Continuous flow-analysis

Gunasingham, H. and Fleet, B., Hydrodynamic voltammetry in continuous-flow analysis, in Electroanalytical Chemistry A Series of Advances, Vol. 16, Bard, A. J., Ed., Marcel Dekker, New York, 1989, 89. 

Continuous flow analysis (CFA) (Skeggs), since 1960 Segmented flow Improved ISEs Tubular electrodes (Blaedel) Adapted ISFETs Special sampling requirements in plant and environment control (Sections 5.5 and 5.6)... 

Fig. 5.19 Continuous flow analysis with air segmentation (Skeggs). 

Flow injection analysis is a rapid method of automated chemical analysis that allows for quasi-continuous recording of nutrient concentrations in a flowing stream of seawater. The apparatus used for flow injection analysis is generally less expensive and more rugged than that used in segmented continuous flow analysis. A modified flow injection analysis procedure, called reverse flow injection analysis, was adopted by Thompson et al. [213] and has been adapted for the analysis of dissolved silicate in seawater. The reagent is injected into the sample stream in reverse flow injection analysis, rather than vice versa as in flow injection analysis. This results in an increase in sensitivity. 

This analytical procedure is based on an optimum analysis condition for segmented continuous flow analysis. The sample is combined with a molybdate solution at a pH between 1.4 and 1.8 to form the //-molybdosilicic acid. After an appropriate time for reaction, a solution of oxalic acid is added, which transforms the excess molybdate to a non-reducible form. The oxalic acid also suppresses the interference from phosphate by decomposing phosphomolyb-dic acid. Finally, a reductant is added to form molybdenum blue. Both ascorbic acid and stannous chloride were tested as reductants. 

FTA [5-7] is a version of continuous-flow analysis based on a nonsegmented flowing stream into which highly reproducible volumes of sample are injected, carried through the manifold, and subjected to one or more chemical or biochemical reactions and/or separation processes. Finally, as the stream transports the Anal solution, it passes through a flow cell where a detector is used to monitor a property of the solution that is related to the concentration of the analyte as a... 

Phosphate, iron, sulphate Continuous-flow analysis [30]... 

Some coupled systems allow measurement of the main N and P forms (nitrate, ammonia and orthophosphates) [22,27,29], among which is a system based on membrane technology in combination with semi-micro continuous-flow analysis (pCFA) with classical colorimetry. With the same principle (classical colorimetry), another system [30] proposes the measurement of phosphate, iron and sulphate by flow-injection analysis (FIA). These systems are derived from laboratory procedures, as in a recent work [31] where capillary electrophoresis (CE) was used for the separation of inorganic and organic ions from waters in a pulp and paper process. Chloride, thiosulphate, sulphate, oxalate,... 

The 1950s saw the introduction of a completely new approach to automation, in the form of continuous flow analysis. This made a significant contribution to the advance of automated analysis and subsequent development has been in the form of flow injection analysis. The original instruments were single channel and capable of measuring only one constituent in each sample. Multichannel instruments were then developed which could simultaneously carry out several different measurements on each sample. These were useful in laboratories where many samples required the same range of tests. 

Segmentation of the liquid streams by air bubbles is a feature of continuous flow analysis. 

Quantitation in continuous flow analysis is based on the direct comparison of the peak heights on a recorder trace of the samples and standards. It is not necessary for the reactions to go to completion, because all the measurements in a particular method are made after the same fixed reaction time, which is determined by the length of analyser tubing and relies on a constant pump speed. 

The mixing of the sample and reagents in predetermined proportions rather than specific volumes is a fundamental principle of continuous flow analysis and is achieved by using a constant-speed roller bar peristaltic pump and flexible plastic delivery tubes of different internal diameters. These pump tubes are stretched between two plastic end-blocks, which hold them taut between a... 

In continuous flow analysis, samples follow one another through the tubing and interaction of adjacent samples, which is called carry-over , does occur. Carry-over manifests itself on the recorder trace as peaks that are not completely differentiated. This is most noticeable when a sample of low concentration follows one of high concentration and is either seen as a shoulder on... 

In air segmented continuous flow analysis, air bubbles help to reduce the interaction between adjacent samples (carry-over)... 

Matrix effects in the analysis of nutrients in seawater are caused by differences in background electrolyte composition and concentration (salinity) between the standard solutions and samples. This effect causes several methodological difficulties. First, the effect of ionic strength on the kinetics of colorimetric reactions results in color intensity changes with matrix composition and electrolyte concentration. In practice, analytical sensitivity depends upon the actual sample matrix. This effect is most serious in silicate analysis using the molybdenum blue method. Second, matrix differences can also cause refractive index interference in automated continuous flow analysis, the most popular technique for routine nutrient measurement. To deal with these matrix effects, seawater of... 

The problem of selectivity can often be efficiently solved when using flow techniques, such as continuous-flow analysis or flow-injection analysis (see section 5.3). 

Routine analyses of large numbers of similar samples can readily be automated and the sample throughput considerably increased (sometimes up to about 200 samples per hour) by carrying out the analyses in a continuously flowing medium. At present there are two basic approaches to the problem, the older technique of continuous-flow analysis (CFA) introduced more than 25 years ago [145] and widely developed by the Technicon Company (Auto-Analyzer), and more recent flow-injection analysis (FIA for a recent literature review see [123]). For a brief comparative survey of the two methods see [148]. 

Liquid-liquid Extraction in Continuous Flow Analysis... 

Valcarcel, M. Liquid-Liquid Extraction in Continuous Flow Analysis. In Developments in Solvent Extraction, S. Alegret, Ed., Ellis Horwood Series in Analytical Chemistry West Sussex, UK, 1988 p. 135. 

Furman, W. B. Continuous Flow Analysis Theory and Practice, Dekker New York, 1976. 

Unsegmented By aspiration Continuous Continuous Completely continuous-flow analysis (CCFA)... 

Fig. 2.14 Standard response crrrve for continuous flow analysis of discrete samples.

Solvent extraction can be automated in continuous-flow analysis. For both conventional AutoAnalyzer and flow-injection techniques, analytical methods have been devised incorporating a solvent extraction step. In these methods, a peristaltic pump dehvers the hquid streams, and these are mixed in a mixing coil, often filled with glass ballotini the phases are subsequently separated in a simple separator which allows the aqueous and organic phases to stratify. One or both of these phases can then be resampled into the analyser manifold for further reaction and/or measurement. The sample-to-extractant ratio can be varied within the limits normally applying to such operations, but the maximum concentration factor consistent with good operation is normally about 3 1. 

Fig. 4.2 Improved flash distillation apparatus for continuous-flow analysis.

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