ENHANCEMENT OF FLOW-INJECTION OPTOSENSING BY SORBENT EXTRACTION AND REACTION-RATE MEASUREMENT

The molybdenum blue reaction for the determination of phosphate is used as a model to illustrate the extension of the use of hydrophobic sorbents in flow injection analysis for the preconcentration of an anion and for on-column detection, i.e., optosensing. Optosensing provides for real-time monitoring of the rate of signal change, dA/dT, so that the rate of color development during the reduction step of the analysis can be measured. The synergistic relationship between the rates of formation of the phosphate and silicate heteropoly complexes is examined. A kinetic optosensing method has been developed in which the difference in reduction rate for the heteropoly complexes allows simultaneous determination of phosphate, in the parts per billion range, and silicate, in the parts per million range. The system used an inexpensive minispec-20 spectrophotometer with a net path length through the sorbent of less than 2 mm. Partial least-squares analysis was used to analyze the data, and prediction errors of approximately 10% were obtained for both components. © 1990, American Chemical Society. All rights reserved.