Segmented-flow method

Despite the fact that the FIA methodology Involves kinetic, physical and chemical aspects which may a priori lead anyone into thinking that the results obtained will be anything but precise, the reproducibility levels achieved are in fact fairly high, even comparable to those attained by manual and segmented flow methods. This is possibly one of the keys to the success of FIA. 

The alcohol oxidation reactions described herein use inexpensive O2 and NO sources, and there is potential for their use in large-scale industrial processes. Continuous flow methods provide a particularly strategic opportunity for large-scale applications (see Chapter 23). Hermans and coworkers demonstrated a segmented-flow method for their amberlyst-15/NO -catalyzed aerobic oxidation... 

Since 1970, new analytical techniques, eg, ion chromatography, have been developed, and others, eg, atomic absorption and emission, have been improved (1—5). Detection limits for many chemicals have been dramatically lowered. Many wet chemical methods have been automated and are controlled by microprocessors which allow greater data output in a shorter time. Perhaps the best known continuous-flow analy2er for water analysis is the Autoanaly2er system manufactured by Technicon Instmments Corp. (Tarrytown, N.Y.) (6). Isolation of samples is maintained by pumping air bubbles into the flow line. Recently, flow-injection analysis has also become popular, and a theoretical comparison of it with the segmented flow analy2er has been made (7—9). 

The measurement of absorbance of light by a dyestuff-anionic surfactant complex, which has been extracted into an organic solvent is a key feature of many methods, and Sodergren has successfully used segmented flow colorimetry for an automated version of this procedure (2 ). An alternative is the two phase titration technique, pioneered by Herring (3) which uses dimidium... 

Basel et al. [23] have described methods of compensating for chloride, ammonia, and bicarbonate interferences in determining bromide in sabne waters with an automated segmented flow analyser utihsing the phenol red method. 

Two independent groups of scientists developed FIA in the middle of seventies Ruzicka and Hansen in Denmark [3] and Stewart et al. in the United States [15] and since then it has been developed rapidly. The first group developed the method using primarily instrumentation normally associated with segmented flow analyzers (SFA). In contrast, the second group based their initial work on HPLC components. These two different origins of FIA are responsible for several features of the technique related to SFA and /or HPLC that may characterize FIA as a hybrid of SFA and HPLC. 

We will only be dealing with segmented-flow methodology in manual, but there are sure to be equivalent FIA methods available. 

The industrially important nitration of aromatic compounds in a microreactor using two immiscible liquid phases was demonstrated in different studies using either parallel [220] or segmented flow [221]. In all studies, a PTFE capillary microchannel, connected to an inlet junction, was used in which either segmented or parallel flow can be created. The use of PTFE tubing is desirable as it is commercially available and no complicated microfabrication methods are involved. 

Koops, J., Klomp, H., van Hemert, H. 1990. Rapid enzymatic assay of free fatty acids lipolysis in farm tank milk by a segmented continuous flow method. Comparison of the results with those obtained by the BDI procedure. Neth. Milk Dairy J. 44, 3-19. 

Flow injection analysis (FIA) is a continuous flow method in which highly precise sample volumes are introduced into a stream that is segmented or non-segmented. The... 

Flow injection analysis is a continuous flow method in which highly precise sample volumes are introduced into a stream using segmented or unsegmented flow. The method must be accurate, precise and reproducible before it can be considered as a useful technique and the following test proves that this technique does meet all the requirements. Tyson [3], carried out several studies involving flow injection techniques and atomic spectroscopy with considerable success. 

In continuous flow methods, the sample is inserted into a flowing stream, where a number of operations can be performed before it is transported to a flow-through detector. Hence, these systems behave as automated analyzers in that they can perform not only sample processing operations but also the final measurement step. Such sample processing operations as reagent addition, dilution, incubation, mixing, dialysis, extraction, and many others can be implemented between the point of sample introduction and detection. There are two different types of continuous flow systems segmented flow analyzers and flow injection analyzers. 

L. Solujic, E.B. Milosavljevic, M.R. Straka, Total cyanide determination by a segmented flow injection-on-line UV digestion-amperometric method, Analyst 124 (1999) 1255. 

Instant localisation of the sample zone is also useful for improving analytical methods implemented in mono-segmented flow systems. These systems utilise sample insertion between two air bubbles to minimise sample dispersion, and thus carryover (see 5.5.1). Several sample zones can be handled simultaneously, which also improves the sampling rate. Precise and instant localisation of the flowing sample inside the analytical path can be achieved in the presence of the air bubbles, as optical switches can be used to sense the gas—liquid interfaces. Feedback mechanisms relying on changes in the logic status of the switches make... 

Versatility. Flow-injection analysis is much more versatile than segmented-flow analysis the former allows for the implementation of a variety of techniques (titrations, stopped-flow methods, etc) which are inapplicable with the latter. 

In dealing with the most oustandlng features of FIA, only those clearly distinguishing this methodology from segmented-flow or conventional manual methods will be considered. 

---