Glass capillaries

The MCB tubes suit ideally into a concept of using glass capillary X-ray light guides to increase radiation intensity at the measurement spot. 

Perhaps the best discussions of the experimental aspects of the capillary rise method are still those given by Richards and Carver [20] and Harkins and Brown [21]. For the most accurate work, it is necessary that the liquid wet the wall of the capillary so that there be no uncertainty as to the contact angle. Because of its transparency and because it is wet by most liquids, a glass capillary is most commonly used. The glass must be very clean, and even so it is wise to use a receding meniscus. The capillary must be accurately vertical, of accurately known and uniform radius, and should not deviate from circularity in cross section by more than a few percent. 

The general attributes of the capillary rise method may be summarized as follows. It is considered to be one of the best and most accurate absolute methods, good to a few hundredths of a percent in precision. On the other hand, for practical reasons, a zero contact angle is required, and fairly large volumes of solution are needed. With glass capillaries, there are limitations as to the alkalinity of the solution. For variations in the capillary rise method, see Refs. 11, 12, and 22-26. 

Streaming potential measurements are to be made using a glass capillary tube and a particular electrolyte solution, for example, O.OIM sodium acetate in water. Discuss whether the streaming potential should or should not vary appreciably with temperature. 

The Washburn equation has most recently been confirmed for water and cyclohexane in glass capillaries ranging from 0.3 to 400 fim in radii [46]. The contact angle formed by a moving meniscus may differ, however, from the static one [46, 47]. Good and Lin [48] found a difference in penetration rate between an outgassed capillary and one with a vapor adsorbed film, and they propose that the driving force be modified by a film pressure term. 

Using a visible light probe NSOM is the eadiest of the probe scopes, at least in conception, and is another apparent exception to the diffraction-liinited resolution rule, in that NSOM illuminates an object with a beam of visible light smaller than the diffraction limit. The resolution then is limited only by the size of that beam. To achieve this, light issuing from a very tiny aperture at the end of a glass capillary scans a very near sample. The tip must be located on the order of X/2 from that surface. Resolution in the range of 10—20 nm has been achieved (31). 

Generally, alkan olamines are analyzed by gas chromatography or wet test methods. Details on gas chromatography conditions are available in the fiterature (1) for packed or glass capillary columns. 

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

Fig. 24. (a) Ostwald glass capillary viscometer, (b) Cannon-Fenske viscometer, and (c) Ubbelohde viscometer. 

For accurate and precise measurement the glass capillary must be clean. The viscometer must be cleaned thoroughly after each series of operations. Samples being tested and cleaning solvents should be filtered to remove particles that can clog the capillary. 

AH glass capillary viscometers should be caUbrated carefully (21). The standard method is to determine the efflux time of distilled water at 20°C. Unfortunately, because of its low viscosity, water can be used only to standardize small capillary instmments. However, a caUbrated viscometer can be used to determine the viscosity of a higher viscosity Hquid, such as a mineral oil. This oil can then be used to caUbrate a viscometer with a larger capillary. Another method is to caUbrate directly with two or more certified standard oils differing in viscosity by a factor of approximately five. Such oils are useful for cahbrating virtually all types of viscometers. Because viscosity is temperature-dependent, particularly in the case of standard oils, temperature control must be extremely good for accurate caUbration. 

More recent developments in the rolling ball area include an automated micro viscometer, the Paar AMV 200, from Paar Physica. The specimen to be measured is introduced into a glass capillary down which a gold-covered steel ball roUs. The rolling time is measured automatically. The shear stress may be varied by changing the inclination angle of the capillary tube. The shear rate range is 10 1000, which makes the instmment useflil for... 

Capillary Electrophoresis. Capillaries were first appHed as a support medium for electrophoresis in the early 1980s (44,45). The glass capillaries used are typically 20 to 200 p.m in diameter (46), may be filled with buffer or gel, and are frequendy coated on the inside. Capillaries are used because of the high surface-to-volume ratio which allows high voltages without heating effects. The only limitations associated with capillaries are limits of detection and clearance of sample components. 

FIG. 6-12 Correction factor for PoiseiiiUe s equation at low pressures. Curve A experimental curve for glass capillaries and smooth metal tubes. (From Brown, et al, J. Appl. Phys., 17, 802 [1.946].) Curve B experimental curve for iron pipe (From Biggie, Couiiesy of E. I. du Pont de Nemours [Pg.641]

J. W. Jorgenson and K. D. Lukacs, Zone electrophoresis in open-tubular glass capillaries, Awa/. Chem. 53 1298 (1981). 

Glass capillary column (38 m X 0.2 mm i.d.) coated with heptakis (3-0-acetyl-2,6-di-0-pentyl)-/3-cyclodextrin... 

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