Liquid chromatographic determination of urinary metanephrines after pre-column oxidation to vanillin.
Urine after the addition of phenylephrine as internal standard is hydrolyzed and passed through a disposable BondElut SCX column. After washing the column, metanephrines and the internal standard are eluted with dilute ammonia. The eluate is treated with periodate and extracted with toluene. The toluene layer is collected and phenols are extracted into dilute tetramethylammonium hydroxide. An aliquot of the aqueous layer is chromatographed on a nonsilica resin base reversed phase column with an alkaline mobile phase. The peaks are detected by an absorbance detector at 350 nm. There is a baseline separation of vanillin formed by metanephrines and of m-hydroxybenzaldehyde formed by phenylephrine. The procedure is linear from 0.2 mg to 10 mg of metanephrine per liter of urine. The procedure has a high degree of specificity as the commonly prescribed antihypertensive drugs and their metabolites do not interfere.