Transition metal doping for improving the fluorescence response of paralytic shellfish poisoning toxins upon pre-chromatographic derivatization

Paralytic shellfish toxins can be quantified in bivalves by the AOAC-2005 method. This method is sensitive for several toxins, but other toxins suffer from strong matrix effects or fluorescence partitioning by multiple oxidation products, reducing their sensitivity. Metals were assessed as candidate catalysts to improve the oxidation reactions, instead of the oyster matrix modifier prescribed in this method. The N1-H sub-group of toxins exhibited increased fluorescence upon doping with iron sulphate. The increase was inversely proportional to the fluorescence reduction caused by matrix suppression. Iron acted as a catalyst, lowering the activation energy of the reaction, which otherwise required heating to achieve a similar boost in the fluorescence yield. The fluorescence due to the N1-hydroxyl sub-group of toxins GTX1 + 4 and GTX6 increased with zinc but decreased with iron. When doping with a metal solution fraction-2 of the carboxylic acid partitioning, both metals reduced the fluorescence, while the eluent (NaCL 0.05 M) enhanced it. As metals are retained by COOH cartridges, sulphonic acid leachables might contribute to this reduction. The reduction observed for GTX1 + 4 was primarily due to the decrease in the proportion of the secondary peak over the primary oxidation peak, which can be reversed by doping with an oyster matrix. Nickel chloride was able to replace the oyster modifier effectively in all five bivalve matrices tested, while zinc chloride was not similarly effective. For dcNEO spiked in several bivalve matrices, adding an oyster modifier derived from <i>Magallana gigas</i> caused a 16-19% reduction in fluorescence. Doping with several metals could not achieve the same fluorescence as doping with 0.1 mM acetic acid. For NEO, with one particular oyster extract, the yield could be higher than using NiCl₂. Depending on the specific oyster extract, the use of the matrix modifier required in the method might not be adequate.