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This document specifies a method [1] for the quantitative determination of saxitoxin (STX), decarbamoyl saxitoxin (dcSTX), neosaxitoxin (NEO), decarbamoyl neosaxitoxin (dcNEO), gonyautoxin 1 and 4 (GTX1,4; sum of isomers), gonyautoxin 2 and 3 (GTX2,3; sum of isomers), gonyautoxin 5 (GTX5 also called B1), gonyautoxin 6 (GTX6 also called B2), decarbamoyl gonyautoxin 2 and 3 (dcGTX2,3; sum of isomers), N sulfocarbamoyl gonyautoxin 2 and 3 (C1,2; sum of isomers) and N-sulfocarbamoyl gonyautoxin 1 and 4 (C3,4; sum of isomers) in (raw) mussels, oysters, scallops and clams. Laboratory experience has shown that this document can also be applied to other marine invertebrates [2], [3] and processed products of those species, however, no complete interlaboratory validation study according to ISO 5725 2:1994 has been carried out so far. The method described was validated in an interlaboratory study [4], [5] and was also verified in a European Union Reference Laboratory for Marine Biotoxins (EURLMB)-performance test aiming the total toxicity of the samples [6]. Toxins which were not available in the first interlaboratory study [4], [5] as dcGTX2,3 and dcNEO were validated in two additional interlaboratory studies [7], [8]. The lowest validated levels [4], [5], [8], are given in µg toxin (free base)/kg shellfish tissue and also as µmol/kg shellfish tissue and are listed in Table 1. [Table 1] A quantitative determination of GTX6 was not included in the first interlaboratory study but several laboratories detected this toxin directly after solid phase extraction with ion-exchange (SPE-COOH) clean-up and reported a mass concentration of 30 µg/kg or higher in certain samples. For that reason, the present method is applicable to quantify GTX6 directly, depending on the availability of the standard substance. Whenever GTX6 standard is not commercially available, it is possible to determine GTX6 after hydrolysis of Fraction 2 of the SPE-COOH clean-up, described in 6.4, as NEO. The indirect quantification of GTX6 was validated in two additional interlaboratory studies [7], [8]. A study to compare direct and indirect GTX6 quantification was conducted at the EURLMB [16]. A quantitative determination of C3,4 was included in the first interlaboratory study. The present method is applicable to quantify C3,4 directly, depending on the availability of the standard substance. If no standard substances are available, C3,4 can only be quantified as GTX1,4 if the same hydrolysis protocol used for GTX6 (6.4) is applied to Fraction 1 of the SPE-COOH clean-up [10]. A study to compare direct and indirect C3,4 quantification was conducted at the EURLMB [16].