Phenylpyruvic acid decarboxylation during extraction

Wappner, R. S., and Brandt, I. K. Artifact in the Gas Chromatographic Analysis of Urinary Organic Acids from Phenylketonuric Children, Decarboxylation of Phenylpyruvic Acid During Extraction Clin. Chim. Acta 61 (3) 367-374 (1975) GA 83 128364c... 

Care must also be taken, when solvent extraction is used, to avoid artefacts either from impurities in the solvent themselves (Perry and Hansen, 1974) or by the extraction of some of the non-acidic and neutral components of urine into the solvents at acid pH. These latter components include urea, phenols and alcohols, particularly those derived from dietary sources. To avoid this problem prior extraction of the urine with the same solvent systems at alkaline pH before subsequent acidification and re-extraction can be effective (Mamer et al, 1971), while the first step towards exclusion of artefacts requires the use of high-purity reagents and solvents. Although seldom incorporated into the solvent extraction methods that have been described, the prior stabilization of 0X0 acids as substituted oxime derivatives is also essential to avoid artefact formation during solvent extraction, such as the oxidative decarboxylation of phenylpyruvic acid to phenylacetic acid (Thompson etal., 1975). 

Thompson, R.M., Belanger, B.G., Wappner, R.S. and Brandt, I.K. (1975), An artefact in the gas chromatographic analysis of urinary organic acids from phenylketonuric children decarboxylation of phenylpyruvic add during extraction. Clin, Chim. Acta, 61,367. 

Possible errors due to the methods used have been referred to in the sections of Part I and include the dehydration of hydroxy acids, for example 3-methyl-crotonic acid from 3-hydroxyisovaleric acid, crotonic acid from 3-hydroxy-butyric acid, either on acid steam distillation or alkaline treatment, decarboxylation of 2-oxo acids, for example phenylpyruvic acid to phenylacetic acid (Thompson et al., 1975b) and similar effects with branched-chain oxo acids, during sample preparation, particularly by solvent extraction. Derivatization artefacts produced during methylation have been detailed in Part I, and decarboxylation of free non-volatile acids during the analysis of volatile acids, for example, methylmalonic acid to propionic acid, are also detailed elsewhere. 

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