Modification of the analysis of amino acids in pig plasma

Determination of amino acids in pig plasma with the classical ninhydrin system is influenced by the excessive amount of protein and lipophilic compounds in the sample, leading to a decline in resolution. This problem was eliminated by using 80 mg of sulphosalicylic acid per ml of plasma, and solid-phase extraction with a C18 cartridge as an additional clean up step. The latter resulted in significantly higher quantities of threonine, asparagine, glutamic acid, glutamine, glycine, alanine, valine and lysine, and lower levels of phenylalanine and tryptophan (P < 0.05). The use of a C18 cartridge had a minor effect on the analytical error.