Automatic analytical methods for environmental monitoring and control
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- Multiple-determination and multiple-detection FIA techniques
- III.2.1.4. Sequential Injection Analysis
- Operational parameters in SIA
- III. 2.1.5. Hyphenated Systems
- III.2.2. AUTOMATED FLOW ANALYZERS
Figure III.2.17. Schematic diagrams of FIA gradient techniques. (a) Gradient dilution. Peaks recorded for different concentrations of a dye, he most concentrate one being 1%; t1…4 – different delay times; calibration curves. (b) Gradient calibration. Different injected concentration of a dye solutions corresponding to different delay times, t1..4, on peak of the most concentrated one. (c) FIA stopped-flow: a – continuous pumping; b, c, d - recorded reaction rate curves for different delay times. Multiple-determination and multiple-detection FIA techniques Another trend in FIA development has been the design and optimization of systems for multiple-determination and multiple-detection. These techniques are used for kinetic determinations or for simultaneous determinations of two or more components from the same injected sample. Multiple-detection – a single sample is injected and two or more signals are recorded in different ways:
using a single detector, the signals being delayed; using a dynamic detector, a physical parameter is measured continuously within a certain range (e.g. absorbance vs. wavelength, current vs. potential, etc., along the dispersed sample zone; using more detectors of the same type displaced in series or in parallel. The multiple-detection (Figure III.2.18) may be successively performed, obtaining more data of a certain measured parameter, or simultaneously, using a single detector that performs simultaneous measurements at different working parameters of the instrument. Multiple-determination – of one or more analytes from a sample may be sequentially realized, performing a number of injections equal with the number of analytes to be determined or simultaneously, determining more analytes from the same injected sample (Figure III.2.18.b). Considering the definitions of these two FIA techniques, it results that the multiple-determination may be realized by the multiple-detection, but the multiple-detection does not means multiple-determination, strictly. Two examples of simultaneous determinations are presented below: (a). Simultaneous determination of Fe(III) and Ti(IV) (Figure III.2.19.a) – both analytes react with Tiron (R). Two sample volumes are simultaneously injected into 1 M HCl carrier stream by means of two injection valves (Vi1, Vi2). The injected sample zone with Vi1 passes through the reduction column (RedC) where Fe(III) is reduced to Fe(II), which does not react with Tiron. The injected sample zone injected with Vi2 is treated with Tiron and the mixture zone goes to the spectrometer where the absorbance due to Fe(III)+Ti(IV)–complex with Tiron is recorded. For each double injection two peaks are recorded, first corresponding to the complex absorbance of Fe(III)+Ti(IV)–Tiron and the second corresponding to the complex absorbance of Ti(IV)–Tiron, only. On the bases of heights of these two recorded peaks, the concentrations of Fe(III) and Ti(IV) can be calculated by using calibration curves. (b) Simultaneous determination of nitrite and nitrate (Figure III.2.19.b) – is based on the measuring the absorbance of the reaction product formed between the nitrite and sulfanilamide (R1) and N-(1-naphthyl)-ethylendiamine (R2). The injected sample zone is split into two sub-zones using a “T” connector. One of the sub-zones is treated with R1; the nitrite ion is diazotized with sulfanilamide and then, the diazotization product is coupled with R2 to form a colored azo-dye for which the absorbance is recorded. The other sub-zone passes through a reduction column (RedC) that contains copperized cadmium and here, the nitrate is reduced to the nitrite. The sub-zone is then treated in the same manner as the other sub-zone and the overall mixture passes to the spectrometer where the absorbance due to nitrite and nitrate is measured. The concentrations of nitrite and nitrate are evaluated from the peaks h  eights by using calibration curves.  (a) (b)
Figure 2.1.18. Schemes of FIA systems for successive multidetection technique. (a) separation of the injected zone in three equal sub-zones that reach the detector after different periods of time. Each injection produces 5 calibration values, 3 peaks (P1, P2, P3) and 2 troughs (T1, T2). (b) more detectors displaced in series, their response being computed by a microprocessor. P – peristaltic pump; Vi – injection valve; RC – reaction coil; D – detector; C – carrier; S – sample; R – reagent, W – waste.  (  a) (b)
Figure 2.1.19. (a) Scheme of FIA system used for simultaneous determination of Fe(III) and Ti(IV) using Tiron as reagent. (b) Scheme of FIA system used for simultaneous determination of nitrite and nitrate. P – peristaltic pump; Vi – injection valve; RedC – reduction column; D – detector; C – carrier; S – sample; R – reagent, W – waste (see explanations in text). III.2.1.4. Sequential Injection Analysis Sequential injection analysis (SIA) was first reported in 1990 by J. Ruzicka and G.D. Marshall at the University of Washington, as an evolution of the flow analysis process. This technique, for automatic sample analysis, is based on the same principles as FIA, namely controlled partial dispersion and reproducible sample handling, and it offers different possibilities with a series of advantages and disadvantages in relation with its parent technique. The instrumental simplicity, robustness, ease and efficiency with which hydrodynamic variables can be controlled, and high flexibility and modes maintenance requirements of this modified technique have turned it into a very popular choice with both research and industrial analysis laboratories. On the other hand, SIA has proven to be a technique that can be designed to operate in a multi-parametric way, which is of special interest when considering the design of the environmental monitors. Thus, the monitoring of wastewater has been proposed for ammonium, nitrate, nitrite, total nitrogen, orthophosphate, total phosphorus, detergents, etc., and it can be determined in 15 min. However, in spite of these advantages, SIA presents two major disadvantages: the sample throughput is lower then that of the usual flow systems and major difficulties in the mixture of sample and reagents. SIA is a single-line system, completely microcomputer controlled, that can be configured to perform most operations of conventional FIA, with no or minimal physical reconfigurations of the manifold, allowing to perform determinations of different analytes. Principles An operational manifold design for SIA is illustrated in Figure III.2.20. The heart of the system is a multi-port selection valve, used in place of a conventional injection valve and this is the primary difference between FIA and SIA. This valve delivers accurately measured volumes of carrier, sample, standards and reagent solution to a holding coil by connecting its common port to a reversible pump featuring a precisely controlled forward-stop-backward motion. The common port can access any of the other ports by electrical rotation of the valve. The holding coil placed between the valve and the pump prevents the aspirated (injected) solutions from entering the pump. Initially, the system is filled with washing or carrier solution, which is aspirated into the holding coil by the pump moving in a forward motion. Each measuring cycle begins by switching the multi-port valve to the sample line and aspirating a precise measured volume (few L) of sample into the holding coil by the pump moving in a backward motion; the pump is stopped during the rotation of the valve to avoid pressure surges. Next, the valve is switched to the reagent line and a precisely measured volume of reagent is drawn into the holding coil. Thus, the sample and reagent solutions are sequentially injected into the holding coil next to one another, hence the name of this technique. A second reagent may be aspirated on the other side of the sample. Finally, the valve is switched to the detector port and the pump propels the sequenced zones forward through the reaction coil to the detector. The cores of sequenced zones penetrate each other via laminar flow and diffusion. If the radial mixing is promoted by a suitable choice of coil geometry, the analyte and reagent zones mix and produce detectable species and a transient signal as in conventional FIA is recorded. The complex reagent/product/sample zone can be either transported through the detector continuously, or stopped within detector, resulting in measurement of the rate of formation of the reaction product. In this way, kinetic information can be extracted from the SIA signal. There is no limit to how many solutions or devices (reaction coils, mixing chambers and detectors) can be nested around the multi-port valve. A series of standards can be permanently nested around valve, being ready for automated recalibration whenever is necessary.  
Figure III.2.20. Schematic diagram of a SIA system. P – single-line reversible (high-precision bi-directional) pump; HC – holding coil; Vp – multi-port valve; D – detector; S – sample; R – reagent; St – standard solution; W – waste. The biggest advantage of SIA over FIA is that is not necessary the physical reconfiguration of the flow path. The injected sample volume, reaction time, sample dilution, reagent/analyte ratio or system calibration are controlled from a computer keyboard. Indeed SIA is fully computer-compatible and allows the configuring of the system to perform complex chemistries. In addition, SIA requires very low sample reagent volumes (L/analysis). Operational parameters in SIA “How does SIA differ form FIA and what are its drawbacks?” T   he characteristic feature of FIA patterns is that the injected sample zone flows to a confluence point where two streams merge in a continuous fashion and in this manner; an equal volume of reagent is added to each element of the passing carrier stream. The result is a concentration gradient of an analyte within the constant background of reagent. In contrast, in SIA no confluence points are used. The multi-port line is used to sequence the zones into a holding coil. This valve does not serve, as a confluence point as it connects only two ports (not three) at a time. The result is a sharp boundary between adjacent zones and only a partial overlap of analyte and reagent picks is possible (Figure III.2.21.b).
(b) Figure III.2.21. (a) Formation of sample zone concentration gradient, S – sample line concentration, in FIA mode via a confluence point supplying a steady reagent, R – reagent line concentration. (b) Mutually penetration of sample, S, and reagent, R, zones in SIA mode. Reaction product - shaded zone. In SIA mode it is needed a computer control otherwise injected zones and their intermixing would be poorly reproduced. Thus, the parameter of prime importance in SIA is the degree of penetration (or overlay) of the adjacent zones. This is dependent on the relative volumes, in addition to the usual parameter of tubing size and length, reaction coil geometry and the flow rate delivered by the peristaltic pump. The degree of penetration and the dispersion determine the signal to be recorded: Figure III.2.21.b shows the mutual zone penetration in SIA. A small zone volume results in increased overlapping, but the dispersion is relatively large. By increasing one or both volumes (sample and reagent) the overlapping decreases, but the dispersion at the maximum overlapping is less. In general, the injected sample volume should be less than the volume at half-maximum signal and the reagent volume should be at least twice the sample volume. The reaction is instantaneous if the concentration of reagent is higher then analyte concentration, the recorded signal maximum should occur at the isodispersion point (the point of maximum overlapping). Tube diameters of 0.9 – 1.5 mm decrease the backpressure compared with 0.5 mm, resulting in improved precision without excessive decrease in the zone penetration. Straight reactors allow a greater zone penetration through axial dispersion than the coiled reactors preferred in FIA.
In previous SIA works a low-pressure syringe was used as liquid driver that provided a sinusoidal flow as a result of non-uniform motion of the piston in the cam-driven piston pump, illustrating that the flow is not constant but reproducible. Peristaltic pumps of high precision were tested for propelling the flow and their performances were compared with the above described piston pumps. Thus, their sampling cycle is shorter than that of the piston pumps as the pump needs to be refilled periodically and also peristaltic pumps are much more commonplace in the laboratories than piston pumps. On the other hand, the disadvantage of the peristaltic pump arises from the need of fairly elastic tubes, which have a much shorter life. In order to circumvent this inconvenient, an auto-burette was used to propel the flow in SIA. It has been demonstrated that a small-volume syringe pump provides the highest precision, of 0.3 %, and the peristaltic pumps and the auto-burette provide a precision of 1 %. In conclusion, a SIA system is assembled using a multi-port (usually 10 ports) electrically activated selection valve, a high-precision peristaltic or syringe pump, a suitable flow-cell detector, tubing/reaction coils and connectors (as those used in FIA) and a personal computer. Appropriate software must be available to control the flow direction, rate and timing of the pump, the position of the multi-port valve and to collect and process the data. At the present there are several commercialized software for SIA users, including: Atlantis, MATLAB, Microsoft QuicBasic, Labview, FlowTEK, DARRAY, FIALab, etc.
The acceptance of the existence of a correlation between environmental preservation and standard of living has led to the need of vigilance and continuous control of a large number of environmental parameters. A large number of environmental samples are submitted to routine laboratories every day in order to satisfy these increasing demands. Traditionally analyses have been performed by off-line methods, which mean all the samples must be carried to a centralized laboratory. These off-line measurements may lead to a significant delay between submission of a sample and analysis result, particularly when demands upon staff and instrumentation are great. One of the current trends in environmental parameters analysis involves avoiding the sampling by utilizing automated, unattended, analytical instrumentation, which may not be very selective but allows the detection of alarm situations and the performing of a complete analysis in those samples where analysis is required. With this instrumentation fast process monitoring is achieved, thus facilitating processes regulation and enabling quality assurance and quality control. High quality chemical information attainable in “real-time” requires having a rapid, accurate, reliable, robust, without demanding the continuous presence of the analyst, with a low consumption of reagent and whenever possible multi-parametric system like the continuous flow analyzers available. Certain analyses of environmental samples are difficult to achieve in a completely automated manner, but aqueous samples are especially well adapted to be analyzed by flow techniques. Among all the continuous flow techniques, flow injection analysis (FIA) is a well-established, powerful, sample handling procedure for laboratory analysis and on-line process analytical chemistry, with a wide range of applications in environmental situations. Maybe this is the reason for which FIA enthusiasts agree on: “This is a technique that allows chemists to easily automate and optimize well-developed wet chemical methods for routine laboratory use. You can even program an analyzer to switch from one analyte to another during the analysis of a batch of sample… But FIA was never been a popular product of larger instrument companies, so smaller firms produce most of these analyzers”. In spite of all, the technique is surprisingly under-used by laboratory chemists and this perhaps automation was too expensive or took too long when FIA was introduced in 1975. FIA offers several advantages over the manual handling of solutions, such as: it is computer-compatible, allows automated handling of solutions, and provides strict control of reaction conditions. Also, because of its versatility in sampling handling, FIA serves as front end to practically all spectrophotometric and electrochemical detectors and to various environmental, clinical and industrial assay. Other applications include “real-time” monitoring of chemical processes, automated renewal of the sensing layer in chemical transducers, and electrochemical methods, such as hydrodynamic voltammetry and ion-selective electrode measurements. The recent results achieved in computerization, microfluidics, and hardware have facilitated the further development of new flow injection techniques. New on-line UV-digestion technique, combined FIA-sequential injection analysis (SIA) techniques, the incorporation of ion-selective electrodes and improvements of data handling are some highlighting examples. The company Global FIA, Inc. adds that mixed media (beads, bubbles, immiscible liquids) in flow systems have opened new applications. FIA methods are also presently undergoing standardization protocols established by the Organization of International Standards and the U.S. Environmental Protection Agency. Table III.2.2 lists some commercial continuous flow analyzers and their features, which can be different in technology, costs and sample types or number.
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