A part of our laboratory\'s response to the sea lion mortality event off of Monterey Bay, California (Summer 1998) was the quantitation of domoic acid (DA) in event related samples by a variety of techniques. For a definitive confirmation of the levels of DA, samples associated with the event were subjected to reversed phase liquid chromatography was coupled with tandem mass spectrometry (LC-MS/MS). With minimal cleanup, low levels (~10 nM concentrations) of DA could be determined from a variety of matrices. The LC-MS/MS analysis allowed unambiguous chemical determination of the presence of DA in various components of the food web in an area surrounding a bloom of the diatom Pseudo-nitzschia. Levels of DA in sea lion excrement from intoxicated animals and food web components, as determined by the LC-MS/MS technique were in the 1 uM-10 nM range. The specificity of the LC-MS/MS technique allowed the distinction between DA and coeluting components. These coeluting components, having a similar UV chromaphore and an identical molecular weight to DA, were shown to be chemically distinct compounds by giving different fragmentation behavior in tandem mass spectrometry experiments.