Flow systems for electroanalytical measurments
Autor:
Prof. Ernest Beinrohr,
CEO, ISTRAN, Ltd., Bratislava, Slovakia
Introduction
The pressure to increase work productivity, reduce analysis costs, and improve metrological parameters has led to the development of analytical flow methods, which have now become an indispensable part of clinical, environmental, and agrochemical laboratories. Flow methods are particularly significant in process analysis, as they are ideally suited for automation, long-term unattended operation, and low maintenance requirements. In practice, various technical solutions are used, but they all share one common feature: solutions are transported via narrow inert tubing, and all components are connected online, so that at the end of the flow system, a specific product of a chemical, physico-chemical, or physical process can be selectively detected. Essentially, this means that all steps of the chemical analysis – including, in many cases, sampling and sample preparation, possible separation steps, and chemical reactions – are carried out within a closed system. From the perspective of electrochemical analysis, flow systems represent a major advancement, as they help eliminate manual work (such as pipetting solutions, separation steps, solution mixing, bubbling, rinsing, etc.), thereby allowing electroanalytical methods to achieve a level of automation comparable to modern spectroscopic and chromatographic techniques.
In voltammetric, coulometric, and chronopotentiometric measurements, the flow system does not operate continuously, as it does in systems with photometric, potentiometric, and amperometric detectors. While the latter techniques provide a practically continuous signal (absorbance, potential, current), the former techniques measure in cycles consisting of several discrete steps: filling the electrochemical cell with a treated sample solution – this is the primary role of the flow system – followed by signal recording (voltammetric, current-time curve, chronopotentiogram) and data evaluation. The measurement may take several seconds to many minutes. From this perspective, these techniques are more accurately considered electroanalytical methods in flow, while the others are flow methods with continuous detection. Nevertheless, all these techniques use similar principles and flow components.
1 Flow System with Segmented Flow (SFA)
The SFA (segmented flow analysis) technique, also referred to as continuous flow analysis (CFA), is the oldest analytical flow method used in practice. Today, it is one of the most widely applied flow techniques, primarily used in clinical analysis and in the analysis of water samples, soil extracts, and fertilizers. The method was developed in 1951 by Leonard Skeggs [1, 2] to simplify clinical sample analysis, and within a few years, equipment for this method began to be produced in the USA. It usually uses photometric and potentiometric detection. Voltammetric and coulometric measurements are rare, due to the presence of air bubbles and the rapid sequence of sample zones.
The principle of the method lies in mixing the sample with a reagent solution during flow through tubing, where the individually prepared samples are separated by air bubbles and continue moving as distinct packets. The inner diameter of the usually Teflon tubes is about 1.5 mm. If necessary, the samples are further processed directly in the flow by dialysis, extraction, and similar methods, and finally pass through a detector. Since the individual dosed samples are separated by air bubbles, dispersion does not occur, and moreover, due to the larger diameter of the tubing, the flow of liquids tends to be more turbulent, which helps to achieve better mixing of the sample with the reagents.
2 Flow Injection System (FIA)
Flow injection analysis (FIA) was developed in 1975 by Ruzicka and Hansen in Denmark [3], and to a certain extent it represents a “European” alternative to the “American” SFA method. Unlike the SFA method, in FIA the solutions are pumped through thinner teflon tubes (0.5 to 1 mm) without air bubbles (on the contrary, there is a strong effort to avoid the formation of bubbles in the system, as they cause problems). The flow of liquids is mostly laminar. Individual samples are separated from each other by a reagent solution or water. The sample and reagent are mixed as a result of so-called controlled dispersion, i.e., the laminar flow profile of the liquids is used, which causes the spreading of the zones of injected solutions and thus their mixing. This controlled dispersion can also be used to simply achieve the necessary, often large, dilution of the sample, which the SFA method cannot do. At the end of the flow system, there is also a detector, most commonly potentiometric, photometric, or amperometric. Besides dispersion and chemical reaction, virtually any process can be used to modify the sample – diffusion, sorption, ion exchange, extraction, thermal decomposition, and so on. The analysis of the modified sample can be carried out without problems using voltammetric, coulometric, and chronopotentiometric methods, which is why this technique is most often associated with electroanalytical methods. The principle of controlled dispersion works in practice only if the flow of the liquids is truly laminar and not disrupted, for example, by bubbles. However, this condition is difficult to achieve in practical applications. Temperature differences, chemical reactions, and variable real samples can cause bubble formation and significantly impair, especially, the reproducibility – if not the accuracy – of the analysis. This is particularly risky in automated and industrial applications, where it is not as easy to resolve as in a laboratory, for example, by manually tapping bubbles out of the system when they appear. Therefore, if the FIA system is to be used for such applications, it is essential to ensure that air bubbles either do not interfere with the measurement or can be automatically removed. Neither option is easy, but the problem is not unsolvable.
In combination with electroanalytical methods, flow systems serve to draw up the sample, process it, transport it to the measuring cell, and flush the cell after measurement. In some cases, the system is also used to control the dilution of overly concentrated samples through controlled dispersion. However, as stated above, the flow of liquid in the system must be laminar and free from bubbles.
The flow system can be most simply implemented by hydrostatic pumping of a suitable electrolyte through a manual injection valve and a measuring cell. (Fig. 1). Electrolyte flows through the cell, then a defined volume of the sample is injected via a dosing loop, and the measurement is conducted in the cell – e.g., by stripping voltammetry or chronopotentiometry. By using two cells in series, the background level in voltammetric measurements can be significantly reduced by applying the method of anodic stripping coulometry with collection [4]. The analyte deposits in the first cell, while the potential of the working electrode in the second cell is maintained at a constant value, thereby minimizing residual currents. Then, the deposit in the first cell is dissolved, and the dissolved analyte moves to the second cell, where it is re-deposited, generating a corresponding reduction current. Due to the very low background, very low concentrations can be measured. The disadvantage of these simple solutions is the strong dependence of the flow rate on the hydrodynamic resistance of the flow system and, naturally, on the electrolyte level in the elevated reservoir.
Fig. 1 Flow system with hydrostatic pumping
A more advanced solution involves using a peristaltic pump and dosing via inert solenoid valves [5] (Fig. 2), where the entire pumping process and valve switching can be controlled electronically – enabling automation via a microprocessor or computer. This system enables precise dosing of both the sample and the standard for calibration, as well as online treatment such as filtration, ion exchange, diffusion, and controlled dilution.
Fig. 2 Flow system with peristaltic pumping
The manifold refers to the module for online solution treatment (filtration, mixing, bubble removal, etc.)
Most practical applications have been developed using a similar system, and this simple solution has also been used in many industrial applications. The number of solutions, as required by the application, can be expanded by adding additional valves. The integration of an autosampler results in a fully automated system capable of analyzing a high number of samples without any operator involvement.
3 Sequential Injection System (SIA)
To overcome some disadvantages of the FIA method – especially high reagent consumption, complexity when using multiple modules, and frequent manual interventions – Ružička and Marshall [6] developed a new flow method in 1990 called Sequential injection analysis (SIA). They replaced the peristaltic pump with a more precise piston pump that can aspirate and dispense solutions bidirectionally, and instead of a dosing valve, they used a multi-position switching valve. In the simplest configuration, the analysis is carried out by drawing the sample, and possibly also the reagent, into the reaction loop using the reverse flow of the pump. After the reaction occurs, the valve is switched toward the detector, and the sample is delivered to the detector by reversing the pump flow. This configuration reduces reagent consumption, accelerates the analysis, and enables virtually any method of sample treatment and calibration.
4 Sequential and Flow Injection System (SaFIA)
For industrial applications, the SIA system is not sufficiently robust. Therefore, we replaced the sensitive piston pump and three-way valve with a less precise but far more robust bidirectional peristaltic pump, which also does not require priming like the piston pump. If a selection valve or multiple valves are placed before the pump, the system can also operate as an FIA system. We named it SaFIA – Sequential and Flow Injection Analyzer (Fig. 3).
Fig. 3 Block diagram of the SaFIA flow system. In the “Blank” position, an additional reagent can be inserted.
The selection valve allows switching between six positions: toward the measuring cell, four reagent/sample solutions, and waste. During sample measurement, a new sample is pumped into the sample tubing and partially into the reactor to remove the previous sample from the tubing. Then, by reversing the pump flow and switching the valve toward the waste line, the contents of the reactor are flushed out. In the next step, the required sample volume is drawn in, and if necessary, mixed with a reagent. This solution is then pumped from the reactor to the measuring cell and measured.
The same procedure applies to calibration when, instead of the sample, a standard is analyzed. If needed, especially when measuring extremely low concentrations, a “blank solution” can also be measured. The reactor is typically a simple coil made of teflon tubing. Additional modules can be integrated into the system, such as a membrane separator, a column filled with sorbent or ion exchanger, etc., which facilitates analysis even of very complex samples.
References:
1 http://www.sciencemuseum.org.uk/broughttolife/people/leonardskeggs.aspx (2012).
2 Skeggs L. J., Amer. J. Clinical Pathol., 28, 311–322 (1957).
3 Ruzicka J., Hansen E.H., Anal. Chim. Acta, 78, 145–157 (1975).
4 Beinrohr E., Tschöpel P., Tölg G., Nemeth M., Anal. Chim. Acta, 273, 13 (1993).
5 Beinrohr E., Čakrt M., Dzurov J., Jurica E., Broekaert J.A.C., Electroanalysis, 11, 1137 (1999).
6 Ruzicka J., Marshall G. D., Anal. Chim. Acta, 237, 329–343 (1990).
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