The basic components of a flow injection analyzer are shown in Figure 13.16 and include a unit for propelling the carrier stream, a means for injecting the sample into the carrier stream, and a detector for monitoring the composition of the carrier stream. These units are connected by a transport system that provides a means for bringing together separate channels and that enables an appropriate mixing of the sample with the carrier stream. Separations modules also may be incorporated in the flow injection analyzer. Each of these components is considered in greater detail in this section.
The propelling unit is used to move the carrier stream in the main channel, as well as any additional reagent streams in secondary channels, through the flow injection analyzer. Although several different propelling units have been used, the most common is a peristaltic pump. A peristaltic pump consists of a set of rollers attached to the outside of a rotating drum (Figure 13.20). Tubing from the reagent reservoirs is placed in between the rollers and a fixed plate. As the drum ro- tates the rollers squeeze the tubing, forcing the contents of the tubing to move in the direction of the rotation. Peristaltic pumps are capable of providing a constant flow rate, which is controlled by the drum’s speed of rotation and the inner diame- ter of the tubing. Flow rates from 0.0005–40 mL/min are possible, which is more than adequate to meet the needs of FIA, for which flow rates of 0.5–2.5 mL/min are common. One limitation to peristaltic pumps is that they produce a pulsed flow, particularly at higher flow rates, which may lead to oscillations in the signal.
The sample, typically 5–200 μL, is placed in the carrier stream by injec- tion. Although syringe injections through a rubber septum are used, a more com- mon means of injection is the rotary, or loop, injector used in HPLC. This type of injector provides reproducible injection volumes and is easily adaptable to automation, a feature that is particularly impor- tant when high sampling rates are desired.
Detection in FIA may be accomplished using many of the electrochemi- cal and optical detectors used in HPLC. These detectors were discussed and are not considered further in this section. In addition, FIA detectors also have been designed around the use of ion-selective electrodes and atomic ab- sorption spectroscopy.
The heart of a flow injection analyzer is the transport system used to bring together the carrier stream, the sample, and any reagents that must react with the sample to generate the desired signal. Each reagent reservoir con- nected to the flow injection analyzer is considered a separate channel. All channels must merge before the carrier stream reaches the detector, with the merging points determined by the chemistry involved in the method. The completed assembly of channels is called a manifold.
The simplest manifold includes only a single channel, the basic outline of which is shown in Figure 13.21. This type of manifold is commonly used for direct analyses that do not require a chemical reaction. In this case the carrier stream only serves as a means for transporting the sample to the detector rapidly and repro- ducibly. For example, this manifold design has been used as a means of sample in- troduction in atomic absorption spectroscopy, achieving sampling rates as high asDW700 samples/h. This manifold also is used for determining a sample’s pH or the concentration of metal ions using ion-selective electrodes.
A single-channel manifold also can be used for systems in which a chemical re- action generates the species responsible for the analytical signal. In this case the car- rier stream both transports the sample to the detector and reacts with the sample.
Because the sample must mix with the carrier stream, flow rates are lower than when no chemical reaction is involved. One example is the determination of chlo- ride in water, which is based on the following sequence of reactions.
The carrier stream consists of an acidic solution of Hg(SCN)2 and Fe3+. When a sample containing chloride is injected into the carrier stream, the chloride displaces the thiocyanate from Hg(SCN)2. The displaced thiocyanate then reacts with Fe3+ to form the reddish colored Fe(SCN)2+ complex, the absorbance of which is moni- tored at a wavelength of 480 nm. Sampling rates of approximately 120 samples/h have been achieved with this system.
Most flow injection analyses requiring a chemical reaction use a manifold containing more than one channel. This provides more control over the mixing of reagents and the interaction of the reagents with the sample. Two configura- tions are possible for dual-channel systems. The dual-channel manifold shown in Figure 13.22a is used when a mixture of the reagents is unstable. For example, in acidic solutions phosphate reacts with molybdate to form the heteropoly acid H3P(Mo12O40). In the presence of ascorbic acid the molybdenum in the het- eropoly acid is reduced from Mo(VI) to Mo(V), forming a blue-colored complex that is monitored spectrophotometrically at 660 nm. Solutions of molybdate and ascorbic acid are maintained in separate reservoirs because they are not sta- ble when mixed together. The two reagent channels are merged and mixed just before the point where the sample is injected.
A dual-channel manifold can also be used to mix a reagent in a secondary chan- nel with a sample that has been injected into the primary channel (Figure 13.22b). This style of manifold has been used for the quantitative analysis of many analytes, including the determination of chemical oxygen demand (COD) in wastewater.24 The COD is a measure of the amount of oxygen required to completely oxidize the organic matter in the water sample and is an indicator of the level of organic pollu- tion. In the conventional method of analysis, COD is determined by refluxing the sample for 2 h in the presence of acid and a strong oxidizing agent, such as K2Cr2O7 or KMnO4. When refluxing is complete, the amount of oxidant consumed in the re- action is determined by a redox titration. In the flow injection version of this analy- sis, the sample is injected into a carrier stream of aqueous H2SO4, which merges with a solution of the oxidant from a secondary channel. The oxidation reaction is kineti- cally slow. As a result, the mixing and reaction coils are very long, typically 40 m, and are submerged in a thermostated bath. The sampling rate is lower than that for most flow injection analyses, but at 10–30 samples/h it is substantially greater than that for the conventional method.
More complex manifolds involving three or more channels are common, but the possible combination of designs is too numerous to discuss in this text. One ex- ample of a four-channel manifold is shown in Figure 13.23.
Incorporating a separation module in the flow injection manifold allows separations, such as dialysis, gaseous diffusion, and liquid–liquid extraction, to be included in a flow injection analysis. Such separations are never complete, but are reproducible if the operating conditions are carefully controlled.
Dialysis and gaseous diffusion are accomplished by placing a semipermeable membrane between the carrier stream containing the sample and an acceptor stream (Figure 13.24). As the sample stream passes through the separation mod- ule, a portion of those species capable of crossing the semipermeable membrane do so, entering the acceptor stream. This type of separation module is common in the analysis of clinical samples, such as serum and urine, for which dialysis separates the analyte from its complex matrix. Semipermeable gaseous diffusion membranes have been used for the determination of ammonia and carbon diox- ide in blood. For example, ammonia is determined by injecting the sample into a carrier stream of aqueous NaOH. Ammonia diffuses across the semipermeable membrane into an acceptor stream containing an acid–base indicator. The re- sulting acid–base reaction between ammonia and the indicator is monitored spectrophotometrically.
Liquid–liquid extractions are accomplished by merging together two immis- cible fluids, each carried in a separate channel. The result is a segmented flow through the separation module, consisting of alternating portions of the two phases. At the outlet of the separation module, the two fluids are separated by taking advantage of the difference in their densities. Figure 13.25 shows a typical configuration for a separation module in which the sample is injected into an aqueous phase and extracted into a less dense organic phase that passes through the detector.