Column porous-layer

PLOT Columns. Porous-layer open tubular columns wherein the internal wall is coated with a layer of adsorbent support. If the support is then coated with a liquid phase it is referred to as a SCOT Column (i.e., support-coated open tubular column). 

One type of column is the wall-coated open tubular column (WCOT) in which the stationary phase is applied and bound directly to the walls of the column. Porous-layer open tubular columns (PLOT) are columns in which the stationary phase is deposited on fine particles of absorbent, absorbed on the walls of the column, increasing the available surface area of the column wall. Support-coated open tubular (SCOT) columns are those... 

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. 

The overall method for gasoline requires no sample preparation other than the addition of internal references. The method is fast since samples may be run at 3-min intervals, interference-free and specific for MMT. Analysis time comparison is favourable with respect to atomic absorption analysis which typically required about 15 min per sample when run in batches. Further, the procedure is readily adapted for use with open tubular columns. Porous layer open tubular columns (PLOY) are best suited to accommodate the direct injection of gasoline samples. The improvement in analysis time for MMT determination by the latter method may outweigh the somewhat poorer limit of detection resulting from limitations on the sample size. 

Finally, the useful life of an analytical column is increased by introducing a guard column. This is a short column which is placed between the injector and the HPLC column to protect the latter from damage or loss of efficiency caused by particulate matter or strongly adsorbed substances in samples or solvents. It may also be used to saturate the eluting solvent with soluble stationary phase [see Section 8.2(2)]. Guard columns may be packed with microparticulate stationary phases or with porous-layer beads the latter are cheaper and easier to pack than the microparticulates, but have lower capacities and therefore require changing more frequently. 

Although capillary columns are generally preferred for most applications, packed and porous layer open tubular (plot) GC columns provide the best separation of low-boiling fluorinated compounds. 

Figure 6, High pressure liquid chromatogram of creatine kinase isoenzymes. First peak, MM second peak, BB. Conditions 50 cm X 4.8 mm (i.d.) column with yydac porous layer bead anion exchange mobile phase, step gradient Solvent A, 10 mmol/liter Tris buffer, pH 8.3 solvent B, 10 mmol/liter Tris buffer, pH 7.0,0.5 mol KCl flow rate, 2 ml/min detection, collected fractions assayed (45).

Porous-Layer Open Tubular (PLOT) and Support-Coated Open Tabular (SCOT) Columns... 

Porous-layer- open tubular (PLOT) and support-coated open tubular (SCOT) columns are prepared by extending the inner surface area of the capillary tube. A layer of particles can be deposited on the surface or the column wall can be chemically treated to create a porous adsorbent layer. Obviously some of the wall-modified open tubular columns discussed in section 2.3.3 could be... 

Figure 2.15 Separation of a mixture of saturated and unsaturated hydrocarbons on a 50 m x 0.32 mm fused silica porous-layer open tubular column coated with alumina modified wl potassium chloride. The separation was performed by temperature programming from 70 to 200°C at 3 C/mln. (Reproduced with permission fr

PA(S) Photoacoustic (spectroscopy) PLOT Porous-layer open-tubular column... 

For use with 25 cm x 4.6 mm analytical columns, guard columns and scavenger columns are often 4.6 mm internal diameter and 3-10 cm in length. They can be packed with microparticulate stationary phases or with porous layer beads. Porous layer beads are cheaper than microparticulates and are easier to pack, but they have lower capacities and will require changing more often. It is usually difficult to know how long a pre-column will last before it requires changing. In routine work, precolumns are usually repacked or replaced to a fixed schedule. 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

The fourth type of mechanism is exclusion although perhaps inclusion would be a better description. Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout. Separations occur because of variations in the extent to which the solute molecules can diffuse through an inert but porous stationary phase. This is normally a gel structure which has a small pore size and into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical size are excluded from the gel and move unhindered through the column or layer whilst smaller ones are retarded to an extent dependent on molecular size. 

Porous layer open tubular (PLOT) columns, 4 615 6 379... 

These have now been superseded by capillary columns, which offer greatly improved separation efficiency. Fused silica capillary tubes are used which have internal diameters ranging from 0.1 mm (small bore) to 0.53 mm (large bore) with typical lengths in excess of 20 m. The wall-coated open tubular (WCOT) columns have the internal surface of the tube coated with the liquid (stationary) phase and no particulate supporting medium is required. An alternative form of column is the porous-layer open tubular (PLOT) column, which has an internal coating of an adsorbent such as alumina (aluminium oxide) and various coatings. Microlitre sample volumes are used with these capillary columns and the injection port usually incorporates a stream splitter. 

An open-tubular column is a capillary bonded with a wall-supported stationary phase that can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The inner diameters of open-tubular CEC columns should be less than 25 pm that is less than the inner diameters of packed columns. The surface area of fused silica tubing is much less than that of porous packing materials. As a result, the phase ratio and, hence, the sample capacity for open-tubular columns are much less than those for packed columns. The small sample capacity makes it difficult to detect trace analytes. 

FIGURE 6 Scanning electron micrograph of a polymeric porous layer in capillary column. (Reproduced with permission from reference 33.)... 

Huang, X., Zhang, J., and Horvath, C. (1999). Capillary electrochromatography of proteins and peptides with porous-layer open-tubular columns.. Chromatogr. A 858, 91-101. 

These phases are composed of adsorbing materials molecular sieves, alumina, porous glass and gels (such as Chromosorb 100, Porapak and PoraPLOT1 ), and graphitised carbon black. They are mainly used to separate gases or volatile compounds. Capillary columns made by deposition of these materials in the form of very fine particulates are called PLOT (porous layer open tubular) columns. 

The wall-coated column in Figure 24-2c features a 0.1- to 5-p.m-thick film of stationary liquid phase on the inner wall of the column. A support-coated column has solid particles coated with stationary liquid phase and attached to the inner wall. In the porous-layer column... 

---