Abstract
Vitamin B 12 is essential as a cofactor in several vital enzymatic reactions. Determination of methylmalonic acid (MMA) in urine is considered as the standard test for vitamin B 12 deficiency and GC-MS as the preferred analytical technique. Quantitation is improved by using isotope dilution, but accurate determination of dicarboxylic acids as their trimethylsilyl derivatives is difficult because electron ionization yields low molecular-ion peaks, while other fragment ions are common to all. It is shown that chemical ionization with isobutane as the reactant gas improves molecular-ion formation. MMA is separated from urine by solid-phase extraction and derivatized with trimethylchlorosilane/ hexamethyldisilazane. With the ion-trap detector, naturally occurring MMA yields a peak at m/ z 263 (M + 1) +; the corresponding peak for deuterated MMA is m/ z 266. The calibration curve covers the range 1–50 μg ml −1 MMA. Recoveries were in the range 90–110%.
Published Version
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