Chromatography porous-layer open tubular columns

Gas-solid chromatography is performed with both packed and open tubular columns. For the latter, a thin layer of the adsorbent is affixed to the inner walls of the capillary. Such columns are sometimes called porous-layer open tubular columns, or PLOT columns. Figure 31-16 shows a typical application of a PLOT column. 

Wall-coated open tubular columns (WCOT columns), or simply capillary columns, and classical packed columns dominate the practice of gas-liquid chromatography today. Porous layer open tubular columns (PLOT columns) and classical packed columns dominate the practice of gas-solid chromatography. WCOT columns are typically up to 100 m... 

Some common adsorbents used for gas-solid chromatography are given below. [Some of these materials are commercially available as porous-layer open tubular columns (PLOT). PLOT columns yield higher sensitivity than do the traditional packed columns because of their higher column permeability and lower pressure drop (than packed columns).]... 

Porous layer open tubular (PLOT) columns were used to separate basic proteins and peptides [15]. The use of these types of columns was prompted by their high permeability and by the relatively high loading capacity due to an increased surface area by the porous layer. The authors showed that under conditions of reversed-phase chromatography at acidic pH, the EOF mobility was over 8-fold higher than that in raw fused-silica capillary, which is an indication of the high surface charge present in the porous layer. As expected, the EOF... 

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. 

The third type, porous layer open-tubular (PLOT) columns, have solid-phase particles attached to the column wall, for adsorption chromatography. Particles of alumina or porous polymers (molecular sieves) are typically used. These colunms, like packed GSC columns, are useful for separating permanent gases, as well as volatile hydrocarbons. The resolution efficiency of open-tubular colunms is generally in the order WCOT > SCOT > PLOT. Wide-bore (0.5-mm) open-tubular... 

Gas chromatography columns for interplanetary exploration should be very efficient, due to the stringent demands imposed on payload. Porous-layer open tubular (PLOT) columns loaded with a styrene-divinyl benzene copolymer showed better performance than Cromosorb 103 or Porapak Q columns in the analysis of low molecular weight hydrocarbons and nitriles. Potassium chloride-deactivated alumina PLOT columns, on the other hand, were not good for the intended purpose, because low molecular weight nitriles were difficult to elute. Wall coated open tubular (WCOT) columns with a stationary chemically bonded phase of dimethyl siloxane have the mechanical resilience to endure the conditions of extraterrestrial exploration and separated efficiently C1-C4 nitriles. Permanent gases were analysed in the presence of hydrocarbons and nitriles with a PLOT capillary column. The equilibrium constant for the interaction of nitrile groups with various solutes was determined by GLC. ... 

Ji Z, Majors RE, and Guthrie EJ (1999) Porous layer open-tubular capillary columns preparations, applications and future directions. Journal of Chromatography A 842 115-142. 

The adsorption (non-porous) wall open tubular column (AWOT) and the adsorption layer open tubular column (ALOT), both used in gas-solid chromatography, should be recognised as separate groups because their structure and chromatographic characteristics differ markedly from those of the other columns. 

Peltonen, K., Vaaranrinta, R. Sampling and Analysis of 1,3-Buta-diene in Air by Gas Chromatography on a Porous-Layer Open-Tubular Fused-Silica Column J. Chromatogr., A, 1995, Vol. 710,... 

Gas-solid chromatography comprises the techniques with an active solid as the stationary phase. Separation depends on differences in adsorption of the sample components on inorganic adsorbents (i.e., silica, alumina, carbon black) or on organic adsorbents such as styrene-divi-nylbenzene copolymers. Separation can also occur by a size exclusion mechanism, such as the separation of gases on synthetic zeolites or molecular sieves. GSC is performed on packed columns or on open tubular columns on the walls of which a thin layer of the porous material is deposited [porous layer open tubular (PLOT) columns], GSC nowadays is used only for special separation problems. and GSC columns are, therefore, referred to as tailor-made columns,... 

Three different types of open tubular columns are in use the wall-coated open tubular (WCOT) (Figure 2.6), porous-layer open tubular (PLOT), and support-coated open tubular (SCOT) columns. WCOT columns are by far the most used. In these columns the liquid stationary phase is coated as a thin film, 0.1-0.5 pm (typically 0.25 pm), on the inner wall of the capillary. The types of stationary phase are described in Section 2.6.2. These columns are used for partition chromatography (GLC). PLOT columns contain a porous layer (or a layer of porous particles) on the inner wall of the capillary and are used for gas adsorption chromatography (CSC), where the porous layer constitutes the stationary phase. The third type is the SCOT columns, where a liquid stationary phase is coated on the porous layer/porous particles at the inner wall of the capillary and hence GLC separations can be carried out. The advantage of the SCOT column compared to the WCOT column is its higher sample capacity. Its disadvantage is lower efficiency. 

Tock and co-workers [101] suggested a very interesting method for preparation of porous silica layers in open tubular columns. This method was worked out for fused silica capillaries used in liquid chromatography. However, in our opinion it is also promising for ALOT columns in gas chromatography. 

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