all modified the conditions for colour development, and V. G. Oberholzer (1969, personal communication) and Dale (1972) also corrected for background absorbance by measurement at several wavelengths. Green (1968) extracted into ether/ethanol from urine saturated with ammonium sulphate before ionexchange chromatography, while Degrazia et al. (1969) extracted into ether and followed the ionexchange chromatography by a gel-filtration step.In the present study, inclusion of extraction and ion-exchange procedures was found to be necessary; methylrnalonic acid was also extracted back into aqueous ammonia before application to the resin column. After elution and reaction with diazotised p-nitroaniline, the coloured product was extracted into amyl alcohol and stabilised with tetra-methyl ammonium hydroxide.Annals (}l CliniCCII Biochemistry, 1979,16,161-164 Colorimetric method for determination of urinary methylmalonic acid SUMMARY The colorimetric method for the determination of methyl malonic acid by coupling with diazotised p-nitroaniline has been re-examined. Extraction of the diazotised product into amyl alcohol and tetra-methyl ammonium hydroxide considerably increases its stability and sensitivity and reduces background absorbance. Preliminary solvent extraction and ion-exchange chromatography are desirable but can be accomplished simply and quickly using very small quantities of urine.In this form, the method is very reliable and gives recoveries of about 95~Ia and a coefficient of variation of about 7 %.
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