Theory and Practice
Flow injection analysis (FIA) was developed in the mid-1970s as a highly efficient technique for the automated analyses of samples. Unlike the centrifugal analyzer described earlier, in which samples are simultaneously analyzed in batches of limited size, FIA allows for the rapid, sequential analysis of an unlimited number of samples. FIA is one member of a class of techniques called continuous- flow analyzers, in which samples are introduced sequentially at regular intervals into a liquid carrier stream that transports the samples to the detector.
Figure 13.16 is a schematic diagram detailing the basic components of a flow injection analyzer. The reagent serving as the carrier is stored in a reservoir, and a propelling unit maintains a constant flow of the carrier through the system of tub- ing comprising the instrument. The sample is injected directly into the flowing car- rier stream, where it travels through a mixing and reaction zone before passing through the detector’s flow-cell. Figure 13.16 is the simplest design for a flow injec- tion analyzer, consisting of a single channel with one reagent reservoir. Multiple- channel instruments, in which reagents contained in separate reservoirs are combined by merging channels, also are possible. A more detailed discussion of FIA instru- mentation is found in the next section.
When a sample is injected into the carrier stream it has the rectangular flow profile (of width w) shown in Figure 13.17a. As the sample is carried through the mixing and reaction zone, the width of the flow profile increases as the sample dis- perses into the carrier stream. Dispersion results from two processes: convection due to the flow of the carrier stream and diffusion due to a concentration gradient between the sample and the carrier stream. Convection of the sample occurs by laminar flow, in which the linear velocity of the sample at the tube’s walls is zero, while the sample at the center of the tube moves with a linear velocity twice that of the carrier stream. The result is the parabolic flow profile shown in Figure 13.7b. Convection is the primary means of dispersion in the first 100 ms following the sample’s injection.
The second contribution to the sample’s dispersion is diffusion due to the con- centration gradient between the sample and the carrier stream. Diffusion occurs parallel (axial) and perpendicular (radial) to the flow of the carrier stream, with only the latter contribution being important. Radial diffusion decreases the linear velocity of the sample at the center of the tubing, but the sample at the edge of the tubing experiences an increase in its linear velocity. Diffusion helps to maintain the integrity of the sample’s flow profile (Figure 13.17c), preventing samples in the car- rier stream from dispersing into one another. Both convection and diffusion make significant contributions to dispersion from approximately 3–20 s after the sample’s injection. This is the normal time scale for a flow injection analysis. After approxi- mately 25 s, diffusion becomes the only significant contributor to dispersion, result- ing in a flow profile similar to that shown in Figure 13.17d.
An FIA curve, or “fiagram,” is a plot of the detector’s signal as a function of time. Figure 13.18 shows a typical fiagram for conditions in which both convection and diffusion contribute to the sample’s dispersion. Also shown on the figure are several parameters used to characterize the fiagram. Two parameters are used to de- fine the time required for the sample to move from the injector to the detector. The travel time, ta, is the elapsed time from the sample’s injection to the arrival of the leading edge of its flow profile at the detector. Residence time, T, on the other hand, is the time required to obtain the maximum signal. The difference between the resi- dence time and travel time is given as t’. The value for t’ approaches 0 when convec- tion is the primary means of dispersion and increases in value as the contribution from diffusion becomes more important.
The time required for the sample to pass through the detector’s flow cell, and for the signal to return to the baseline, is also described by two parame- ters. The baseline-to-baseline time, ∆t, is the elapsed time between the arrival of the leading edge of the sample’s flow profile to the departure of its trailing edge. The elapsed time between the maximum signal and its return to the baseline is called the return time, T ‘. The final characteristic parameter of a fi- agram is the peak height, h, which is equivalent to the difference between the maximum signal and the signal at the baseline.
Of the six parameters shown in Figure 13.18, the most important are peak height and return time. The peak height is related, directly or indirectly, to the analyte’s concentration and is used for quantitative work. The sensitiv- ity of the method, therefore, is also determined by the peak height. The return time determines the frequency with which samples may be injected. Figure 13.19 shows that when a second sample is injected at a time T ‘ after injecting the first sample, the overlap of the two FIA curves is minimal. By injecting samples at intervals of T ‘, the maximum sampling rate is realized.
Peak heights and return times are influenced by the dispersion of the sample’s flow profile and are influenced by the physical and chemical properties of the flow injection system. Physical parameters affecting the peak height and return time in- clude the volume of sample injected; the flow rate; the length, diameter, and geom- etry of the mixing and reaction zone; and the presence of mixing points where sep- arate channels merge together. The kinetics of any chemical reactions involving the sample and reagents in the carrier stream also influences the peak height and re- turn time.
Unfortunately, there is no good theory that can be used to consistently predict the peak height and return time for a given set of physical and chemical parameters. The design of a flow injection analyzer for a particular analytical problem still oc- curs largely by a process of experimentation. Nevertheless, some general observa- tions about the effects of physical and chemical parameters can be made. In the ab- sence of chemical effects, sensitivity (larger peak height) is improved by injecting larger samples, increasing the flow rate, decreasing the length and diameter of the tubing in the mixing and reaction zone, and merging separate channels before the point where the sample is injected. Except for sample volume, an improvement in the sampling rate (smaller return time) is achieved by the same combination of physical parameters. Larger sample volumes, however, lead to longer return times and a decrease in sample throughput. The effect of chemical reactivity depends on whether the species monitored by the detector is a reactant or a product. For exam- ple, when the monitored species is a reactant, sensitivity is improved by selecting a combination of physical parameters that enables the sample to reach the detector more quickly. Adjusting the chemical composition of the carrier stream in a man- ner that decreases the rate of the reaction also improves sensitivity in this case.