work where accurate and precise amino acid concentrations are required.