ABSTRACT
Flow-injection analysis (FIA) may be defined as an automated or semiautomated analytical process consisting of a sequential insertion of discrete sample solutions into an unsegmented continuously flowing liquid stream with subsequent detection of the analyte. It is a relatively new analytical process, which shows considerable potential for high-speed precise analysis of discrete samples [1]. This definition, however, was soon considered obsolete and was revised to describe a technique for “information gathering from a concentration gradient formed from an injected, well-defined zone of a fluid, dispersed into a continuous unsegmented stream of a carrier” in order to accommodate new developments in stopped-flow FIA, merging zones, zone sampling and other gradient techniques. This new definition was soon challenged by FI systems which were segmented in one way or another, or which dealt with samples eluted from columns without well-defined boundaries. Furthermore, FIA defined as “A flow analysis technique performed by reproducibly manipulating sample and reagent zones in a flow stream under thermodynamically nonequilibrated conditions.” The main important parts of a flow-injection analytical system are a unit for propelling liquids through the manifold system for introducing a sample into a continuously flowing stream of a carrier, an appropriate detector [2], and a transport system linking various elements which make up the FIA system and allowing the sample to attain a suitable degree of dispersion or mixing as it travels through it. When the extent of dispersion is not suitable for the experiment concerned and a reaction or further splitting of the flowing stream is required, the system can be supplemented with accessories such as mixing chambers, reactors and merging points (Fig. 10.1) [3]. In the case of the FIA technique the physical equilibrium (flow homogenization) is never reached at the moment of detection. Moreover, it is not necessary for the chemical equilibrium to be obtained at the moment of detection. The concept of FIA depends on a combination of three factors: reproducible sample injection volumes, controllable sample dispersion, and reproducible timing of the injected sample through the flow system. Except for detector warm-up, the system is ready for instant operation as soon as the sample is introduced. FIA offers several advantages in term of considerable decrease in sample (normally using 10 to 50 μL) and reagent consumption, high sample throughput (50 to 300 samples per hour) reduced residence times (reading time is about 3 to 40 s), shorter reaction times (3 to 60 s), easy switching from one analysis to another (manifolds are easily assembled and/or exchanged), reproducibility (usually less than 2% RSD), reliability, low carry over, high degree of flexibility, and ease of automation. Perhaps the most compelling advantage of the FIA technique is the great reproducibility in the results obtained by this technique that can be set up without excessive difficulties and at very low cost of investment and maintenance. These advantages have led to an extraordinary development of FIA, unprecedented in comparison to any other technique.
