Fused silica electrical conductivity

CE, another high performance separation technique, was also proved to be a powerful tool and an alternative for HPLC in the analysis of natural dyestuffs, even if its application in this field is still considerably limited. It could play an important role especially in the analysis of artworks, as it requires a very small volume of a sample solution (a few dozen nanolitres). In CE[ 10 14] separation of charged species is based on their different migration properties along the capillary tube which is in a constant electric field. Two platinum electrodes and both ends of a narrow bore (i.d. 25 100 pm) flexible fused silica capillary (usually 60 100 cm long) filled with a suitable conducting buffer are immersed in two... 

The relatively good electrical conductivities of metals are due almost entirely to the presence of the free electrons in the electron cloud. They can be caused to move, i.e. an electric current can be made to flow, by the application of much less energy—i.e. a louver electromotive force—than would be needed to bring about actual separation of the electrons from their outer orbits. Electrical insulators have crystal lattices in which there are no free electrons. Thus, for example, in fused silica all the valency electrons are tied up in holding the silicon and oxygen atoms together. Here we have an explanation of the fact that the electrical conductivity of the metal copper is about 1024 times greater than that of the insulator fused silica. 

CE is mostly performed in fused silica (FS) capillaries, which form a suitable separation compartment because of their reproducible physical dimensions, good electrical isolation properties, high thermal conductivity, and transparency in the short wavelength UV radiation used for detection. However, the dissociation of silanol groups created on a... 

Fused silica has a mass density of 2200 kg.m while vitreous silica is slightly denser with a density of 2210 kg.m Mechanically, fused silica is a relatively strong but brittle material with a tensile strength of 28 MPa, a compressive strength of 1450 MPa, and a Mohs hardness of 5. Both grades exhibit an extremely low coefficient of thermal expansion (e.g., 0.6 x 10 K from 20 to 1000°C) and a remarkable thermal shock resistance together with a low thermal conductivity. Fused silica, with a dielectric field strength of 16 MV.m , exhibits also excellent electrical insulation capabilities up to 1000 C. When heated above 1150 C, fused silica converts irreversibly into a-cristoballite as follows ... 

Capillary electrophoresis is an analytical technique which enables efficient and rapid separation of ionic compounds under the influence of an electric field. The analysis is conducted in a fused silica capillary that is filled with an electrolytic solution, or buffer. Each end of the capillary is placed in a buffer reservoir along with electrodes attached to a high-voltage power supply. The power supply is then used to apply an electric field, up to 30 kV, between the two reservoirs. A schematic of a typical CE instrument is shown in Fig. 10. 

The instrumentation of HPCE is uncomplicated (see the schematic drawing in Figure 1). Briefly, both ends of the narrow-bore fused silica capillary are immersed into reservoirs containing a buffer solution that also fills the capillary. The reservoirs also contain electrodes that provide electrical contact between the high-voltage power supply and the capillary. The sample is loaded onto the capillary by replacing one of the buffer reservoirs by a sample reservoir and applying external pressure (hydrodynamic injection) or an electric field (electrokinetic injection). After the injection, the reservoir is replaced, the electrical field is applied, and the separation starts. The detection is usually performed at the opposite end of the capillary (normal polarity mode). UV/vis detection is by far the most common detection technique in HPCE. Other techniques include fluorescence, amperometry, conductivity, and mass spectrometry. Modem HPCE instruments are fully automated and thereby allow easy operations and precise quantitative analyses. 

In traditional electrophoresis, electrically charged analytes move in a conductive liquid medium under the influence of an electric field. Introduced in the 1960s, the technique of CE was designed to separate species based on their size to charge ratio in the interior of a small capillary filled with an electrolyte. The technique uses fused silica capillaries under an electrical field, and separation occurs either solely on the basis of molecular weight and charge, electrophoretic mobility, or associated with differential solubilisation in surfactant micelles. Associated with preconcentration, pre-derivatisation or incapillary derivatisation techniques in order to overcome the problem of inherent low sensitivity, this method has shown potential for excellent separation efficiency (Asthana et al. 2000 Cavallaro et al. 1995 Sun et al. 2009). 

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