Topics - Column Diameter
The column diameters employed in chromatography vary widely with the type of chromatography that is being used and the size of the sample being separated. In general, analytical columns have small diameters and preparative columns have wide diameters, but this is a generalization. For example, materials prepared for assessing biological activity may be handling only a few micrograms of material and, thus, can be prepared using columns 1 mm - 2 mm in diameter. The smallest diameter columns are those used in capillary GC for analytical purposes. They are normally constructed from fused quartz (although metal columns are still sometimes used in the hydrocarbon industry) and range in diameters from 50 m ID to 500 m ID and in length from 5 - 10 m to 100 - 200 m. Such columns are used to, either produce very fast separations or, very high efficiencies and, as a consequence, high resolution for complex mixtures. Larger columns (but in this case packed) having diameters between 0.5 - 2 mm are employed for similar purposes in LC to produce very fast separations or high efficiencies. These LC columns range in length from 10 cm to 2 m, the longer columns being rather difficult to pack. Packed columns having diameters ranging from 2 to 6 mm ID are used in both GC and LC. The columns 2 mm - 4.6 mm ID, 50 cm to 2 m long) are used in gas solid chromatography (GSC) largely for the analysis of gas samples or low boiling hydrocarbons. LC columns 2 mm – 4.6 mm ID can be 3 cm to 50 cm long and are used mainly for analytical purposes or where only relatively small samples are required for preparative work. Due to the high flow impedance of packed columns, short LC columns are packed with small particles (ca 3 m) and the longer columns with larger particles (ca 20 m). The diameters of preparative columns both in GC and LC can be as large as the economical use of the mobile phase can permit. GC and LC preparative columns have been constructed a meter or more in diameter which demands an enormous (and costly) amount of stationary and mobile phases. There appears to be no practical limit to the diameter of a preparative column but there will be an economic limit ultimately determined by the value of the product.