Sample Preparation—Electrodes

Silver and gold, which have relatively high vapor pressures, can be easily applied by vacuum evaporation, while metals of lower vapor pressure, such as platinum, palladium, and stainless steel, can be deposited by radio frequency sputtering in this case, however, the film thicknesses are limited to about 100 nm. Before electrode deposition, the samples must be cleaned and heat treated. Failure to ranove surface impurities will result in loss of adhesion when samples are subsequently heated during the measurement. 

The preparation of electrodes is only the first step towards a successful experiment. The measurement system must also be kept free of elements such as silicon or phosphorus, which, under high temperature reducing conditions, form volatile compounds that attack the electrodes, for example platinum, to produce brittle and poorly-conducting compounds. 

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Species/Sample matrix Sample preparation Electrode Analysis technique Literature... 

When working with metal electrodes, the energy of the electrons in the metal is lower than the vacuum level by the work function of the metal, which tends to be 3-5 eV. Work functions of some materials relevant to LED devices are collected in Table 10.2 [11]. The work function can vary depending upon the crystal facet from which emission is measured (or if the metal is amorphous), and sample preparation details. The photoelectric (PE) effect is exploited in XPS (ESCA) or UPS to measure the work function. It is very critical to realize that, in these experiments, what is measured is the energy required to remove an electron to a point just outside the surface of the solid, not to infinity. At this range, the dipolar forces at the surface are still active, and one can learn about surface dipoles in the material. 

FIA star 5010 Modular, semi- or fully automatic operation. May be operated with process controller microprocessor. Can be set up in various combinations with 5017 sampler and superflow software which is designed to run on IBM PC/XT computer 60-180 samples h Dialysis for in-line sample preparation and in-line solvent extraction.Thermostat to speed up reactions. Spectrophotometer (400-700nm) or photometer can be connected to any flow through detector, e.g. UV/visible, inductively coupled plasma, atomic absorption spectrometer and ion-selective electrodes... 

Fig. 7.4 XRD signal for powder samples prepared from bulk Si (top) and micro PS films produced on p-type electrodes using different anodization current densities, as indicated in...

The TOF experiment suffers from the special sample preparation and the potential influence of the electrode. As we will show, these difficulties do not arise in XTOF. 

In recent years, with the emergence of nonimpact printers for electronically processed or stored information, a less familiar technique has been found very useful. The technique is a nondestructive method of investigating transport properties of photoreceptors that are used in these systems. The technique is called XTOF, and it can be conveniently employed in parallel with the conventional xerographic measurements for photoreceptor characterization. Here we note that the TOF experiments suffer from the special sample preparation needed and the potential influence of the electrode. These difficulties do not arise in XTOF. 

Choosing the Right Electrode. Choosing the correct dimensions of the electrodes for your experiment is straightforward since it will dependent on your sample size and sample container. However, the type of electrode used in your method will depend on the matrix of your sample and the measurement conditions for the sample preparation. Ideally, sample solutions will have a pH in the range 2 to 12, a temperature between 10 and 50°C, and an ion concentration between... 

Electrode Preparation. Electrodes were prepared by mixing the samples with equal parts of pure graphite. To insure homogenous mixing and to determine the plate sensitivity, 50 ppm indium was added as an internal standard to the sample-graphite mixture. (In this paper, ppm are given in terms of weight.) The mixtures were pressed into... 

The source unit must vaporize and excite a portion of the sample, which is generally used as one of the electrodes between which the electric discharge takes place. No single excitation source is ideally suited for all applications of emission spectrochemistry. Trace impurities in metals, alloying constituents in high concentrations, biological substances, ceramics, slags, oils, nonconductors, refractories—all may require different excitation techniques and sample preparation procedures. Table 1 summarizes the important characteristics of the commonly used spectrochemical source units. 

The combination of preconcentration by electrodeposition with stripping by voltammetry is probably the most sensitive electroanalytical method in common use today. Consequently, this popular technique is discussed in more detail in Chapter 24. Preconcentration of material by an electrode reaction has been used in sample preparation for atomic absorption, neutron activation, x-ray fluorescence, microprobe, and several other spectroscopic techniques. 

The first step in sample preparation is the deposition of a thin metal film on an insulating substrate (e.g. a glass microscope slide). This base electrode is deposited by conventional vacuum deposition techniques with the electrode geometry defined by a shadow mask. Next, this electrode is oxidized either by exposing the film to room air or oxygen, or by establishing an oxygen plasma within the vacuum chamber. In the case of Al-electrodes, a remarkably uniform oxide layer is formed, typically 1-2 nm thick. The oxide film may then be dosed with the compound of interest this is achieved in one of three ways. 

Turning to assays using sensors that require more than a simple cut in the sample, Bergann et al. [37] paid considerable attention to sample preparation in attempts to measure lactate in meat with a sensor (not thick film) based on reaction of hydrogen peroxide with a platinum electrode or on the reaction of ferrocene carboxylate with the active site of lactate oxidase. Sample preparation entailed extraction into buffer following grinding. In some cases, ground samples were left to allow the lactate to diffuse into the buffer solution. This was quite effective but slow (up to 90 min). Ultimately, the quality of the assay was dependent on the method of sample preparation. 

A brief sample preparation procedure based on solid-phase extraction was developed [45]. Wool was extracted in acetonitrile for 30 min, the fluid then being filtered and passed through a C18 solid-phase cartridge. The acetonitrile was removed under vacuum and replaced with methanol for application of the putative inhibitors to enzyme-based electrodes. 

The instruments discussed earlier are the primary ones used in toxicant analysis, but an enormous number of analytical techniques are used in the field. Many of the instruments are expensive (e.g., Raman spectrometers, X-ray emission spectrometers) and few laboratories possess them. Many other instruments are available, however, such as the specific-ion electrode, which is both sensitive and portable. Specific-ion electrodes have many other advantages in that sample color, suspended matter, turbidity, and viscosity do not interfere with analysis therefore many of the sample preparation steps are not required. Some of the species that can be detected at ppb levels are ammonia,... 

The polarographic and potentiometric methods are not HPLC-run. The polarographic method relies upon the measurement of half-wave potentials of various sulfur compounds reacting with a mercury electrode. It is sensitive to submicromolar concentrations (Luther, pers. com.). While sulfide, thiosulfate, polysulfide and polythionates can be measured, the initial sample must be subdivided and pretreated in different ways. The disadvantages are that sample preparation ana analysis are time-consuming and there is no way to preserve samples for later analysis nor to study organic thiols with precision., ... 

Nondrug-related impurities can be extracted from various components employed in the sample preparation of a low-dose drug product. Classic sources of this type of contamination are glassware,20 filters, centrifugation tubes, HPLC vials and caps and transfer pipettes.20 Chemicals from the pH electrode can also be extracted into the sample diluent while adjusting the pH of the solution. In addition, impurities in reagents used in sample preparation1 can pose issues. 

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