Foundation of flow-injection analysis

The physical foundation of FIA Is related to the behaviour of the sample plug Inserted In the flow, which Is characterized mathematically by means of the so-called dispersion. This, In turn, is defined by the shape of the profile yielded by the Injected sample portion along the system —particularly at the flow-cell. 

Although the nil mixing between successively Injected samples was formerly attributed to the turbulent regime under which FIA experiments were believed to be conducted [1,2], It has been demonstrated beyond doubt [3,4] that most FIA systems operate under laminar flow conditions (Fig. 6.3). 

When the sample is injected Into the flowing stream, its dilution within it is a function, of, among other factors, the time considered along the transport process. Initially, dilution Is chiefly convective In nature later it becomes convective-diffusional and eventually develops a purely dlffuslonal character. The intermediate situation (l.e. convective-diffusional transport) is by far the most common In FIA. 

It has been demonstrated that radial dispersion contributes more significantly to the dilution of the sample in the flow than does axial dispersion. This type of fluid movement, termed secondary flow by TIJssen [4], results in a washout effect accounting for the low mutual contamination of samples successively Injected Into a carrier stream. This advantageous feature Is a result of the use of low flow-rates and small tubing bores, and results In decreased peak-width and hence to increased sampling rate. 

Theoretical studies on FIA have been aimed at the establishment of accurate relationships between the geometric (length and diameter) and hydrodynamic (flow-rate) characteristics of an FIA system and the parameters defining the profile obtained (travel time, coordinates of the peak maximum and peak width). 

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