Paralytic shellfish poison reference materials: an intercomparison of methods for the determination of saxitoxin

Within the framework of the European Commission's Measurements and Testing Programme (BCR) a project has been undertaken to develop shellfish reference materials for Paralytic Shellfish Poisons (PSP). In a preliminary phase of the project, an intercomparison study of methods was undertaken. In this exercise 18 laboratories were asked to analyse solutions of saxitoxin and PSP-containing shellfish extracts with a method of their choice. The study revealed that: all the methods considered (four HPLC methods, one ELISA method) were in principle adequate for the quantification of saxitoxin in solution in the absence of interfering substances (Coefficient of variation (CV) 33% at a concentration of 0.5 microgram/ml); three of the HPLC methods used were able to quantify saxitoxin in PSP-positive mussel extract, the fourth method gave significant overestimation; the CV of all HPLC results was 53% at a mean saxitoxin mass fraction of 2.06 mg/kg mussel meat, the recoveries varied from 59-173%; and the ELISA method grossly overestimated the saxitoxin content in mussel extract, probably due to cross reactions of the antibodies with other PSP. The feasibility of preparing a homogeneous batch of ampouled mussel extracts (CV 3.5% at a saxitoxin concentration of approximately 1.5 mg/kg shellfish), sufficiently stable for at least 4 months storage both at 4 degrees C and approximately 20 degrees C, was demonstrated. The performance of the different methods for the analysis of PSP other than saxitoxin has not yet been evaluated, due to the current lack of PSP standards. Some of the problems observed in the intercomparison study were partly due to the nature of the chromatographic columns used, the composition of the HPLC mobile phase (pH, ion strength), non-optimal conditions for derivatization and matrix interference. Following the outcome of this study, a three year multistage project involving 15-20 European laboratories has been initiated, aimed at improving the accuracy and comparability of PSP measurements as well as preparing reference materials for PSP.