Bradley et al. (1990) discussed the need for a standard reference material for ammonium\ud determinations. Subsequently, it was decided to collect a bulk sample and prepare this first\ud as a BGS in-house standard; later, if resources permitted, full international certification would\ud be obtained. The material chosen needed to have a fairly high ammonium content; to be\ud available in quantity within a reasonable distance of Keyworth; to have a similar matrix to\ud the majority of samples currently analysed for ammonium at BGS and, ideally, to be fairly\ud fine-grained to enable easy homogenisation with the available sample preparation equipment.\ud These factors, coupled with the author's knowledge of possible suitable areas of the Lake\ud District, led to the decision to collect a sample of the Skiddaw Slate Group
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