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GC Columns The first type of GC column was described by James and Martin in the early 1950s and consisted of a straight tube, about 1m long, 3-4 mm ID packed with celite (a diatomaceous earth) carrying about 15% of stationary phase. This device aroused tremendous interest and provoked the introduction of different mobile phases, squalane, dinonyl phthalate, polyethylene glycol and apiezon grease etc. It also provoked the introduction of different support material such as ground calcined celite (firebrick), glass beads and deactivation processes were also introduced. Column lengths were increased, the limit in packed glass columns being 50 ft long operated at an inlet pressure of 200 psi providing an efficiency of over 40,000 theoretical plates. Probably the longest packed metal column constructed was 100 ft and gave an efficiency of over 50,000 theoretical plates. The problem with packed GC columns was their high flow impedance which was solved by Golay when he introduced the capillary column. The capillary column is a simple tube with the stationary phase lining the tube walls as a thin adherent film. The tube diameter can range from about 50 micron to about 500 micron and the column can be many hundreds of meters long. The low pressure drop across open tubular columns allowed very long columns to be used with the consequent extremely high plate count. The first capillary columns were made from copper, these were replaced by cupronickel, which, in turn, was replaced by stainless steel columns. Then D. H. Desty introduced a technique for drawing very long glass columns which were used very successfully for a number of years. The most recent capillary columns are the fused silica columns which are very strong, very inert (in contrast with glass columns) and can be used over a wide range of temperatures. Today fused silica capillary columns are used in probably over 90% of all GC analyses. |
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Author: RPW Scott
Book:Gas Chromatography to 1 mm thick. The Packed GC Column Packed columns are usually constructed from stainless steel or Pyrex glass. Pyrex glass is favored when thermally labile materials are being separated such as essential oils and flavor components. However, glass has pressure limitations and for long packed columns, stainless steel columns are used as they can easily tolerate the necessary elevated pressures. The sample must, of course, be amenable to contact with hot metal surfaces. Short columns can be straight, and installed vertically in the chromatograph. Longer columns can be U-shaped but columns more than a meter long are usually coiled. Such columns can be constructed of any practical length and relatively easily installed. Pyrex glass columns are formed to the desired shape by coiling at about 700˚C and metal columns by bending at room temperature. Glass columns are sometimes treated with an appropriate silanizing reagent to eliminate the surface hydroxyl Author: RPW Scott
Book:Extra Column Dispersion (e) which is generally accepted (2). The properties of the three columns are shown in table 1 including the peak volume, (4sv), which has more significance to the practicing chromatographer It is seen that the dead volume peak widths are very narrow (assessed as (4sv(k'=0)) and range from about 5 ml for the peak from the microbore column to about 15 ml for the 3 mm I.D. and 4.6 mm I.D. columns. Dispersion in Contemporary GC Columns The same mathematical arguments apply to GC columns as to LC columns except that, as the mobile phase is compressible, the pressure correction factor must be applied. viz. Consequently, for a GC packed column, from equation (1) and In addition, for a capillary column, which has no packing, the column volume will be . where (rc) is the radius of the capillary column. Furthermore, the approximate value of (Hmin.) (see Dispersion in Chromatography Columns ) will be Author: RPW Scott
Book:Preparative Chromatography GC Columns Small GC columns, up to 2 or 3 ft long and up to 0.5 in. in diameter can be hand packed using techniques similar to those used in the preparation of analytical columns. The system used is shown on the right hand side of figure 14. The column is fitted with a pre-column so that the column proper can be packed 3 or 4 inches longer than is actually required. The column is packed in a vertical position by adding small quantities of packing (about 2 ml) at a time and tapping the sides of the column until Author: RPW Scott
Book:Preparative Chromatography Author: RPW Scott
Book:Extra Column Dispersion GC Capillary Columns The maximum permissible extra column dispersion for a GC capillary column must be corrected for the compressibility of the mobile phase and the alternative geometry of the tube, Thus, for a GC capillary tube, equation (3) becomes and equation (4) becomes In a similar manner, the maximum sample volume that can be placed on a GC capillary column, assuming half the permissible extra column dispersion is allotted to sample volume, follows directly |