Sample extraction should be performed as soon after collection as is possible. Ideally, this should be done at sea if conditions and space permit. Otherwise it should be done as soon as the vessel reaches shore. Sample should never be stored more than 3^-4 hour before solvent addition and they should be kept in the dark and cool during storage. The extraction is performed with pure hexane of nanograde or HPLC grade. Note further considerations on purity as described in section 2.3. If there is some doubt as to the quality of the solvent after long storage. It can be cleaned by adding 5 ml of concentrated H2OSO4 per litre. Shaking, and allowing it to stand overnight. The acid will sink and the solvent can be poured off the top.