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# The use of porous disks in flow injection analysis for advancements in sample introduction and liquid-liquid extraction

Abstract (Summary)
A porous disk device is described that was used for the introduction of sample solutions into a flow system and for on-line extractions. A 0.5$\mu$m 316L stainless steel disk with a 6.35mm diameter and 1.6mm thickness was placed in a Teflon piece of the same thickness and then sandwiched between two blocks of Lucite. Flow channels were made on either side of the disk by cutting oval-shaped holes in Teflon spacers inserted between the disk and the blocks. The disk's void volume was determined with the device placed in a computer-controlled flow manifold. Samples of As(III) were injected, collected in a titration cell and titrated with electrogenerated Ce(IV). The generated titration curves were used to determine the microequivalents of Ce(IV) at the endpoint which were then used with the titration reaction stoichiometry to determine the volume of As(III) injected. The y-intercept of a plot of As(III) volume injected vs. injection times adjusted for flow rates was the void volume of the porous disk. Three determinations using this experiment yielded an average disk volume of 20.60 $\pm$ 0.32$\mu$L. Reproducible injections of a nickel sulfate solution were demonstrated. The device was adapted to perform on-line extractions of caffeine from aqueous solutions into methylene chloride. The aqueous solutions were flowed past the porous disk so that the caffeine was extracted into the CH$\sb2$Cl$\sb2$ contained in the disk. A dispersion of 6.6 was determined for the injection of the plug of solution in the disk into the flow stream and delivery to the detector. A calibration curve of the steady state absorbance vs. concentration of caffeine solutions in $\rm CH\sb2Cl\sb2$ yielded a molar absorptivity coefficient of 6785 M$\sp{-1}$cm$\sp{-1}$. Extraction efficiencies from 0.53 to 2.1%, were obtained for a series of aqueous caffeine solutions ranging in concentration from 0.10002mM to 10.002mM and using a sample volume of 393$\mu$L. The effect of increasing the time that the aqueous caffeine solution was exposed to the $\rm CH\sb2Cl\sb2$ was investigated. The extraction efficiencies increased for hold times up to 180s, after which a decrease in extraction efficiencies was observed. Finally, the feasibility of analyzing real samples containing caffeine was demonstrated through the on-line analysis of a cola sample.
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