Membrane-dialyzer injection loop for enhancing the selectivity of anion-responsive liquid-membrane electrodes in flow systems Part 2. A selective sensing system for salicylate

Abstract

An electrode-based flow-injection system suitable for the direct determination of salicylic acid is described. The system utilizez a tubular polymer membrrane electrode based on manganese(III) tetraphenylporphyrin chloride to sense salicylate ions formed in a recipient buffer solution held within the upper channel of a flow-through membrane dialyzer assembly. Samples containing salicylic acid are manually intoduced into the lower channel of the dialysis unit, in which a thin silicone rubber membrane separates the two channels. The analyte is trapped across the membrane as salicylate ions within a static layer of an appropriate recipient buffer. After a fixed trapping time, the recipient plug is flushed to the electrode in a conventional flow-injection manner. Peak potentials observed are logarithmically related to the salicylic acid concentrations in the original sample. Without the dialysis unit, the electrode response to salicylate is nearly Nernstian over the range 2 x 10-6-10-2 M. In the complete flow/dialysis system, near Nernstian response was achieved for 10-4-10-2 M salicylate with a 2-min trapping time. Detection limits can be altered by changing the trapping time. Anionic salicylate can be determined by acidifying the sample. The resulting system offers very high selectivity for salicylate (as salicylic acid) over most inorganic and organic anions normally found in blood. Preliminary studies demonstrate the practical application of this system for the determination of salicylate in serum.