Determination of paralytic shellfish toxins in Portuguese shellfish by automated pre-column oxidation

Automated pre-column oxidation (the method of Lawrence) was implemented on a routine basis since the end of 1996 to study paralytic shellfish poisoning (PSP) toxins in Portuguese shellfish. Liquid chromatography confirmed the presence of PSP toxins when the known toxic algae were present: Gymnodinium catenatum and/or Alexandrium cf. lusitanicum. On the other side, it has eliminated PSP toxins as a possible recurrent contaminant in oysters from Sado estuary. These oysters were already known to contain high levels of some metals (mainly zinc, copper and cadmium) due to their location in a contaminated area and their particular physiology prone to accumulate metals. The presence of PSP toxins in Scrobicularia plana from Mondego estuary and Tellina crassa from the northern coast, during the absence of the above toxic microalgae in the water column, was confirmed. Unlike other shellfish, these two genera have the feeding habit of aspirating more sediment than organisms in suspension, and probably ingest from the sediment resting cysts of PSP producing microalgae. This is another route of contamination that may help to explain why after a bloom certain shellfish species maintain toxicity for long periods. The method revealed to have a fast implementation on a daily basis, short analysis time (around 20 min between samples), high sensitivity and robustness, and therefore, it is one of the best HPLC methods for screening a large number of shellfish samples for monitoring purposes.