Packed, Open-Tubular, and Preparative Columns

It was indicated earlier that column lengths of up to 300 ft are not unusual. It should be mentioned here that the longer a column with the stationary phase tightly packed (a so-called packed column), the greater the gas pressure required to sustain the flow of carrier gas. A 20-ft length is approximately the 

Cross sections of (a) a packed column and (b) an open-tubular capillary column. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

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