| Microcystins, potent toxins produced by some species of cyanobacteria, are a growing problem in drinking water supplies worldwide. A simple, rapid method for quantitation of microcystins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was developed. Four potential internal standards were tested, including an 15N-labeled microcystin; nodularin proved to be the most effective internal standard. For microcystin-LR (MC-LR) in mixed solutions simulating field collection conditions, a linear range of 0.19--5.0 muM (R2 = 0.99) with a method detection limit of 0.058 muM was obtained. The ionization efficiency and analyte-analyte suppression for four microcystins of varying polarity were assessed; the three polar congeners exhibited good ionization efficiency and acceptable levels of analyte-analyte suppression.;A novel method for simplifying adduct patterns to improve the detection and identification of peptides using MALDI-TOF MS was developed. Addition of 20 muM zinc sulfate heptahydrate (ZnSO4•7H2O) to samples prior to spotting on the target enhanced detection of the protonated molecule while suppressing competing adducts. This produced a highly simplified spectrum with the potential to enhance quantitative analysis, particularly for complex samples. The resulting improvement in total signal strength and reduction in the coefficient of variation (from 31.1% to 5.2% for MC-LR) further enhanced the potential for sensitive and accurate quantitation.;MALDI-TOF MS offers several advantages over other available methods for analysis of cyanotoxins, including speed, reduced sample handling, and congener identification, but has previously been limited to qualitative analysis of microcystins. Addition of MALDI matrix to intact cell samples provided simple, rapid identification and quantitation of microcystins. Consistent results were obtained across a wide range of cyanobacterial and diatom densities, with recoveries near 100%. Samples extracted using a more traditional approach were analyzed in parallel by PPIA and MALDI. The initial correlation between the MALDI and PPIA results (R2 = 0.69) improved significantly (R2 = 0.81) when the MALDI results were adjusted based on the mouse bioassay toxicity of each identified congener. The results confirm the utility of MALDI-TOF MS as a quantitative screening method for microcystins, and the potential for its use as a stand-alone technique. |
