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.