The Flow Injection Analyser

When a long sample residence time is required, e.g., for slow reactions, the option is normally a segmented flow system, whereas unsegmented flow systems, such as the flow injection analyser, are generally preferred when enhanced versatility is required. 

The most outstanding characteristics of the flow injection analyser are the beauty of simplicity and its worldwide acceptance as an analytical tool [23,24], 

Axial dispersion is the main process that governs the continuous mixing of the sample with the surrounding solution in a flow injection 

The dispersion process can be better understood by considering a model single line system into which a solution of dye A is inserted into an inert carrier stream. At the time of insertion (time zero), the plug of this solution is ideally a perfect cylinder and the associated concentration/ time function is hypothetical and rectangular in shape (Fig. 5.9a). 

As the sample plug starts being pushed forward, axial diffusion is the main component of the dispersion process, due to the high concentration gradients at the sample/carrier stream interface. The hypothetical peak shape associated with the flowing sample is shown in Fig. 5.9b, which corresponds to the first theoretical Taylor solution [28,29] for the diffusive-convective equation (Eq. 3.4). Situations associated with Fig. 5.9a,b never occur in practice in flow injection analysis. 

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There are many variants of analytical flow systems, e.g., segmented flow analysis, flow injection analysis, sequential injection analysis, multisyringe flow injection analysis, batch injection analysis, mono-segmented flow analysis, flow-batch analysis, multi-pumping flow analysis, all injection analysis and bead injection analysis, all of which have acronyms [176]. In view of the existence of several common features, however, all flow analysers can be broadly classified as either segmented or unsegmented, with the most common example of the later mode being the flow injection analyser. 

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 sampling 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. Because the volume of the flow injection manifold is small, typically less than 2 mb, consumption of reagents is substantially less than with conventional methods. This can lead to a significant decrease in the cost per analysis. Flow injection analysis requires additional equipment, beyond that used for similar conventional methods of analysis, which adds to the expense of the analysis. On the other hand, flow injection analyzers can be assembled from equipment already available in many laboratories. 

Most flow injection analyses use peak height as the analytical signal. When there is insufficient time for reagents to merge with the sample, the result is a split-peak, or doublet, due to reaction at the sample s leading and trailing edges. This experiment describes how the difference between the peak times can be used for quantitative work. 

Luminol-based chemiluminescence methods have also been employed for detection of analytes in flowing stream analytical techniques such capillary electrophoresis (282), flow injection analyses, and hplc (267). AppHcations of the enhanced luminol methodology to replace radioassay methods have been developed for a number of immunological labeling techniques (121,283). 

K.A. Law and S.PJ. Higson, Sonochemically fabricated acetylcholinesterase micro-electrode arrays within a flow injection analyser for the determination of organophosphate pesticides. Biosens. Bioelectron. 20, 1914-1924 (2005). 

Sakamoto [243] determined picomolar levels of cobalt in seawater by flow injection analysis with chemiluminescence detection. In this method flow injection analysis was used to automate the determination of cobalt in seawater by the cobalt-enhanced chemiluminescence oxidation of gallic acid in alkaline hydrogen peroxide. A preconcentration/separation step in the flow injection analysis manifold with an in-line column of immobilised 8-hydroxyquinoline was included to separate the cobalt from alkaline-earth ions. One sample analysis takes 8 min, including the 4-min sample load period. The detection limit is approximately 8 pM. The average standard deviation of replicate analyses at sea of 80 samples was 5%. The method was tested and inter calibrated on samples collected off the California coast. 

The flow injection AAS system with online preconcentration will challenge the position of the graphite furnace technique, because it yields comparable sensitivity at much lower cost by using simpler apparatus and separation mode. The method offers unusual advantages when matrices with high salt content (e.g., seawater) are analysed, because the matrix components do not reach the nebuliser. 

Flow injection methodologies are highly suitable for implementing CL analyses using low-pressure continuous mixing. There are many reported applications of this type including immobilized reactants [13] or enzymes [14], One recent example is the flow injection manifold used for the determination of poly-... 

Flow-injection analysers available range from relatively low-cost unsophisticated instruments such as those supplied by Advanced Medical Supplies, Skalar and ChemLab to the very sophisticated instruments such as the FIA star 5010 and 5020 supplied by Tecator (Table 1.2). 

Methods based on flow-injection analyses have been described for the determination of extractable [62, 64-66] and available [67] phosphorus in soils. 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

Automation is especially advantageous for analysing large numbers of samples on a routine basis. The flow injection method requires low sample volumes, hut even the recommended 600 pi loop size can be reduced to approximately 100 pi without substantial losses in sensitivity, accuracy or precision. In certain applications involving ICPs samples as small as 20 pi have been reported [7]. There is Httle doubt that sample introduction with a flow-injection valve and driven by a peristaltic pump or another... 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

Three recent reviews specifically cover HPLC methods for quantitating riboflavin in foods. In addition to HPLC methods, Nielsen (81) summarized paper chromatography, TLC, and open-column chromatography procedures for quantitating total riboflavin and the individual vitamers in foods, pharmaceuticals, and biological samples. Russell (44) included a brief discussion of the standard methods, along with HPLC and flow injection analyses published between 1990 and 1994 for total riboflavin and the individual vitamers in foods. Ball (45) reviewed HPLC methods for quantitation of riboflavin, as well as chemical and microbiological riboflavin assays for foods. 

The advantages of pulsed ultrafiltration-mass spectrometry include the variety of different applications that may be carried out, the convenience of on-line screening, solution-phase screening, the ability to screen either combinatorial libraries or natural product extracts, the diversity of receptors that may be screened, and the freedom to use either volatile or non-volatile binding buffers. For metabolic and toxicity screening, flow injection analyses have the additional advantages that product feedback inhibition is prevented so that the metabolic profile more closely approximates the in vivo system ( 70). Finally, the... 

Flow injection procedures are very useful for performing trace analyses in highly concentrated salt solutions. Fang and Welz [270] showed that the flow rate of the carrier solution can be significantly lower than the aspiration rate of the nebulizer. This allows even higher sensitivities than with normal sample delivery can be obtained. Despite the small volumes of sample solution, the precision and the detection limits are practically identical with the values obtained with continuous sample nebulization. The volume, the form of the loop (single loop, knotted reactor, etc.) and the type and length of the transfer line between the flow injection system and the nebulizer considerably influence the precision and detection limits that are attainable. 

One can then infer that flow injection analysis relies on three cornerstone features sample injection, controlled dispersion and reproducible timing [50]. A typical flow injection analyser and the related recorder output are shown in Fig. 2.7. 

FIGURE 2.7 How diagram of a typical flow injection analyser and the associated recorder tracing. S = sample C — sample/ wash carrier stream R = reagent Rc = coiled reactor D — detector arrows = sites where pumping is applied. 

Development of the sequential injection analyser, as well as intensive studies exploiting beads (including magnetic beads) [80—82], led to the appearance of flow analysers incorporating bead injection [83] and lab-on-valve [84] devices. 

Another alternative for designing multi-commuted flow systems operating in the pumping mode is to exploit syringes as fluid propulsion devices. This led to the proposal of multi-syringe flow injection analyser by Albertus et al. in 1999 [109] as an advanced means of managing multichannel flow analysis. 

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