Continuous-flow segmentation

Figure 21.1 A typical continuous flow segmented manifold where sample, air, and reagent are pumped to provide segmentation between the air and sample-reagent segments. The bottom picture depicts the mixing process. (Reprinted by permission of John Wiley and Sons.)...

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). 

Sulphate In Waters, Effluents and Solids (2nd Edition) [including Sulphate in Waters, Effluents and Some Solids by Barium Sulphate Gravimetry, Sulphate In waters and effluents by direct Barium Titrimetry, Sulphate in waters by Inductively Coupled Plasma Emission Spectrometry, Sulphate in waters and effluents by a Continuous Flow Indirect Spectrophotometric Method Using 2-Aminoperimidine, Sulphate in waters by Flow Injection Analysis Using a Turbidimetric Method, Sulphate In waters by Ion Chromatography, Sulphate in waters by Air-Segmented Continuous Flow Colorimetry using Methylthymol Blue], 1988... 

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 has been adapted to automatic air-segmented continuous flow systems, e.g., the Technicon AutoAnalyzer system. Several reducing... 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

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. 

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

In this technique, which was developed in the 1970s, microlitre volumes of liquid sample are injected, at intervals, into a continuously flowing carrier stream which is not air-segmented. Various reagent streams are introduced as required and controlled mixing of reagents and sample occurs. The fact that flow injection analysis does not involve air-segmented streams makes it possible to include such separation steps as solvent extraction and gas diffusion. 

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

Determination of Ammonia in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimetric... 

Continuous segmented methods avoid carry-over by use of air bubbles establishing physical separations (segments) along the continuous flowing stream. These methods were invented by Skeggs [1] and formed the basis of the Technicon AutoAnalyzer. They are now also implemented on Skalar assembhes. Samples are introduced sequentially by aspiration with a moving articulated pipette. 

There is usually no problem of access to basic laboratory instruments and associated glassware, however, the only means of handling large numbers of tests is to apply some form of automation. An added advantage is that it improves the analytical precision and reproducibility. The most suitable technique has been based on the segmented continuous-flow principle invented by Skeggs (1957), and which was first marketed as the Technicon AutoAnalyzer. The system consists of a number of modules powered from a stabilized 110 V supply, and a typical layout is shown in Fig. 1.1. 

Fig. 1.1. (a) Modular layout of a typical segmented continuous flow system. 

Figure 3.10 — Flow manifolds for implementation of flow-through biosensors. (A) Flow injection merging-zones manifold for the bioluminescence detennination of ATP. ATP standards (30 fiL) and luciferin (30 fiL) are injected into the buffered carrier streams, each pumped at 0.7 mL/min and synchronously merged 12.5 cm downstream. Distance from merging point to immobilized enzyme coil, 2.2 cm. (Reproduced from [59] with permission of Elsevier Science Publishers). (B) Completely continuous flow manifold for the determination of NADH. (Reproduced from [71] with permission of the Royal Society of Chemistry). (C) Segmented-flow manifold for the determination of L-(+)-lactate. (Reproduced from [65] with permission of Marcel Dekker, Inc.). (D) Single-channel flow injection manifold with immobilized reagent for the detennination of glucose. (Reproduced from [77] with permission of Elsevier Science Publishers).

Extraction in liquid/liquid segmented flow has been developed. A continuous-flow extraction system coupled online with an RP-HPLC with C-l 8 and a UV detector was used to study the extraction of fenthion and azinphos methyl in water (82). 

Ratanathanawongs and Crouch [19] have described an on-line post-column reaction based on air-segmented continuous flow for the determination of phenol in natural waters by high performance liquid chromatography. The reaction used was the coupling of diazotised sulphanilic acid with the phenol to form high coloured azo dyes. The detection limit for phenol was 17pg L 1 which represents a 16-fold improvement over determination of phenol with ultraviolet detection. 

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