binding affinity to the sodium channel receptors which results in different toxicities relative to saxi- toxin (Genenah and Shimizu 1981). Therefore, the CONTAM Panel of EFSA recommended that the regulatory limit to be expressed as mg STX-equiva- lents per kg of shellfish meat (EFSA 2009), which was accepted by the European Community. This method of expression of total toxicity or total level of contamination is therefore a simple concept for the biological methods whereby toxin levels correlate to toxicity and therefore the regulatory limit directly. However, this causes severe complications for both biochemical and analytical methodologies whereby conversion factors are required especially when certain toxin standards are limited or unavailable for all detectable PSP toxins in a contaminated sample. Toxicity-equivalence factors (TEFs) (Figure 1), originally determined by Oshima (1995) and provided as supplementary material for available toxin stan- dards by the National Reference Centre in Canada (NRC), are used for the conversion of the individual toxin amounts to STXdiHCl equivalents. However, the EFSA working group on STX has recommended new TEFs that are due to be implemented later in 2010 in Europe with the major difference being that the TEF for dcSTX will double in value. The use of TEFs has supported the development of analytical methods for PSP toxin analysis that can then be correlated to the mouse bioassay and the regulatory limits. Analytical methods such as high- performance liquid chromatography (HPLC) with fluorescence detection, using post-column (Oshima 1995; Asp et al. 2004) and pre-column oxidation methods (Lawrence and Niedzwiadek 2001; Lawrence et al. 2004, 2005), have been developed for the determination of PSP toxins. The method of Lawrence has now been AOAC accredited and accepted following rigorous validation studies by the European Union as a first action method for the toxins for which it is validated (AOAC 2005b). However, HPLC methods are slow, labour intensive and analytical standards are difficult to obtain. Oxidation of PSP toxins is required for HPLC with fluorescence detection because they lack a chromo- phore. For the AOAC HPLC method two oxidation steps (peroxide and periodate) are required resulting in two analytical runs. Lack of standard material, lengthy purification procedures and insufficient detectability has inhibited the progress in the devel- opment of other techniques such as mass spectro- metry for routine monitoring. Though advances in mass spectrometry are being made through the use of a zwitterionic hydrophilic interaction chromatog- raphy column (Diener et al. 2007), nevertheless mass spectrometry methods are more suitable for confir- matory analysis rather than screening methods due Food Additives and Contaminants 713 to their high expenditure and the requirement of dedicated skilled scientists for processing the data for quantitative analysis. Hence, alternative screening methods are still sought that are effective and suitable for routine on-site monitoring by unskilled personnel. Different functional assays, biochemical and biosensor-based technologies employing various binding molecules have shown some potential as alternative screening methods, but surface plasmon resonance (SPR) optical biosensor technology has shown considerable promise in this field in a relatively short period of time. This review describes SPR optical biosensor technology and the progress and problems to date in developing an SPR assay for PSP toxin detection for full acceptance as a regula- tory tool. Highlighted is a feasibility study of how combining binders has the potential to overcome certain issues and discussed are the possibilities of new novel binders with this technology. Surface plasmon resonance (SPR) optical biosensor technology SPR is a powerful physical technique that measures changes in refractive index and, therefore, in the resonance angle by which polarised light is reflected from a surface which is in turn related to a change in mass on that surface (Subrahmanyam et al. 2002). It is a phenomenon used to measure biomolecular interactions in real time in a label-free environment. These biomolecular interactions can be used to identify the binding of two or more interactants to each other to determine the affinity of the interac- tions, to measure the actual association and dissoci- ation rates of the interaction, and for the purpose of PSP toxin detection can be exploited to quantify the concentration of one of the interactants (Karlsson 2004). Optical biosensors based on SPR technology are now an established tool in biomedical research for exploring molecular interactions but are increasingly being investigated as monitoring tools by academic and industrial scientists interested in food safety for the detection of additives and contaminants. As Biacore biosensors had been proven to be a successful platform for detecting low-level chemical contaminants and toxins in food products, the Biacore Q platform was selected for assay develop- ment for the monitoring of PSP toxins (Campbell et al. 2007; Fonfria et al. 2007). This biosensor-based assay measured the real-time interaction of a specific biological recognition element or binder with a target toxin immobilised onto the sensor chip surface. A typical sensorgram, a measure of the binding response against time, is illustrated in Figure 2 (Abery 2001). The running buffer flowing over the