High-performance, flow-based, sample pre-treatment and introduction procedures for analytical atomic spectrometry . Plenary Lecture

Abstract

Recent and on-going work in the author's laboratory is described with particular reference to the use of flow injection, continuous flow and HPLC procedures for the development of improved analytical methodology for (a) the determination of trace concentrations of As, Cd, Pb and Se and (b) the determination of various arsenic and selenium compounds. The methods have been applied to the analysis of several different sample matrices, including urine, soil, sediments, waters, plants (garlic, onion, apple leaves), yeast, fruit juices, wine, calcium supplements and marine plankton. The dependence of the LOD of an FI HG method on sample volume is examined and the validity of the proposed rectangular hyperbolic relationship established for a number of different analyses. The use of immobilized tetrahydroborate in conjunction with preconcentration of the analyte on the same anion-exchange resin is described as a possible procedure for improving the LOD. When used in conjunction with ETAAS, an LOD of 0.004 µg l^–1 for both As and Se was obtained for a sample volume of 10 ml. The procedure was also used in a method for the determination of inorganic arsensic and methylated arsenic(V) species. Methods for the determination of Pb by HG in the presence of hexacyanoferrate(III) were developed and applied to the analysis of urine, soils, waters and apple leaves. In the case of urine, the interference from the chelating agents used in the treatment of patients with elevated lead was overcome by the addition of Sc. The best LOD of 0.03 µg l^–1 was obtained for a procedure in which the lead hydride was trapped on the interior of a flame-heated slotted quartz tube under fuel-lean conditions with subsequent atomization when the flame was made fuel rich (by the injection of a small volume of isobutyl methyl ketone). It has been confirmed that it is possible to determine Cd by a ‘cold vapour’ procedure and it was shown that the nature of the atom cell surface played no part in the atomization process, which appeared to be the spontaneous decomposition of the species evolved from acid solution of cadmium on the addition of sodium tetrahydroborate solution. A modest increase in signal was obtained in the presence of nickel and thiourea. An LOD of 0.02 µg l^–1 was obtained for a sample volume of 300 µl and the procedure was used for the analysis of NIST SRM 2711 (Montana Soil) in which the interference from the high lead content was overcome by coprecipitation with barium sulfate. Three examples of procedures using manifold designs incorporating an ‘eight-port’ rotary valve are given to illustrate the versatility of this component: the separation of high concentrations of uranium (5000 mg l^–1) from trace concentrations (1 µg l^–1) of Al, Be, Li and Mg for determination by ICP-MS, the automated implementation of the co-immobilization of analyte and tetrahydroborate on an anion-exchange resin and the stopped-flow microwave digestion of human urine for the determination of Se by HG and ETAAS. Improvements in the ion-pair (with trichloroacetate) reversed-phase (C8) HPLC procedure (with ICP-MS detection) for the separation of selenoamino acids (and closely related compounds) were made with a new stationary phase (Waters Symmetryshield RP₈) and a small-volume spray chamber. The results of extraction procedures indicated that much of the selenium in yeast and garlic is bound in high molecular mass material. So far only a few of the compounds in the extracts have been identified by retention time matching. A reversed phase (C₁₈), ion-pair (tetrabutylammonium) HPLC procedure for the separation of four arsenic species (arsenite, arsenate, monomethylarsinate and dimethylarsonate), with detection by post-column HG-AAS, has been devised and applied to the extracts of soils spiked with these four species. Low recoveries of arsenite were obtained and microwave energy significantly accelerated the oxidation of arsenite to arsenate.

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