Segmented continuous flow analysis

Unlike with discrete or batch configurations, the nomenclature used with continuous configurations Is not quite correct taking into account the clear distinction between the terms analyeie and determination , establlehed by Pardue [9] in his hierarchical view of Analytical Chemistry (see Chapter 1). Thue, terms such as continuous-flow analysis , segmented-flow analysis or flow-injection analysis are not meant to describe the overall analytical process insofar as they do not include the preliminary sampling and sample treat-... 

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. 

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

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

Zhang, J. Z. (2000). Shipboard automated determination of trace concentrations of nitrite and nitrate in oligotrophic water by gas-segmented continuous flow analysis with a liquid waveguide capillary flow cell. Deep Sea Res. 147, 1157—1171. 

J. Snyder and H. Adler, Dispersion in Segmented Flow through Glass Tubing in Continuous Flow Analysis The Ideal Model, Anal Chem. 45 1017 (1976). 

TTHE BENEFITS OF AUTOMATING NUTRIENT ANALYSES IN SEAWATER have been recognized and utilized for several decades. Automated analyses allow samples to be processed faster and generally with better precision and accuracy than is possible with most manual methods. The only technique that was available at a reasonable cost for the automated analysis of seawater nutrients, until recently, was segmented continuous-flow analysis (CFA). Segmented CFA is characterized by the use of air bubbles to segment the liquid in the reaction tube so that dispersion of the sample is limited. 

C.J. Patton, R. Crouch, Experimental comparison of flow-injection analysis and air-segmented continuous flow analysis, Anal. Chim. Acta 179 (1986) 189. 

L.R. Snyder, H.J. Adler, Dispersion in segmented flow through glass tubing in continuous-flow analysis the ideal model, Anal. Chem. 48 (1976) 1017. 

Z.-L. Zhi, Segmental flow injection analysis, a hybrid technique of segmented continuous flow analysis and flow injection analysis, Trends Anal. Chem. 17 (1998) 411. 

An overview of the different types of discontinuity used in automatic methods and their characteristics is presented in Table 7.1. The most common discontinuity in discrete and robotic methods is the absence of flow, which involves keeping the samples in separate vessels for measurement. On the other hand, automatic continuous methods use very different kinds of discontinuity or do not use one at all. The discrete nature of segmented methods is determined by the presence of bubbles and wash cycles as a means of avoiding carryover, whereas that of unsegmented methods is dictated by the manner in which the sample —and reagent— is introduced into the system. There is only a single type of method using no discontinuity completely continuous flow analysis (CCFA). 

W. D. Basson and J. F. van Staden, Use of Non-Segmented High-Speed Continuous Flow Analysis for the Determination of Calcium in Animal Feeds. Analyst, 103 (1978) 296. 

Thb definition differentiates FIA from segmented continuous flow analysis in the node of manipulation of samples and/or reagents in fluid zones (the latter applies con-tmoous sample introduction), but more importantly, in the non-equilibrium conditions of operation and measurement for FIA (the latter applies steady state equilibrium conditions for measurements). 

The first application of on-line dialysis to a flow system seems to be that made by Skeggs [ 1 ] in his pioneering work on segmented continuous flow analysis. The first report on using on-line dialysis in a non-segmented flow system was that made by Kadish and Hall [2], whereas Hansen and Ruzicka [3] were the first to report such applications in FIA. Despite its early implementation in HA, apyplications of on-line dialysis in this field have been rather few compared to other separation techniques, and mostly dedicated to the analysis of blood serum. This may be due to the fact that dialysis is a slow separation procedure compared to the speed of most FI procedures, and the dialysis efficiencies are usually quite low. 

The initial development of air-segmented flow analysis (also known as CFA, from continuous flow analysis ) was restricted to Technicon Corporation Inc., owner of the main patents until the mid-1970s. The first air-segmented systems were marketed in clinical chemistry with the AutoAnalyzer trade... 

FI versus LC, Segmented Continuous-Flow Analysis, and Sequential-Flow Analysis... 

The colorimetric Nesslerization method uses alkaline phenol and hypochlorite to react with ammonia to form indophenol blue in an amount proportional to the ammonia concentration. The blue color is intensified with sodium nitroprusside, and the concentration is measured using a calibrated colorimeter. The titration of basic ammonia is accomplished with standard sulfuric acid using a mixed indicator. Determination of ammonia may also be based upon the indophenol reaction adapted to automated gas-segmented continuous flow analysis. Potentiometric determination of ammonia is performed by ion-selective ammonia electrodes. 

Sjosten and Blomqvist [90] have reported that the rate of formation of phosphoantimonyl blue was reduced by decreasing temperature and decreasing phosphate concentrations. At low temperatures (<5°C) and concentrations (5 pg P L ), reaction times of ca. 50 min were required to reach complete color development. The authors note that these effects may cause significant nonlinearity in the calibration of automated instruments (FIA, segmented continuous flow analysis, SCFA) at low concentrations, or underestimation in samples that have not been allowed to reach ambient temperature prior to analysis. 

Continuous flow techniques have been widely used for automated phosphorus analysis of waters since the introduction of the segmented continuous flow analysis systems in the 1950s. Segmented continuous flow manifolds described include those which are suitable for the determination of phosphorus in water in the presence of high silica [145], for highly sensitive detection of FRF in the presence of mercuric chloride preservative [146],... 

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