Gas Chromatography - GC Columns > The Capillary or Open Tubular Column > Page 27


Figure 14. The Separation of a "Benzole" Mixture on a Packed Column, 40 ft Long


The column was packed with 5% w/w polyethylene glycol adipate coated on deactivated fire brick and operated isothermally at 130˚C with an inlet gas pressure of 140 psi. The analysis time was about 3.5 hours. The column efficiency was about 40,000 theoretical plates and all the xylene isomers are separated. The two previous off-scale peaks are benzene and toluene. This separation could be achieved equally well on a open tubular column and probably in less than half the time. The advantage of the packed column would be that much higher sample loads can be placed on the column and thus the dynamic range of the analysis can be made much greater. Components present at a level of 0.001% can be easily separated and determined quantitatively without any preliminary fractionation or concentration.

The Capillary or Open Tubular Column

Capillary columns are fabricated from stainless steel or quartz. Metal capillary columns must be carefully cleaned to remove traces of extrusion lubricants before they can be coated, usually by washing with methylene dichloride, methanol and then water. After removing oil and grease, the columns are washed with dilute acid to remove metal oxides or other corrosion products that may remain adhering to the walls, washed with water and the again washed with methanol and methylene dichloride. Finally the column is dried in a stream of hot nitrogen. Metal columns provide the high efficiencies expected from open tubular columns and were used for the analysis of petroleum and fuel oils, etc. Metal columns, however, have some disadvantages as although easily coated with dispersive stationary phases (e.g., squalane, Apiezon grease etc.) they are not so easily coated with the more polar stationary phases such as CARBOWAX. In addition, hot metal surfaces can cause decomposition or molecular rearrangement of many thermally labile materials such as the terpenes contained in essential oils. Metal can also react directly with some materials by chelation and adsorb polar material which results in asymmetric and tailing peaks. Nevertheless, metal columns are rugged, easy to handle and easy to remove and replace in the chromatograph consequently, their use has persisted in many application areas despite the introduction of fused silica columns.