Marine brown algae such as Ascophyllum nodosum and Fucus vesiculosus accumulate polyphenols composed of phloroglucinol units. These compounds are of ecological importance and, due to their antioxidative activity, of pharmacological value as well. In this study four methods for the quantitative determination of phlorotannins are compared: spectrophotometric determinations using Folin-Ciocalteu's phenol reagent or 2,4-dimethoxybenzaldehyde (DMBA), quantitative (1)H NMR spectroscopy (qHNMR), and gravimetrical measurements. On the basis of the relative standard deviation and the F-test, the determination using Folin-Ciocalteu's phenol reagent and qHNMR proved to be the most reliable and precise methods.

Thalli of the brown alga Ascophyllum nodosum were collected on the Scottish west coast and the phlorotannin content was measured once a month between January and December 2005. Due to the potential use of A. nodosum extract as a nutritional supplement or for medical purposes, our aim was to determine the most favourable time for collection of the algal material. For this purpose two different methods for quantitative phenol determination were used, i.e. the well-known spectrophotometric measurements with Folin-Ciocalteu's phenol reagent (FC-method) and for the first time quantitative

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