Fused-silica capillary columns popularity

Conventional gum phases, methylpolysiloxanes (SE-30, OV-1), 5% phenylmethylpolysiloxane (SE-52), 5% phenyl-95 % dimethylpolysiloxane, and 5% phenyl-1% vinylmethylpolysiloxane (SE-54) are popular stationary phases in capillary column GC, by which separation of PAHs up to coronene (relative molecular mass = 300) may be achieved. The stationary phase giving the best separation for a maximum number of PAHs, however, is SE-54 bonded to fused silica capillary columns. It is a slightly more selective stationary phase than SE-30, its selectivity arising from the polarizability of its phenyl groups. 

A particle beam interface (PBI) better serves the purpose of LC-EI-MS [55-57]. The PBI removes the solvent by nebulization into an evacuated desolvation chamber from where the evolving microscopic sample particles are transferred into an El ion source via a jet separator (Fig. 5.14). Designs different from PBI have also been developed [57]. The PBI is comparatively robust and attained popularity in particular for low- to medium-polarity analytes, but has some drawbacks such as poor sensitivity especially with water-rich mobile phases, moderate linearity with polar compounds, and low tolerance for heat-sensitive compounds [58,59]. The most recent addition to LC-EI interfaces, also the simplest and most elegant solution, makes use of the very low liquid flow rates of nano-LC equipment [60,61]. The flow from a 30 pm i.d. fused silica capillary column is passed... 

The use of packed glass or stainless steel columns has been abandoned long ago and 30-60-m wall-coated open mbular fused silica capillary columns are used for essential oil analysis. For fast GC, shorter but more efficient columns are used to achieve analysis in a much shorter time. In capillary columns, gums or gum-like phases are preferred. Apolar silicone phases such as SE-30, SE-52, OV-1, OV-73, OV-101, DB-1, and DB-5 have become popular for essential oil analysis. Polyethylene glycol (PEG), polypropylene glycol, carbowax phases, and their nitroterephthalic esters also are used. PEG 20M and Car-bowax 20M are polar phases. 

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

Fused silica capillaries are the most popular capillaries and are commercially available with internal diameters ranging from 10 to 200 pun, although the most commonly used capillary dimensions are 25, 50, and 75 /xm I.D. and 350-400 /xm O.D. To minimize distorting refractive index effects and light scattering with on-column optical detection, the ratio of the outer... 

The most popular column diameters available for fused silica capillaries are 0.18, 0.25 narrow bore), 0.32 and 0.52 mm (magabore or wide bore). Other diameters are also available from various manufacturers. The following guidelines apply to selection of a column diameter. 

Despite the several detailed procedures reported for the fabrication of packed columns for CEC [14,17,20,27,30,47-50], column fabrication may still be regarded as an art. A reliable and reproducible performance of a column depends on the column fabrication. Poorly packed columns can lead to low efficiency, poor resolution, and asymmetric peak shapes. The capillary tubes typically used to fabricate CEC columns are fused silica tubes with inner diameters of 100 pm or less, with 50 and 75 pm I.D. being the most popular. The small inner diameter allows for heat dissipation, which is generated by the applied electric field. Packing such columns is an elaborated process and a skill that requires experience. 

For a period of time, OT columns that had characteristics intermediate between those of WCOT and packed columns were popular. There were two types, but they were similar. Support coated open tubular (SCOT) columns had a thin layer of solid support coated on the inside wall of a capillary tube of larger diameter than that used for WCOT columns. This layer was coated with stationary liquid similar to packed columns. Porous layer open tubular (PLOT) columns were similar but made differently for example, the solid support was added while the capillary tube was being drawn. With a few exceptions, SCOT and PLOT columns are no longer popular because wide diameter WCOT fused silica columns are as good, more stable (no layer to flake off), and easier to use. Subsequent discussion will be restricted to WCOT columns. 

CEC columns are generally made of fused-silica tubing, usually packed with the appropriate stationary phase. Today, the most commonly used CEC columns have i.d. of 100 p,m or less, with 50 and 75 p,m i.d. being the most popular. The stationary phase is retained in the column by two frits. Column designs can be categorized into two major types OT columns and packed structures, which include packed columns, monolithic columns, and microfabricated stractures (open or continuous beds). Packed capillary columns are most commonly used, as has been demonstrated in numerous papers [9-11]. They can be subdivided into three different categories columns packed with particles, columns with continuous beds fabricated in situ creating a rod-like monolithic structure, and columns with immobilized or entrapped particulate materials. 

The heart of the chromatograph is the column the first ones were metal tubes packed with inert supports on which stationary liquids were coated. Today, the most popular columns are made of fused silica and are open tubes (OT) with capillary dimensions. The stationary liquid phase is coated on the inside surface of the capillary wall. The two types are shown in... 

Two types of stationary phases are most popular the polysiloxanes and polyethylene glycol phases. Both types of phases may be characterized as having the necessary high viscosity and the capability for crosslinking and/or chemical bonding with fused silica. One should note the presence of more recent additions to the capillary column family, namely, specialty columns designed for selected EPA methods, chiral separations, and gas-solid chromatographic separations. These specialty phases are considered in Section 3.11.6. 

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