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This paper introduces a novel methodology for downscaling reagent based assays to micro-and submicroliter level. It is shown that sample handling in the sequential injection mode, which employs forward, reversed and stopped flow, can be programmed to accommodate a wide variety of assays within the same microfluidic device. Solution metering, mixing, dilution, incubation and monitoring can be executed in any desired sequence in a system of channels, integrated with a multipurpose flow cell. The channel system and flow cell are fabricated as a monolithic structure mounted atop a conventional multiposition valve. In addition to compactness, the advantage of this 'lab-on-valve' system is the permanent rigid position of the sample processing channels that ensures repeatability of microfluidic manipulations, controlled by conventional sized peripherals. With the exception of the integrated microconduit system, that has been designed and mesofabricated by computer aided design (CAD) technology, all peripherals (sequential injection system, fiber optic UV/VIS spectrophotometer-fluorometer) are conventional sized and commercially available components. This provides proven robustness and reliability of operation, and makes the microfluidic system compatible with real life samples and peripheral instruments. The system has been characterized by dye injection, to provide guidelines for method development. Its versatility is documented by a phosphate assay, enzymatic activity assay of protease and by a bioligand interaction assay of immunoglobulin G (IgG) based on its interaction with protein G immobilized on Sepharose beads.

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A rapid, automated, microanalytical procedure for the determination of 90 Sr in aged nuclear waste has been developed. It is based on a sequential injection analysis (SIA) system which rapidly separates 90 Sr from 90 Y, 137 Cs, and other radionuclides. The isolated 90 Sr is then detected on-line with a flow-through liquid scintillation counter. The separation is achieved using a sorbent extraction minicolumn containing a resin (EIChrom, Sr-Spec) that selectively binds 90 Sr as a crown ether complex under acidic conditions. The 90 Sr is eluted with water, mixed with liquid scintillation cocktail, and detected in the flow cell of the counter. Sample 90 Sr activity can be quantified from peak areas, giving linear calibration curves. The instrument can also be operated in a stoppedflow mode for longer counting times. Analyses of aged nuclear waste samples from the Hanford site by the SIA method and a manual method were in excellent agreement: correlation coefficient R ) 0.994. Sample analysis by the SIA method is complete in less than 40 min. The advantages of the new 90 Sr analyzer include faster analysis, greater precision, reduced labor costs, and reduced secondary waste. Worker safety is improved because solution handling operations are fully automated and contained.

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