Flow Injection Analysis

(adapted from McKelvie, I.D., Analytical Testing Technology 20, 20-24 (1999).

General description

Flow Injection Analysis (FIA) is a continuous flow technique which is ideally suited to rapid automated analysis of liquid samples. The technique was first reported by Ruzicka and Hansen in 1975, and research on this technique has subsequently been rapid and widespread, resulting in the publication of more than 10000 papers during the period 1975-2000.

In a flow injection analyser, a small, fixed volume of a liquid sample is injected as a discrete zone using an injection device into a liquid carrier which flows through a narrow bore tube or conduit. The sample zone is progressively dispersed into the carrier, initially by convection, and later by axial and radial diffusion, as it is transported along the conduit under laminar flow conditions. Reagents may be added at various confluence points and these mix with the sample zone under the influence of radial dispersion, to produce reactive or detectable species which can be sensed by any one of a variety of flow-though detection devices. The height or area of the peak-shaped signal thus obtained can be used to quantify the analyte after comparison with the peaks obtained for solutions containing known concentrations of the analyte.

The whole process of sample/standard injection, transport, reagent addition, reaction and detection can be accomplished very rapidly (seconds to 10's of seconds), using minimum amounts of sample and reagents, and with excellent reproducibility (e.g. coefficient of variation, CV, generally < 2%).

Although complete equilibrium may not be achieved during this process, quantitation is possible because both standards and samples are dispersed to the same extent and processed in an identical manner.

FIA differs markedly from the other most common continuous flow analysis method, segmented continuous flow analysis (SCFA), a technique which involves the separation of sample and a wash solution by air bubbles in order to avoid cross contamination between samples. In SCFA, turbulent flow conditions apply, complete sample dispersion occurs, and a steady-state condition is attained prior to analyte detection. Sample thoughput in SCFA is therefore generally slower than in flow injection analysis, although the former has until recently been more amenable to multi-channel, multi-parameter analysis.