Flow injection analysis carrier stream

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Flow injection analysis (FIA) was developed in the mid-1970s as a highly efficient technique for the automated analyses of samples. °> Unlike the centrifugal analyzer described earlier in this chapter, in which samples are simultaneously analyzed in batches of limited size, FIA allows for the rapid, sequential analysis of an unlimited number of samples. FIA is one member of a class of techniques called continuous-flow analyzers, in which samples are introduced sequentially at regular intervals into a liquid carrier stream that transports the samples to the detector. ... 

Another interesting development, in which continuous flow was combined with discrete sample titration, is continuous flow titration by means of flow injection analysis (FIA) according to Ruzicka and co-workers70. Fig. 5.16 shows a schematic diagram of flow injection titration, where P is a peristaltic pump, S the sample injected into the carrier stream of diluent (flow-rate fA), G a gradient chamber of volume V, R the coil into which the titrant is pumped (flow-rate fB), D the detector and W waste. 

A more recent development is a technique known as flow injection analysis, in which a discrete volume of a liquid sample is injected into a carrier stream. Reagents required for the development of the analytical property of the analyte, e g. colour developing reagents for spectrophotometry, are already present in the stream. The stream then flows straight to the detector and the technique depends upon the controlled and reproducible dispersion of the sample as it passes through the reaction zone. Thus the reaction does not necessarily need to develop to completion,... 

Flow-injection analysis is based on the introduction of a defined volume of sample into a carrier (or reagent) stream. This results in a sample plug bracketed by carrier (Fig. 1 (a)). 

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. 

Figure 6.11 Examples of Tecator Chemifold types for flow injection analysis. S, sample injection port C, carrier stream R1 R2, R3, reagent streams D, detector W, waste.

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

Dialysis units provided highly efficient means for increasing selectivity in a dynamic system by placement in front of a lithium-selective electrode constructed by incorporating 14-crown-4 ether 3-dodecyl-3 -methyl-1,5,8,12-tetraoxacyclotetradecane into a PVC membrane that was in turn positioned in a microconduit circuit by deposition on platinum, silver or copper wires. The circuit was used to analyse undiluted blood serum samples by flow injection analysis with the aid of an on-line coupled dialysis membrane. For this purpose, a volume of 200 pL of sample was injected into a de-ionized water carrier (donor) stream and a 7 mM tetraborate buffer of pH 9.2 was... 

Gine et al. [29] has described a semi-automatic determination of manganese in plant digests using flow injection analysis. This technique utilises the introduction of the sample into a continuously flowing carrier stream of formaldoxime reagent. When injected, the sample is pushed by this stream and dispersed into the reagent stream, whereupon the required reaction takes place. The coloured complex is then carried into a spectrophotometric flow cell, where the absorbance is measured after an exactly defined time interval. 

The focusing bi-laminating micro mixer was realized in the framework of the development of a flow injection analysis (FIA) system [114,115]. The mixer is placed downstream of the two-fold injection of sample and reagent streams into the carrier. Thereafter, the mixed stream enters a reaction chamber and finally passes a detector. An easy integration is required, as the mixing element is part of an integrated system which has the minimization of size as one issue. 

Flow-injection analysis is a versatile technique to evaluate the performance of a detector system. CHEMFETs may have an advantage over ISEs because of their small size and fast response times. We have tested our K+-sensitive CHEMFETs in a wall-jet cell with a platinum (pseudo-)reference electrode. One CHEMFET was contineously exposed to 0.1 M NaCl and the other to a carrier stream of 0.1 M NaCl in which various KC1 concentrations in 0.1 M NaCl were injected. The linear response of 56 mV per decade was observed for concentrations of KC1 above 5 x 10"5 M (Figure 9). When we used this FIA cell (Figure 10) for determination of K+ activities in human serum and urine samples, excellent correlations between our results and activities determined by flame photometry were obtained (Figure 11). 

Fig. 19. Schematic design of a flow injection analysis (FIA) system. A selection valve (top) allows a selection between sample stream and standard(s). The selected specimen is pumped through an injection loop. Repeatedly, the injection valve is switched for a short while so that the contents of the loop are transported by the carrier stream into the dispersion/reaction manifold. In this manifold, any type of chemical or physical reaction can be implemented (e.g. by addition of other chemicals, passing through an enzyme column, dilution by another injection, diffusion through a membrane, liquid-liquid extraction, etc. not shown). On its way through the manifold, the original plug undergoes axial dispersion which results in the typical shape of the finally detected signal peak...

Takeuchi et al. published a mechanized assay of serum cholinesterase by specific colorimetric detection of the released acid [40]. The cholinesterase reaction was carried out on a thermostatted rack at 30° C with a reaction mixture of serum (10 pL), 50mM barbitone-HCl assay buffer (pH 8.2 140 pL), and 12.5 mM acetylcholine solution (50 pL). The solutions were prepared by programmed needle actions, and a sample blank was also prepared. The reaction was stopped after 9.7 min by injection of the mixture into a flow injection analysis system to determine the quantity of acetic acid formed. The carrier stream (water, at 0.5mL/min) was merged with a stream (0.5mL/min) of 20 mM 2-nitrophenylhydrazine hydrochloride in 0.2 M HC1 and a stream (0.5 mL/min) of 50 mM 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide hydrochloride in ethanol containing 4% of pyridine. The sample was injected into this mixture (pH 4.5), passed through a reaction coil (10 m x 0.5mm) at 60°C, 1.5M NaOH was added, and, after passing through a second reaction coil (lm x 0.5 mm) at 60°C, the absorbance was measured at 540 nm. 

In flow injection analysis [32] with electrochemical detection a sample is injected into an electrolyte carrier stream dispersion of the sample plug into the carrier stream occurs so that electrolyte is effectively added to the sample—with consequent sample dilution—before reaching the electrode. Even so, by using a capillary flow injection system nanolitre sample volumes can be investigated [33]. In continuous flow systems, electrolyte often has to be added to the sample beforehand, also leading to sample dilution. 

A useful and rapid method of automated analysis is the technique of flow-injection analysis (FIA). The sample or a reagent is injected into the stream of a solution of constant composition. Calibration of FIA systems requires the injection of standard solutions, equal in volume to that of the sample, into the carrier stream. The backgrormd chemical composition of the standards should be equal, as nearly as possible, to that of the samples. Frequent standardization is not necessary because the measurement of peak height, albeit on a sloping base line, is relatively unaffected by cell voltage drift. Some difficulties can appear with peristaltic pumps, owing to extraneous potentials caused by pulsation of the stream. Cells with a small volume (<20 pi) or the cells of the wall-jet type are the most acceptable for continuous measurements. ... 

Flow-injection analysis is also well-suited for the automation of anodic stripping voltammetry. Metals can be plated from the sample solution as it passes over the electrode. Stripping is then carried out in the deox-ygenated carrier stream (15, 34). The sample itself does not have to be deox-ygenated. Detection limits of 3 nM have been reported for lead by this technique (34). 

Flow Injection Analysis [10,11], Flow injection analysis involves injecting a known volume of sample solution into a continuous flowing liquid carrier stream usually of the same solvent that the sample is dissolved in (Figure 2.18). A loop of fixed volume is attached to a rotating valve which can be connected and disconnected manually or by computer to a flowing stream between sample analyses. As the loop is fixed the volume... 

Flow injection analysis (FIA), which was introduced by Ruzicka and Hansen (iz ) and by Stewart et al (iQ), is based on the concept of controlled dispersion of a sample zone when injected into a moving and nonsegmented carrier stream. In continuous flow analysis (CFA), successive samples are mixed and Incubated with reagents on the way toward a flow through detector. The greatest difficulty to overcome in CFA was intermixing of adjacent samples during transport from the injection valve to the detector. In the past, it was widely believed that there are only two ways to prevent carryover in CFA either by the use of turbulent flow or by air... 

Flow injection analysis (FIA) is an automated method which consists in the injection of the sample solution to a continuous stream of an inactive carrier (e.g., a pH buffer or water) [49-51]. The diluted analyte is transported to a reaction spiral where a chromogenic reagent is added to the mixture. The dimensions of the spiral, the volume of the sample injected, and... 

In 1985, mono-segmented flow analysis was proposed [64] as a means of achieving extended sample incubation times without excessive sample dispersion. The sample was inserted between two air bubbles into an unsegmented carrier stream therefore the innovation combined the favourable characteristics of both segmented and unsegmented flow systems. Further development revealed other potential applications, especially with regard to relatively slow chemical reactions, flow titrations, sample introduction to atomic absorption spectrometers, liquid-liquid extraction and multi-site detection (Chapters 7 and 8). This innovation was also referred to as segmental flow injection analysis [65]. 

In single-line (also called straight or single channel) flow systems, the required reactants are present in the sample carrier stream and are added to the sample zone as a consequence of dispersion. The configuration is associated with the inception of flow injection analysis and is characteristic of the sequential injection analyser. Flow injection systems comprising two or more streams that converge to form the main carrier stream into which the sample is inserted [134] are also considered as single-line flow systems. 

FIGURE 4.13 Schlieren signals recorded for different solutions with the same refractive index. Carrier stream = water coiled reactor length = 100 cm a. b, c, d = 2.0 mol L-1 HC1,11.2% (m/v) sucrose, 14% (m/v) glycerol, and 24.13% (m/v) ethanol respectively. Other conditions are as in Fig. 4.13. Reprinted from Anal. Chim. Acta 234 (1990) 153, E.A.G. Zagatto, M.A.Z. Arruda, A.O. Jacintho, I.L. Mattos, Compensation of the Schlieren effect in flow-injection analysis by using dual-wavelength spectrophotometry, with permission from Elsevier (Ref [28]). 

The determination of zinc in plants involving solid-phase extraction was the first application of this strategy to real samples in flow injection analysis [98]. The pronounced Schlieren noise arising from the insertion of the ion-exchange resin mini-column into the eluent carrier stream was successfully minimised by DWS. 

As the sample plug starts being pushed forward, axial diffusion is the main component of the dispersion process, due to the high concentration gradients at the sample/carrier stream interface. The hypothetical peak shape associated with the flowing sample is shown in Fig. 5.9b, which corresponds to the first theoretical Taylor solution [28,29] for the diffusive-convective equation (Eq. 3.4). Situations associated with Fig. 5.9a,b never occur in practice in flow injection analysis. 

FIGURE 5.15 Influence of the confluence stream flow rate QCf- h and s = recorded peak height (A) and width (mL) carrier stream flow rate — 1.6 mL min-1 a, b, c = 5,50 and 250-cm loop lengths (ca 25, 250 and 1250 pL injected volumes) other experimental conditions as in Ref. [84]. Adapted from Anal. Chim. Acta 198 (1987) 153, E.A.G. Zagatto, B.F. Reis, M. Martinelli, F.J. Krug, H. Bergamin-Filho, M.F. Gine, Confluent streams in flow injection analysis, with permission from Elsevier. 

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