Mixing at Confluence Point
The characteristic feature of Flow Injection system is the confluence point, where two streams merge at a T formed junction. In contrast, Sequential Injection systems are single stream systems, with no confluence points (Chapter 2).
The advantage of mixing by T configuration is that it promotes, when well designed, an instant radial mixing and also an even reagent concentration along the entire length of the injected sample zone. It also allows addition of several reagent in sequence, as it may be required by a given assay protocol. The chief drawback of this flow system is that each T junction has to be served by a separate pumping device. This is why almost all FI instruments employ multichannel peristaltic pumps to move carrier and reagent streams that merge at confluence points.
The sample is injected by means of a two-position injection valve with a fixed injection loop. In the traditional (cFI) systems the pump moves solutions at a constant flow rate forward, thus providing a defined time frame for samples and standards to react, as they are serially injected. In the flow programmed (pFI) systems a fast flow is applied when sample moves past the confluence point to promote mixing, while slower flow is applied to move reacting mixture towards the detector in order to increase the incubation time. In this way, all samples and standards are processed in exactly the same way, and the standards yield readout for the construction of a calibration curve.
1.2.15.
