The µSI Methodology
The µSI methodology, the successor to the flow injection (FI) methodology, was introduced by Ruzicka and Marshall et al. [Ref. 4]. It introduces computer control capability to the system, allowing for better integration with modern process automation. A barebones µSI system comprises a bi-directional syringe pump, a holding coil, a multi-position valve, a reactor coil, and a detector. Conventional FI protocols require physical reconfiguration of the flow manifold to perform different assays; however, a µSI system allows all experimental protocols to be scripted and selected by users.
In a reagent-based µSI assay protocol, the sample and the reagent are injected sequentially through the solvent selector valve into the holding coil, forming a layered solution profile in the tubing. The solution profile is then transported through the mixing coil and into the detector by the flow reversal action of the syringe pump after the valve switches to the flow cell. A flow-through detector records the changes in a desired physical parameter when the reaction product reaches the flow cell.
Flow programming, along with random access to sample and reagent solution ports by the valve, provides versatility since all operating protocols such as sample injection, solute dispersion, and timing are controlled by software protocols. Assays with similar characteristics can be implemented on the same µSI system without physical reconfiguration of the µSI system.