Total microcystin determination using erythro-2-methyl-3-(methoxy-d3)-4-phenylbutyric acid (MMPB-d3) as the internal standard Original Research Article

MMPB (erythro-2-methyl-3-methoxy-4-phenylbutyric acid) from the Adda moiety of microcystin was determined by gas chromatography/chemical ionization mass spectrometry (GC/CI-MS) using MMPB-d3 [erythro-2-methyl-3-(methoxy-d3)-4-phenylbutyric acid] as the internal standard. In the selected ion monitoring of GC/CI-MS, m/z 223 for MMPB methyl ester and 226 for MMPB-d3 were monitored; the retention times of the methyl esters of MMPB and MMPB-d3 were almost the same. Methyl MMPB was determined from peak ratios of m/z 223/226. No peak interfered. The detection limit of methyl MMPB was 0.1 ng (0.43 pmol). Supposing the average of molecular weights of microcystin variants are 1000, the value (0.1 ng of MMPB) of the detection limit is equivalent to 0.43 ng of microcystin. These results indicated that this method is the most sensitive method for determination of total microcystin, and it could be applied to the monitoring of microcystins in drinking water, reservoirs and freshwater lakes. As an additional feature, this method need not use toxin (microcystin) standards.