Metanephrines analytical methods

The oxidation reaction has been carried out with (a) iodine at more alkaline pH values than are used in the case of the catecholamines, (b) potassium ferricyanide, but with a higher concentration of zinc ions needed to catalyse the reaction, which works in the case of metanephrine, but not for the nor compound, and (c) periodates. The periodate oxidation gives amino-chromes only at low pH values [131]. Under alkaline conditions, periodate oxidation leads to degradation of the side-chain to give vanillin. This reaction had, in fact, been the basis of earlier analytical methods for (27) and (28) [132, 133]. 

In contrast to the catecholamines, measurements of urinary metanephrines and VMA are still based in some routine laboratories on the early spectrophotometric assays developed by Pisano, Crout, and others in the late 1950s and early 1960s. Despite subsequent development of a variety of preanalytical cleanup and extraction procedures, these assays remain susceptible to analytical interference. They are also restricted to measurements in urine. Another limitation for spectrophotometric or fiuorometric assays of urinary metanephrines is that these methods do not allow separate (fractionated) measurements of normetanephrine and metanephrine. 

As with the catecholamines, fluorescence methods have also been reported for urinary metanephrine analysis. Fluorescent derivatization of the metanephrines NM and MN by chemical oxidation was based on modification of the trihydroxyindole reaction used for catecholamines. The individual metanephrines were measured following chromatographic separation and fluorescent derivatization or through the formation of differential fluorescent compounds by oxidation at different pH levelsSince the stability of the fluorescent products was variable, with some products decomposing within 10 min,this method has limited application in current practice. Other early methods for analysis of NM and MN included electrophoresis and paper and thin-layer chromatography. These assays were technically complex and had poor analytical sensitivity. 

As with other analytes, the earlier mass spectrometric measurements of urinary amines, particularly the metanephrines, were developed using gas chromatography-mass spectrometry (GC-MS), as the coupling of GC to MS technology was technically more straightforward than LC coupling to Nonetheless, the number of published GC-MS methods... 

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