Wall-coated capillary columns

Wallach transformation orgchem By the use of concentrated sulfuric acid, an azoxy-benzene is converted into a pura-hydroxyazobenzene. wal ak. tranz-far.ma-shan wall-coated capillary column analychem Acaplllary column characterized bya layer of stationary liquid coated directly on the Inner wall of a colled capillary tube. wol kod-3d kap-3 ler-e. kal am )... 

A recent modification of the atmospheric pressure ionization technique involving a special low dead volume interface was described by Thomson etal. [27]. They employed packed microbore columns (170 p, 320 p, and 500 p I. D. with lengths ranging from 5 to 15 cm) in conjunction with a low-volume, wall-coated capillary column as an interface. The total ion current chromatogram of the tryptic digest sample of about 1 picomole of human growth hormone is shown in figure 29. The column was packed with an octadecyl bonded phase... 

The phase ratio, l.e. the ration of the volumes occupied by the gas phase and the stationary phase. Is given the symbol H. For wall coated capillary columns, Is approximated by ... 

Table III. Efficiency of Wall Coated Capillary Columns...

For gas chromatographic analysis a 50 m PONA fused silica wall coated capillary column (BP-1) was applied in temperature-programmed mode (-50 to 220 °C) with flame ionisation detection (FID) of hydrocarbons. 

Wall-coated capillary columns are therefore increasingly used in SFC [30]. These have the great advantage of high efficiency because of greater length consequent on the permeability of capillaries to the flow of low-viscosity supercritical fluids. The low volume flow rates of capillary columns facilitate the use of toxic, corrosive, inflammable, or expensive fluids. Capillary columns are also compatible with a variety of detectors (section 9.3), permit maximum... 

Chen-Ying Liu Cho-Chum Hu Cheng-Lung Yang Synthesis and characterization of the novel wall coated capillary column for the separation of polycyclic aromatic hydrocarbons. J. Chromatogr. A, 1997, 773, 199-208. 

Low White Ultrafine particle size used to coat inside walls of capillary columns typical particle size is 1-4 pm... 

A fourth type of column is the whisker walled (WW) in which the wall of the column has been etched by chemical means, leaving behind whiskers on the surface of the column. These projections of the fused silica significantly increase the available surface area of the column. Wall-coated, porous-layer, and support-coated capillary columns have all been available as whisker walled and have been given the acronyms WWCOT, WWPLOT, and WWSCOT, respectively. 

Open tubular columns are simply capillary tubes in which the inside of the column wall is used as the support for the liquid phase. These wall-coated open tubular columns (WCOT) have the stationary phase distributed in the form of a thin film on the inside surface of the open capillary tube, the walls thus serving as the support. In order to reduce the thickness of the liquid phase film, a porous layer may be formed on the inside wall of the capillary tubing and then coated with the liquid phase to produce a support-coated open tubular column (SCOT). Porous-layer open tubular colunms (PLOT) are similar to SCOT colunms, the difference being that in the former, the stationary phase is deposited on fine crystalline particles or glass powder which is adsorbed onto the walls of the tube. In both cases, the available surface area of the wall is increased, and allows an increased amount of liquid phase to be accommodated in the same length and diameter of tubing. The whisker-walled (WW) colunm consists of whiskers chemically etched on the surface of the wall, which also result in a significant increase in the available surface area. Wall-coated, porous-layer, and support-coated capillary columns are all available as whisker-walled, i.e., WWCOT, WWPLOT, and WWSCOT, respectively. 

There are two basic disadvantages to the coated capillary column. First, the limited solute retention that results from the small quantity of stationary phase in the column. Second, if a thick film is coated on the column to compensate for this low retention, the film becomes unstable resulting in rapid column deterioration. Initially, attempts were made to increase the stationary-phase loading by increasing the internal surface area of the column. Attempts were first made to etch the internal column surface, which produced very little increase in surface area and very scant improvement. Attempts were then made to coat the internal surface with di-atomaceous earth, to form a hybrid between a packed column and coated capillary. None of the techniques were particularly successful and the work was suddenly eclipsed by the production of immobilize films firmly attached to the tube walls. This solved both the problem of loading, because thick films could be immobilized on the tube surface, and that of phase stability. As a consequence, porous-layer open-tubular (PLOT) columns are not extensively used. The PLOT column, however, has been found to be an attractive alternative to the packed column for gas-solid chromatography (GSC) and effective methods for depositing adsorbents on the tube surface have been developed. 

A comparison of efficiency of HPLC and GC columns was made by Wldmer et al (20). For example, 3 ym packed 10 cm column gives 15,000 plates or 150,000 plates/meter. This compares with 11,880 plates obtained with 0.1 mm l.d. wall coated capillary GC columns. These data suggest that greater efficiency In terms of number of plates/meter Is possible with HFLC. 

For packed columns the efficiency is given by the quality and particle size of the column packing. For wall coated capillaries the efficiency is a matter of the coating film properties. Furthermore, the efficiency depends on the injection mode, solvent effects, flow rate and column dimensions. 

According to the tjT)e and form of the coating, capillary column may be subdivided into wall-coated open tubular (WCOT) type, porous-layer open tubular (PLOT) type, and support coated open tubular type. The most... 

Thin films of ILs have been applied for gas separation in GC either by coating the column material of the packed columns or the inner wall of capillary columns. 

Classical El and Cl sources can be readily coupled to GC and much less readily to liquid flows using particle beam (PB) interfaces. Packed column GCs require the used of a jet separator for successful coupling, while wall-coated capillary GC columns operating with helium flow rates in the range 1-2 mL.min can be introduced directly into a mass spectrometer without overloading the vacuum pumps. 

Figure 3 SEM images of (a) the cross section of the MIL-lOl-coated capillary column, the inset shows the thickness of the MIL-101 coating, (b) The MIL-101 deposited on the inner wall of the capillary column, (c) GC chromatograms on the MIL-lOl-coated capillary column (15-m long x 0.53-mm i.d.) for the separation of xylene isomers and ethylbenzene at 160 °C under a Nj flow rate of 3 mL min . (Reproduced with permission from Ref 46. Wiley-VCH Verlag GmbH Co. KGaA, Weinheim, 2010.)...

One of the most important advances in column construction has been the development of open tubular, or capillary columns that contain no packing material (dp = 0). Instead, the interior wall of a capillary column is coated with a thin film of the stationary phase. The absence of packing material means that the mobile phase... 

Capillary columns are of two principal types. Wall-coated open tuhular columns (WCOT) contain a thin layer of stationary phase, typically 0.25 pm thick, coated on the capillary s inner wall. In support-coated open tuhular columns (SCOT), a thin layer of a solid support, such as a diatomaceous earth, coated with a liquid stationary phase is attached to the capillary s inner wall. 

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

The Liquid Phase. The stationary phase in an open tubular column is generally coated or chemically bonded to the wall of the capillary column in the same way the phase is attached to the support of a packed column. These are called nonbonded and bonded phases, respectively. In capillary columns there is no support material or column packing. 

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