Segmented flow analysis, determination

Kerouel, R., and Aminot, A. (1997). Fluorometric determination of ammonia in sea and estuarine waters by direct segmented flow analysis. Mar. Chem. 57, 265—275. 

In segmented flow analysis, axial sample dispersion is not pronounced, being influenced mainly by the characteristics of the thin liquid film established at the tubing inner wall and by the number of segments per sample (see 5.1.2). Reduction in sample concentration is therefore strongly dependent on the addition of confluent streams. Hence, the flow rates of the sample/wash and confluent streams are the main parameters determining sample dispersion, and application of Eqs 3.10 and 3.13 can then make this practical index readily available. 

D.J. Malcome-Lawes, C. Pasquini, A novel approach to non-segmented flow analysis. Part 3. Sequential determination of ammonium, nitrite and nitrate ions, J. Autom. Chem. 10 (1988) 192. 

The addition of confluent streams is the main process that contributes to dispersion in segmented flow analysis. The extent of this process can be determined from Eqs. 3.10 and 3.11, which are presented here again for didactic purposes ... 

Classical transducers such as the photomultiplier tube and the photodiode have been used in most the flow systems [96,97], Since the 1980s, the tendency to use diode array spectrophotometers has increased, mainly for simultaneous determinations [98] and/or for implementing dual wavelength spectrophotometry [99], the latter being more exploited in segmented flow analysis. 

Stream splitting was originally exploited to improve the sampling step in segmented flow analysis for enzymatic assays or simultaneous determinations. 

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-... 

For determination of the elements, mainly spectrometric techniques are used here. Depending on the kind of element and the expected concentration level, the following methods are applied flame atomic emission spectrometry (flame AES), flame atomic absorption spectrometry (flame AAS), inductively coupled plasma optical emission spectrometry (ICP-OES), electrothermal atomisation (graphite furnace) atomic absorption spectrometry (ETA-AAS), inductively coupled plasma mass spectrometry (ICP-MS), spectrophotometry and segmented flow analysis (SFA). Besides, potentiometry (ion selective electrodes (ISE)) and coulometry will be encountered. In many cases, more than one method is described to determine a component. This provides a reference, as well as an alternative in case of instrumental or analytical problems. 

The determination is based on the Bertheiot reaction, in which a phenoi derivative (here saiicyiate) forms an azo dye in the presence of ammonia and hypochiorite. In alkaline medium, the indophenol thus formed has a green-blue colour, of which the absorbance is measured at a wavelength of 660 nm. This is a measure for the concentration of ammonium, formed by the nitrogen compounds in the sample. The determination is carried out as a so-called segmented-flow analysis (SFA). 

An example of use of the above rules for calibration purposes is a procedure called the method of gradient ratios (gradient ratio calibration method, GRCM) [8], which is one way of implementing the dilution method in flow analysis. In this approach, sample and standard solutions are injected one after the other into a flow system. The obtained peaks overlap each other (as shown in Fig. 3.12) and are considered point-by-point from the maximum points along the softer sides of both peaks. On the basis of the values of the two measurement points obtained for standard and sample after the same time, analyte concentrations can be determined in various parts of the sample segment. For the final analytical result, the average of these concentrations is taken (only if their values differ from each other randomly). 

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. 

Conversely, confluence flow injection systems rely on sample insertion into a chemically inert carrier stream and the required reagents are added by confluence. The configuration is characteristic of the segmented flow analyser. The carrier (or background) stream is a solution similar to the sample but without the chemical species under determination. Distilled water, soil extracting solution, ethanol and synthetic seawater are examples of chemically inert carrier streams for the analysis of natural waters, soil extracts, spirits and seawater, respectively. 

Nowadays, the parallel development of the different modes of unsegmented flow analysis has led to innovations in segmented flow systems, e.g., by exploiting both sample insertion into unsegmented carrier stream and air-segmentation, as demonstrated by the potentio-metric determination of fluoride in waters [22],... 

This innovation generally involves modifications to the operation of the sampler and random access reagent selection, and can be implemented in both segmented and unsegmented flow analysers. For unsegmented flow analysis, the spectrophotometric determination of zinc and phosphate in soil extracts [368] is a good example. Zinc was determined only when phosphate was present at concentrations above a threshold level. The number of determinations required was reduced by 30%. Analogously, an expert flow system was proposed for the turbidimetric determination of chloride and sulphate in natural waters [369]. Both methods were implemented in the same manifold, and the need for sulphate determination was dependent on the chloride concentration determined. 

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). 

The instruments with which continuous techniques of clinical analysis are carried out belong to the group of flexible analysers. Segmented-flow instruments used in this field are multi-parameter and allow the simultaneous determination of the different species by means of systems splitting the aspirated sample into as many lines as parameters are to be determined (see Chapter 5). The adaptation for analysis of a new parameter Is readily accomplished by simply changing the corresponding analytical cartridge. Thanks to Its versatility, the FIA technique is adaptable to any type of analysis, whether for one... 

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