Type of capillary

Fig. 19.7 Reference electrodes and capillaries, (a) Reference electrode, salt bridge and Luggin capillary (A) calomel electrode (c) frontal types of capillaries and positions (r/) rearside capillaries (after von Fraunhofer and Banks )...

Figure 5 shows three different types of capillary viscometers often used for viscosity measurements of polymer solutions. The disadvantage of the Oswald viscometer is that the viscometer has to be charged with the solution to a precise level and fine adjustments need to be made at the temperature of measurement. The Ubbelohde viscometer, also frequently referred to as the suspended level viscometer, is particularly useful when a series of different polymer concentrations is to be measured. The filling volume needs not to be adjusted precisely. The largest dilution ratio obtainable is limited only by the ratio of the volume of bulb B to that of the volume between the bottom of bulb B and the top of bulb C. For the compact version (Figure 5(c)) smaller sample volume is needed. There are also capillary viscometers available that can be coupled with liquid... 

Viscometric manipulations are simplified considerably by using the modified Ubbelohde viscometer described by Davis and Elliott.26,126 This type of capillary viscometer can be modified to enable an estimate of the shear correction to be made,127 although a Couette viscometer128 is more satisfactory for this purpose. 

The various types of capillary electrophoresis are performed either in free solution or in gels. The choice of method depends on the nature of the sample and the analytical objective but capillary gel electrophoresis, including iso-electric focusing and SDS electrophoresis, is particularly useful for protein applications. 

The efficacy of CE separation depends considerably on the type of capillary. Fused-silica capillaries without pretreatment are used most frequently. Its outside is coated with a polymer layer to make it flexible and to lessen the occurrence of breakage. The polymer coating has to be dissolved with acid or burned away at the detection point. Capillaries with an optically transparent outer coating have also found application in CE. The objectives of the development of chemically modified capillary walls were the elimination of electro-osmotic flow and the prevention of adsorption on the inner wall of the capillary. Another method to prevent the adsorption of cationic analyses and proteins is the use of mobile phase additives. The modification of the pH of the buffer, the addition of salts, amines and polymers have all been successfully employed for the improvement of separation. 

Selection of capillary length is dictated by the type of capillary used and the required resolution. When using coated capillaries with insignificant EOF, separations can be achieved with relatively short capillaries of 20 to 30 cm effective length... 

To diminish this problem, a capillary with a sharp tip is used [8], which may be prepared by drawing out a normal capillary. To overcome the shielding effect significantly, the overall diameter of the capillary at its end should be considerably smaller than the mean diameter of the mercury drop. It should, however, be mentioned that work with this type of capillary is more difficult, and the production of such capillaries with a uniform drop time requires considerable practice. 

This unit describes a method for measuring the viscosity (r ) of Newtonian fluids. For a Newtonian fluid, viscosity is a constant at a given temperature and pressure, as defined in unit hi. i common liquids under ordinary circumstances behave in this way. Examples include pure fluids and solutions. Liquids which have suspended matter of sufficient size and concentration may deviate from Newtonian behavior. Examples of liquids exhibiting non-Newtonian behavior (unit hi. i) include polymer suspensions, emulsions, and fruit juices. Glass capillary viscometers are useful for the measurement of fluids, with the appropriate choice of capillary dimensions, for Newtonian fluids of viscosity up to 10 Pascals (Newtons m/sec 2) or 100 Poise (dynes cm/sec 2). Traditionally, these viscometers have been used in the oil industry. However, they have been adapted for use in the food industry and are commonly used for molecular weight prediction of food polymers in very dilute solutions (Daubert and Foegeding, 1998). There are three common types of capillary viscometers including Ubelohde, Ostwald, and Cannon-Fenske. These viscometers are often referred to as U-tube viscometers because they resemble the letter U (see Fig. HI.3.1). 

There are several types of capillary inlets in use today which can be divided into two categories direct on-column injection and split/splitless injection techniques. 

Fig. 7.6. RMS roughness measurements as determined by AFM for the four types of capillaries involved in the etching and chemical modification processes.

Because the benzene derivative and nitric acid are immiscible, the impact of mixing/ distribution on slug formation was investigated. Uniform slugs of the aromatic compound/nitric acid were formed in a Y-piece [22]. The capillary attached has a stabilizing effect on the slug flow. The deviation of slug size distribution is very small (about 5%). Hence, interfacial area is nearly constant for this type of capillary flow. 

Determine the viscosity in capillary tubes by measuring the amount of time it takes for a given volume of liquid to flow through a calibrated capillary tube. Calibrate the capillary tube by using liquids of known viscosity. The calibration may be supplied with the viscometer tube when purchased along with specific instructions for its use. Many types of capillary viscometer tubes are available, and exact procedures will vary... 

The most distinguishing phenomenon in capillary electrophoresis is whether the experiments are performed in the absence or in the presence of electroos-motic flow (EOF), (see Chapter 6 for details on EOF). Unlike other types of capillary electrophoresis, isoelectric focusing can be performed under both modes. Since the experimental and theoretical principles governing these modes of CIEF are different, they will be discussed separately. 

Separation columns are the heart of any CEC techniques. In order for CEC to become a versatile analytical technique, it is important to have columns with good stability, high EOF velocity generation, and affinity for different analytes. To this end, several different types of capillary columns have been developed for the separation of biomolecules. In this section, a summary of recent advances in column technology, including open tubular columns, duplex columns, and monolithic columns, will be presented. A comparison of different column configurations is shown in Figure 13. 

Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses GC analyses are performed on a WCOT (wall-coated open tubular) polydimethylsiloxane fused silica capillary column (1 fim film thickness, 50m x 0 32mm ID ) Shorter columns for example 25m, or columns with thinner films, for example 0 2/im, may also be used successfully Injection of the silylated sample (2/d) is performed with a moving needle-type injector, other types of capillary GC injection systems may be used after optimization of the concentration and amount of sample to be injected With a 50-m capillary... 

The optimum operating conditions for GC analysis must be determined prior to the GC-MS analysis. In general, this is accomplished by injecting the sample solution several times while varying such parameters as carrier gas, splitting ratio of carrier gas at the injection port fitted with a split liner, carrier gas flow rate, and oven temperature profile. In addition, the type of capillary column used and the concentration and volume of the sample to be injected also have to be optimized. 

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