Wall-jet systems

One of the benefits of electrochemical batch injection analysis is that dilution of the sample with electrolyte is not necessary, see below. A sample of volume =sl00p.L is injected directly from a micropipette, tip diameter 0.5 mm, over the centre of a macroelectrode exactly as in a wall-jet system. This is equivalent to a flow injection system with zero dispersion. During the injection, and after a short initial period to reach steady-state, the hydrodynamics is wall-jet type and a time-independent current is registered. BIA was first devised using amperometric, e.g., [31], and potentiometric, e.g., [34], detection. A typical amperometric trace is shown in Fig. 16.5. By using a programmable, motorised electronic... 

A further development of the wall-jet system is to insert a packed-bed electrode upstream of the jet to make a packed-bed wall-jet electrode (PBWJE) [5-7]. The packed bed can be used to generate reactant, to generate a fresh electrode surface or, if the bed has immobilised enzyme, to carry out an enzymatic reaction. The packed-bed wall-jet electrode is illustrated in Fig. 3. 

The fact that the wall-jet system has no moving parts means that it is the... 

The flow of solution can be maintained by centrifugal and peristaltic pumps that are placed outside the cell [62], Homemade micro-pumps with a rotating smooth-disk impeller and nozzle, or with a vibrating conically perforated disk, are situated in the cell, closely below the working electrode [67]. The most favorable relationship between the average distance S and the flow rate of the solution is found in the wall-jet systems in which a flux of the solution induced by a centrifugal pump is directed perpendicularly to the surface of a stationary disk electrode. In these systems, the current is linearly proportional to the flow rate of the solution [68],... 

Notes Ii(A) limiting current, cj (moIm ) bulk concentration of the reactant A, for other quantities see Table 6 R, Rj (m) radius of the disc or tubular electrode S (m ) surface of the working electrode in thin-layer, rotating disc and wall-jet system respectively b (m) dimension of the electrode perpendicular to the liquid-flow F (m s ) sample inlet rate a (m) diameter of the inlet nozzle of the wall-jet detector = laminar convection, 1 streaming perpendicular to the electrode R a is assumed. 

Special systems with forced flow Flow directions in tubular and planar systems are illustrated in Figs. 69A and B. The motion of the liquid near the electrode surface in systems with stationary and rotating disc and in the wall-jet system is demonstrated in Figs. 70A-C. The... 

There are many possible combinations of supply and exhaust air. For example, a line jet could be used as a shield in an opening, as a stripping system on surfaces, for blowing contaminants into an exhaust, etc. An enclosure could be designed with a line jet in the opening, with a wall jet inside to increase efficiency, or with a low-momentum jet inside or outside the opening to replace the room air supply. In this section, only some basic combinations are described. 

More recently, in the middle 1990s, the UK s Health and Safety Executive (HSE) also reviewed the push-pull system. Hollis and Fletcher offer a comprehensive literature review on push-pull ventilation and note that the main conclusions of previous work on push-pull ventilation of tanks are that the control is primarily supplied by the inlet jet, forming a wall jet along the surface of the tank, and that the main purpose of the exhaust hood is to remove the air and contaminant contained within the push jet. 

We thus have a means of describing, albeit approximately, the fluid flow induced by the system in terms of the Verhoff formulae and a graphic relationship between the offset jet parameters and the equivalent wall jet parameters. We now wish to be able to calculate the movement of the contaminant in the system... 

Ingham. - This gives the required minimum value for the momentum ol the equivalent wall jet we must also recall the relationship shown in Fig. 10.72 to determine the required momentum of the offset jet in the push-pull system. 

The analytical performance of Prussian blue-modified electrodes in hydrogen peroxide detection were investigated in a flow-injection system equipped with a wall-jet cell. Nano-structured Prussian blue-modified electrodes demonstrate a significantly decreased background, which results in improved signal-to-noise ratio. 

Substrate orientation should be examined to determine if some planes are preferentially etched. If there is preferential etching taking place, what is its dependence on the etching cycle conditions The hardware being used for these studies should also be investigated. Very little has been done to optimize the flow cell. It is anticipated that a hydrodynamic electrode system such as a rotating disk or wall jet should work as well. 

The wall-jet electrode has the general configuration shown in Figure 7.8(a). When using this system, the current is measured while a fine jet or spray of analyte solution is squirted under relatively high pressure towards the centre of the working... 

Like the flow and channel electrodes described above, the wall-jet electrode is not a batch-mode system, so it can be employed as the basis for... 

Convection-based systems fall into two fundamental classes, namely those using a moving electrode in a fixed bulk solution (such as the rotated disc electrode (RDE)) and fixed electrodes with a moving solution (such as flow cells and channel electrodes, and the wall-jet electrode). These convective systems can only be usefully employed if the movement of the analyte solution is reproducible over the face of the electrode. In practice, we define reproducible by ensuring that the flow is laminar. Turbulent flow leads to irreproducible conditions such as the production of eddy currents and vortices and should be avoided whenever possible. 

The sensing microzone of the flow-through sensor depicted in Fig. 5.9.B1 integrates gas-diffusion and detection with two analytical reactions [28], viz. (a) the urease-catalysed formation of ammonium ion by hydrolysis of urea (the analyte), which takes places on a hydrophilic enzyme membrane in contact with the sample-donor stream, which contains a gel where the enzyme is covalently bound and (b) an acid-b reaction that takes place at the microzone on the other side of the diffusion membrane and involves Bromothymol Blue as indicator. This is a sandwich-type sensor including a hydrophilic and a hydrophobic membrane across which the sample stream is circulated —whence it is formally similar to some enzyme electrodes. Since the enzymatic conversion of the analyte must be as efficient as possible, deteetion (based on fibre optics) is performed after the donor and acceptor streams have passed through the sensor. Unlike the previous sensor (Fig. 5.9.A), this does not rely on the wall-jet approach in addition, each stream has its own outlet and the system includes two sensing microzones... 

Scheme of the wall-jet electrode as constructed in our laboratory with (1) solution tank, (2) vessel to control the solution flow rate, (3) pump, (4) flow-rate measuring device, (5) capillary, (6) measuring chamber, (7) counter electrode, (8) reference electrode, (9) working electrode, (10) overflow system to return solution to the solution tank. 

Prior to use for analytical purposes, the developed wall-jet electrode should first be characterised and calibrated. This is described here, where the wall-jet disc electrode is optimised by making use of a reversible, one-electron exchanging, redox system ([Fe(CN)6]47[Fe(CN>,]3 ) in order to obtain the most favourable conditions for the determination of sodium dithionite, sulphite and indigo. 

The portable instrumentation and low power demands of stripping analysis satisfy many of the requirements for on-site and in situ measurements of trace metals. Stripping-based automated flow analyzers were developed for continuous on-line monitoring of trace metals since the mid-1970s [16,17]. These flow systems involve an electrochemical flow detector based on a wall-jet or thin-layer configuration along with a mercury-coated working electrode, and downstream reference and counter electrodes. 

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). 

The use of hydrodynamic electrodes in these experiments has been very important in that they increase sensitivity because of higher mass transport, ensure good reproducibility, and sometimes gives better resolution in solutions of mixtures. The use of cells such as the wall-jet in flow systems is particularly useful, as response is fast and it is easy to introduce them at any point in the flow system. 

The fundamentals of the electrochemical response at electrodes operating in a regime of forced convection, hydrodynamic electrodes, and the information that can be obtained have been reviewed [23, 24]. Some of these electrodes are good candidates for direct introduction into flow systems, in particular tube/channel electrodes and impinging jet (wall-jet and wall-tube) electrodes. Particular practical advantages of these flow-past hydrodynamic electrodes are that there is no reagent depletion while the sample plug passes the electrodes, and there is no build-up of unwanted intermediates or products. Recent advances in instrumentation also mean... 

The wall-jet continuous flow system is readily modified to discrete injection mode as shown in Fig. 16.4. This is the electrochemical version of a general discrete injection technique known as batch injection analysis (BIA) [31] and is a hybrid between flow injection analysis and wall-jet in continuous flow. 

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