Urinary metanephrines as measured by liquid chromatography with an on-line post-column reaction detector

In this method for measuring metanephrine and normetanephrine in urine, they are freed from their conjugates by hydrolysis in acid, neutralized, and isolated on a cation-exchange resin. Both metanephrines are co-eluted with ammoniacal methanol and the eluate is concentrated 10-fold with respect to the original urine volume by evaporation and reconstitution. The recovered metanephrines ae separated by reversed-phase (C18) "high-pressure" liquid chromatography with use of a mobile phase consisting of 10 mmol/L trichloroacetate, pH 2.7, containing 30 mL of acetonitrile per liter. The metanephrines are detected at 365 nm after being converted to vanillin in a post-column reaction with alkaline periodate. Total metanephrine values for most patients were appreciably lower than values obtained by the comparison procedure, that of Pisano (Clin. Chim. Acta 5: 406, 1970); however, results obtained by the two methods for two patients with pheochromocytomas agreed well.