Abstract
A method for saxitoxin (STX) determination is proposed. It consists of target compound extraction with 0,05 M acetic acid, derivatization with 2,4-dinitrophenylhydrazine (DNPH), and subsequent HPLC electrospray ionization MS analysis. The reaction was performed in acetonitrile-trifluoroacetic acid (98,5:1,5 v/v) at 65 °C; the half-life of STX was ca. 3,6 hours. The method was applied to analysis of natural cyanobacteria samples and a cultivated Nostoc Pruniforme strain. Detection was performed by measuring total ion current, and chromatograms for STX were obtained using an extracted ion procedure at m/z 462,15 (STX hydrazone). A frequent complication of the derivatization with DNPH is the formation of two stereoisomers, E and Z, giving two separate peaks under HPLC. Quantum-chemical calculation predicted that, due to steric hindrance, only one isomer (Z) of STX adduct is formed, and hence only one peak of the analyte is present in the chromatogram. The following metrological parameters were achieved: limits of determination (20 ng/g of dry cyanobacterial biomass), reproducibility (4%), and medium-term precision (± 7%).
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