This thesis describes a series of experiments, firstly into mammalian arsenic urinary metabolites and secondly into the use of phospholipase enzymes as a tool to assist in the characterisation of lipids extracted from the seaweed, <i>Laminaria digitata</i>. HPLC-ICP-MS has been used as the principal analytical instrument, often coupled with ES-MS. The ICP-MS. Provides a ‘hard’ ionisation process and yields data specific to arsenic. ES-MS (electrospray mass spectrometry) is a ‘soft’ ionisation technique that allows analysis of the intact molecules. Analysis of the urine from Scottish Blackface sheep that had been the subjects of a feeding trial in which the sheep routinely ate <i>Laminaria</i> <i>digitata</i> as part of their normal diet revealed the presence of the short chain fatty acids, dimethylarsenopropionic acid and dimethylarsenobutanoic acid. These had previously only been seen in the urine of human volunteers following ingestion of cod liver oil. Further controlled feeding trial experiments were performed in which cod liver oil, <i>Laminaria digitata</i> and aqueous extract of <i>Laminaria digitata</i> was ingested by human volunteers. Similarities and differences in the arsenic urinary metabolites resulting from the different feeding trial regimes were investigated. The continual presence of arsenobetaine in the urine produced by the volunteers, including the control samples, raised questions concerning the accepted retention time of arsenobetaine in the body that were answered by performing another feeding trial, the results of which have been published. Phospholipase D, C, and A2 were used to cleave arsenic containing phospholipids extracted from freeze dried <i>Laminaria digitata</i>. Some valuable information was gained and the technique shows great promise for future study.