Nutrients, and especially micronutrients, may also be added via impurities, and especially via agar. Such impurities may well be beneficial. This is particularly true of Ni, which has recently been shown to be an essential element (Gerendas et al, 1999) but was not known to be when most medium formulations were established. This element is usually not included in the inorganic constituents but can be provided by impurities. Tables 3.2 and 3.3 show impurities of various agar brands and their relative contribution to MS. Agar provides a large addition of sodium but levels of sulphur and copper are also significantly increased. Increases in the other elements in MS, are less than 20 %. Gelrite contains fewer organic impurities but inorganic ones occur at high concentrations (Table 3.2). It should be noted that the data in Table 3.2 are from determinations done more than 15 years ago and that the production process of gelrite has been improved ever since. Gelrite is being used in medicines as an ophthalmic vehicle. Furthermore, minerals are absorbed to a significant percentage by agar (Scholten and Pierik, 1998 Leiffert et al, 1995) and by activated charcoal (Van Winkle et al, 2003) but whether this has a significant effect has not been examined.
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