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Automated methods development in flow injection analysis

University of British Columbia

A versatile computer-controlled flow injection analyzer has been constructed and used to develop and characterize five analytical methods using a variety of manifolds and modes of operation. (1) Palladium(II) was determined by its reaction with sulphochloro-phenolazorhodanine monosodium salt (Na-SCPAR). The reagent was synthesized, purified and characterized. The rate constant and pre-exponential factor were then obtained by non-linear least-squares fitting. Potential interference from other ions {e.g., Pt(II), Zn(II), Hg(II)) was quantified, and the pH dependence of the reaction and a masking agent used to improve specificity. Factor analysis helped determine the number of possible chemical reaction components in the mixture and elucidate the reaction mechanism. (2) Peroxydisulphate was determined via its previously unreported reaction with 3,3'-dimethoxybenzedine. The rate constant and pre-exponential factor were obtained, and potential organic and inorganic interferents assessed. Two "classical" chemistries used in comparative studies were the determination of (3) iron(II) with 1,10-phenanthroline and (4) orthophosphate by the molybdenum blue method. Finally, (5) selenium(IV) was determined via its catalytic effect on reaction of chlorate ion with phenylhydrazine: The intermediate product was coupled with 1,8-dihydroxynaphthalene-3,6-disulphonic acid to form the colored analytical species. Interference studies were undertaken and a masking agent used to improve the specificity. The pseudo-first order rate constant, pre-exponential factor and activation energy were obtained. Automated optimization of each of these chemistries was achieved within 2 - 4 hours by the composite modified simplex algorithm. The apparatus ran replicate experiments under different chemical conditions in a sequential manner and without human intervention. This yielded significant sensitivity improvements {e.g., palladium 58%; persulphate 197%), relative to initial operating conditions. The resulting analytical methods exhibited improved precision and accuracy, faster sample throughput, and decreased cost per analysis over existing manual procedures. Previously, flow injection analysis (FIA) has almost exclusively been used to rapidly and reproducibly measure analyte concentrations. Automated FIA provides a powerful new tool by which chemical interactions (which govern analytical performance) may be thoroughly characterized. Reported here are the first uses of automated chemical response surface mapping in FIA. Selected surfaces were mathematically modelled. The interactions of chemical and/or physical FIA variables were investigated for all five methods; these each required 64 or 100 sets of experimental conditions, with at least four replicates. Response surface maps thus obtained were compared and contrasted with those from the simplex studies, and found to be more reliable. Also, surfaces obtained under conditions of constant and variable total flow rates were compared. Alternative responses were found to be useful for maximizing analytical performance: Examples included peak height repeatability, wavelength of maximum absorbance, and time to peak maximum. Data sets of higher dimensionality (e.g., absorbance vs. time "peak shape curves" as a function of two chemical concentrations) were also generated, and are discussed. The studies required implementation of four FIA modes of operation: Conventional, stopped flow, flow reversal and merging zones FIA. The ability of the instrument to precisely control timing of injection(s), and direction and rates of multiple flows was critical. Flow-reversal FIA provided a useful means to dynamically alter sample residence time. It allowed good control of sample dispersion and facilitated changes in "effective" reaction coil length, whilst maintaining a fixed geometry manifold. Stopped-flow FIA served to further limit sample dispersion, whilst allowing longer reaction times. The analytical performance of the first three modes was critically compared for the selenium chemistry: Flow reversal provided greatest sensitivity, but over the smallest linear dynamic range. Lastly, studies which combined merging zones and flow-reversal modes have attempted to characterize two-variable chemical interactions over a wide range of concentrations within a single complex experiment.

Text

2011-02-18

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http://hdl.handle.net/2429/31513

Chemistry

University of British Columbia

Graduate