Electrode cleaning potential pulse

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

Fig. lOJ Potential pulses used to clean the electrode surface and prepare it in-situ for adsorption measurement. Data from Briefer and Gilman, Trans. Faraday Soc. 61, 2546, (7965). 

Wightman and his group have continued with studies of this type and have found that using a variant of normal pulse voltammetry is probably preferable to using cyclic voltammetry because the period at the base potential between pulses helps to prevent film formation on the electrode and thus keeps the electrode clean. In this way, the behaviour of an implanted microelectrode remains reproducible over a long period of time, which is important as it is very difficult to reposition an electrode in exactly the same spot after it has been removed for cleaning. 

The most severe problem with voltammetric methods is the poisoning of the working electrode surface. It can be cleaned by applying potential pulses that are capable of removing the adsorbed reaction products from the surface. The surface can also be cleaned mechanically by bombardment with abrasive grit or by rapid rotation in the presence of plastic abrasion pellets. Strong ultrasound has also been used to clean the electrode surface. 

One aspect of eluent compatibility with EC detection is that there should be no effect on the components of the detector. Detector cell bodies are now routinely constructed of PTFE, other fluoroplastics, glass or stainless steel, and seem stable to most eluents. Nevertheless electrodes are vulnerable to chemical attack. Problems with the longer term use of some eluents at potentials around +1 V vs Ag/AgCl have been experienced. For example, ammonium acetate buffers have caused flaking of the surface of glassy carbon electrodes held at as little as +0.1 V for one batch of electrodes. Noble metal electrodes are easily contaminated by a number of eluents unless the electrode is cleaned by pulsing the applied voltage as in carbohydrate analysis (Chapter 3, Section 6). 

Reductive cleaning by a large negative potential pulse, or Ejed (-1.6 to -2.0 V) for a period of t ed- In addition, any Au ions in the diffusion layer are reduced to elemental gold, which greatly extends the Ufe of the electrode... 

The pulse sequence with three different potentials that is usually applied for the detection of carbohydrates is shown in Figure 8.12a. However, the cleaning potential E2 being applied for 200 ms leads to an electrode recession that enlarges the cell volume as shown in Figure 8.13. This, in turn, lowers the linear speed of the liquid through the detector cell that consequently results in a decrease of the response factor for a given analyte concentration. 

The main idea of application of variety of potential-time waveforms for polarization of working electrodes is employing a decrease of the activation barrier for oxidation of determined analytes. The electrocatalytic electrode process is frequently based on stabilization of reaction intermediates via the adsorption at clean noble electrode surface. Typical pulse sequences are shown in Figure 4.2. 

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

A Pt electrode in acidic aqueous methanol was cleaned of adsorbed methanol fragments by pulsing the potential to 1.4 V vs. RHE for a few seconds prior to stepping the potential to various values between 0.4 V and 0.05 V for 15min, after which IRRAS spectra were collected. The spectra are shown in Figure 3.34. As can be seen from the figure, the frequency shift is... 

For the liquid chromatographic detection of carbohydrates and alcohols, a pulsed waveform of the type illustrated in Figure 27.16A is employed the technique is called pulsed amperometric detection (PAD). In this case, the potential is stepped to El, where oxidation of the compound takes place. The current is sampled for a short time at the end of the pulse (16.7 ms), where charging current is at a minimum. The potential is then stepped to E2, where the electrode undergoes oxidative cleaning. Last, the oxide-free surface is regenerated at potential E3. 

Pulse amperometric detection (PAD) has been used for the detection on a PDMS chip. This method is useful for analysis of underivatized compounds, such as carbohydrates, amino acids, and sulfur-containing antibiotics, which easily caused electrode fouling. In PAD, a high positive potential (1.4-1.8 V) is first applied in order to clean the electrode (e.g., Au) surface. This is followed by a negative potential step (-0.5 V) to reactivate the electrode surface. A third moderate potential (+0.5 to +0.7 V) is applied for detection of the target analyte [752]. 

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