Ultrasensitive determination with PAS is applicable to various kinds of liquid samples. However, when PAS is combined with an extraction technique, the background due to impurities, excess reagents, and instability of reagents should be low and constant as shown in this paper. When a suitable extraction method, which can highly enrich the chelate and reduce the background, is applied, various materials should be determined at sub-part-per-trillion to parts-per-trillion levels using PAS. ACKNOWLEDGMENTWe are greatly indebted to Junko Fujita of the Laboratory of Analytical Chemistry, Kaken Co., Ltd., for skillful preparation of the reagents and samples and her useful advise regarding technical aspects of the extraction.

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The use of orthogonal collocation in the treatment of the problem of an array of ultramicroelectrodes is presented and compared to existing digital simulation techniques.

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