Commonly Used Techniques

The sections below describe the most commonly used techniques. There are many variations and permutations for all of these. The reader is referred to the software documentation and original literature for clarification of the details. 

The nonquantitative detection of radioactive emission often is required for special experimental conditions. Autoradiography, which is the exposure of photographic film to radioactive emissions, is a commonly used technique for locating radiotracers on thin-layer chromatographs, electrophoresis gels, tissue mounted on sHdes, whole-body animal sHces, and specialized membranes (13). After exposure to the radiolabeled emitters, dark or black spots or bands appear as the film develops. This technique is especially useful for tritium detection but is also widely used for P, P, and 1. 

Polyhalogenobenzene Hydrolysis. The chlorobenzenes can be transformed into chlorophenols by hydrolysis in a Hquid-phase basic medium. The two most commonly used techniques are treatment in aqueous alkaU medium at a temperature between 200 and 350°C (48), or a milder hydrolysis (200—250°C) treatment with dilute sodium hydroxide in the presence of copper. The hydrolysis may be carried out in the vapor phase (250—400°C) on soHd catalysts based on rare-earth phosphates (49) or copper-beating siUca. 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

One commonly used technique which gives high accuracy is Simpson s Rule (more correctly called Simpson s Rule). Here the area under the curve is divided into equal segments of width, h. For an even number of segments, m, we can divide the range of interest (MAX — MIN) into ordinates xq to where the number of ordinates is odd. 

Critical to hazard characterization is the identification of hazards and the assessment of possible worker exposure. This can be accomplished in a variety of ways. As described before, one commonly used technique is a JHA with project teams that include the worker. The information collected is used by the SSHO and the radiation control officer to develop an appropriate hazard control and protection strategy. 

Scrubbers are the most commonly used technique for separating particulates from gas streams. The field of scrubber technology is very mature, with literally hundreds of thousands of scrubbers in operation worldwide, scrrne in operation for more than 50 years. 

The comparison among these techniques is tabulated in Table 22.1. In summary, HdC is a separation technique with low selectivity however, the efficiency can be very high. Especially in PCHdC, high analysis speed can be achieved over a wide MW range. ThFFF performs best for the separation of high MW samples. SEC has an intermediate selectivity between FldC and ThFFF. Practicality makes SEC the most suitable method for the common MW range of synthetic polymers. SEC is by far the most commonly used technique for molecular weight distribution determinations. However, HdC is better for the fast analysis purpose. 

The most commonly used techniques for contact angle measurements are the sessile drop method and the Wil-helmy plate method. Results obtained from these two methods are in good agreement. 

In liquid chromatography, in contrast to gas chromatography [see Section 9.2(2)], derivatives are almost invariably prepared to enhance the response of a particular detector to the substance of analytical interest. For example, with compounds lacking an ultraviolet chromophore in the 254 nm region but having a reactive functional group, derivatisation provides a means of introducing into the molecule a chromophore suitable for its detection. Derivative preparation can be carried out either prior to the separation (pre-column derivatisation) or afterwards (post-column derivatisation). The most commonly used techniques are pre-column off-line and post-column on-line derivatisation. 

Three related methods based on the quasiisostatic method are used to measure permeability. The most commonly used technique allows the permeant gas or vapor to flow continuously through one chamber of the permeability cell. The gas or vapor permeates through the sample and is accumulated in the lower-concentration chamber. At predetermined time intervals, aliquots are withdrawn from the lower cell chamber for analysis. The total quantity of accumulated permeant is then determined and plotted as a function of time. The slope of the linear portion of the transmission-rate profile is related to the sample s permeability. 

The following examples demonstrate commonly used techniques for determining Cpj and K,. 

The technique of AC Impedance Spectroscopy is one of the most commonly used techniques in electrochemistry, both aqueous and solid.49 A small amplitude AC voltage of frequency f is applied between the working and reference electrode, superimposed to the catalyst potential Uwr, and both the real (ZRe) and imaginary (Zim) part of the impedance Z (=dUwR/dI=ZRc+iZim)9 10 are obtained as a function of f (Bode plot, Fig. 5.29a). Upon crossplotting Z m vs ZRe, a Nyquist plot is obtained (Fig. 5.29b). One can also obtain Nyquist plots for various imposed Uwr values as shown in subsequent figures. 

Due to diffraction effects of micron-sized mirrors in a regular array, commonly used techniques for surface characterization based on interferometry are inefficient. To overcome the diffraction effects we have developed a novel surface characterization method with an incoherent light source, based on the Foucault s knife-edge test (Zamkotsian and Dohlen, 1999). Since Leon Foucault introduced the knife-edge test in the last century (Foucault, 1859), it has been widely used for testing optical surfaces (see Ch. 3). The test offers a simple way of obtaining easily understandable, qualitative information of the surface shape. 

Usually, physisorption is carried out using nitrogen as an adsorbate at 77.3 K, the boiling point of liquid nitrogen. The solid material is called the adsorbent. Adsorption can be measured in many different ways. So-called volumetric adsorption, in which volumes of gas are introduced. sequentially while simultaneously measuring the pressure, is a commonly used technique. Fig. 3.41 shows a schematic of the equipment used (referred to as barometric equipment ). 

For illustration we will present some commonly used techniques for chemisorption measurements. Chemisorption can be measured gravimetrically, volumetrically, or spectroscopically. Also, pulse techniques, and Temperature Programmed Desorption (TPD) can be used. 

A survey of the literature with a key phrase tissue residue analysis yielded a distribution of separation and detection techniques as outlined in Table 2. LC with either UV or fluorescence detection was the most common separation and detection technique, representing 61% of the citations. The results are an indication of the maturity of LC as a common, well-understood technique. The second most commonly used technique cited in the literature (13%) was GC with either a mass-selective or electron capture detector. GC is also a mature technology and a good choice owing to the... 

Several extraction techniques are used in the analysis of soil. The following are brief descriptions of some of the most commonly used techniques. 

Several devices are also available to promote airway clearance. Flutter valve devices employ oscillating positive expiratory pressure (OPEP) to cause vibratory air flow obstruction and an internal percussive effect to mobilize secretions. Intrapulmonary percussive ventilation (IPV) provides continuous oscillating pressures during inhalation and exhalation. Finally the most commonly used technique is high-frequency chest compression (HFCC) with an inflatable vest that provides external oscillation. Vest therapy is often preferred by patients because they can independently perform the therapy even from an early age.5,14... 

For small projects, and for simple choices between alternative processing schemes and equipment, the decisions can usually be made by comparing the capital and operating costs. More sophisticated evaluation techniques and economic criteria are needed when decisions have to be made between large, complex projects, particularly when the projects differ widely in scope, time scale and type of product. Some of the more commonly used techniques of economic evaluation and the criteria used to judge economic performance are outlined in this section. For a full discussion of the subject one of the many specialist texts that have been published should be consulted Brennan (1998), Chauvel et al. (2003) and Vale-Riestra (1983). The booklet published by the Institution of Chemical Engineers, Allen (1991), is particularly recommended to students. 

The most commonly used techniques for the separation and purification of miscible liquids are distillation and solvent extraction. In recent years, adsorption, ion exchange and chromatography have become practical alternatives to distillation or solvent extraction in many special applications. 

Solids separation. Methods used for solids separation include most of the commonly used techniques ... 

There are two commonly used techniques for determining the range of process variables where these steps influence conversion rates. The first approach involves carrying out a series of runs... 

One of the simplest, most commonly used techniques for preparing a catalyst involves dis-... 

Mass Loss Rate as a Function of Temperature. The most commonly used technique is thermogravimetry (TG). The basic components of modern TG have existed since the early part of this century (22-2 4). 

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