Sequential injection uses programmable, bi-directional discontinuous flow, precisely choreographed by computer control. Sample (A red) and reagent (B blue) are injected sequentially, by means of a multiposition valve (MPV), into a carrier stream, driven by a single syringe pump, placed upstream of the valve. As the reaction product (C yellow) starts to form at the interface of stacked zones, a flow reversal (D,E) is applied, to further promote mixing and to transport the reaction mixture into the detector for monitoring.
Since the movement of sequentially injected zones is computer controlled, the time interval between sample injection (A) and analyte detection (E) is reproducible. In this way, all samples are processed in exactly the same way, allowing comparison of standards with unknowns.