Flow Injection Analysis: Evaluation

Evaluation

The majority of FIA applications are modifications of conventional titrimetric, spectrophotometric, and electrochemical methods of analysis. For this reason it is appropriate to evaluate FIA in relation to these conventional methods. The scale of operations for FIA allows for the routine analysis of minor and trace analytes and for macro-, meso-, and microsamples. The ability to work with microliter injection volumes is useful when the sample is scarce. Conventional methods of analysis, however, may allow the determination of smaller concentrations of analyte.

The accuracy and precision of FIA are comparable to that obtained by conven- tional methods of analysis. The precision of a flow injection analysis is influenced by variables that are not encountered in conventional methods, including the stabil- ity of the flow rate and the reproducibility of the sample’s injection. In addition, re- sults from FIA may be more susceptible to temperature variations. These variables, therefore, must be carefully controlled.

In general, the sensitivity of FIA is less than that for conventional methods of analysis for two principal reasons. First, as with chemical kinetic methods, measure- ments in FIA are made under nonequilibrium conditions when the signal has yet to reach its maximum value. Second, dispersion of the sample as it progresses through the system results in its dilution. As discussed earlier, however, the variables that in- fluence sensitivity are known. As a result the FIA manifold can be designed to opti- mize the sensitivity of the analysis.

Selectivity in FIA is often better than that for conventional methods of analysis. In many cases this is due to the kinetic nature of the measurement process, in which potential interferents may react more slowly than the analyte. Contamination from external sources also is less of a problem since reagents are stored in closed reser- voirs and are pumped through a system of transport tubing that, except for waste lines, is closed to the environment.

Finally, FIA is an attractive technique with respect to demands on time, cost, and equipment. When employed for automated analyses, FIA provides for very high sam- pling rates. Most analyses can be operated with sampling rates of 20–120 samples/h, but rates as high as 1700 samples/h have been realized.22 Because the volume of the flow injection manifold is small, typically less than 2 mL, consumption of reagents is substantially less than with conventional methods. This can lead to a significant de- crease in the cost per analysis. Flow injection analysis requires additional equipment, beyond that used for similar conventional methods of analysis, which adds to the ex- pense of the analysis. On the other hand, flow injection analyzers can be assembled from equipment already available in many laboratories.