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A packed column most often uses several meters of ⅛‐inch o.d. stainless steel tubing, although early PGCs used larger diameters, and some PGCs now employ ‐inch o.d. “micropacked” columns.

In the traditional packed column, the packing is a granular porous solid with particles about the same size as granulated sugar. These particles pack tightly together inside the tube so that any sample molecules moving with the carrier gas are in intimate contact with them.

The type of column so produced depends on the role of the solid particles:

 An active‐solid column contains solid particles having a large activated surface area to selectively adsorb certain molecules from the sample gas.Since the stationary phase is solid, this technique is gas‐solid chromatography (GSC).

 A liquid‐phase column contains solid particles having a coating of non‐volatile liquid to selectively dissolve certain molecules from the sample gas.Since the stationary phase is liquid, this technique is gas‐liquid chromatography (GLC).

Many columns now use proprietary stationary phases, often made from specialized polymer material. These columns don't easily fit into the old classifications of GSC or GLC, so the terminology is becoming passé.

In a liquid‐phase column, we call the granular solid an inert support. In real life, an inert support might not be completely inert; it sometimes affects the performance of a column.

The thickness of the liquid film coated on the support is an important variable. The liquid loading gives the percentage by weight of liquid on support.

The first gas chromatographs used packed columns, and they are commonly found in PGCs today.

Open‐tubular columns

An open‐tubular or capillary column uses several tens of meters of capillary tubing having an internal diameter ranging from about 100 to 530 μm. The mode of operation differs: the stationary phase adheres to the inner wall of the tube, and the carrier gas flows down the middle. Figure 1.6 illustrates three versions (Harvey 2017):

 A “wall‐coated open‐tubular” or WCOT column uses tubing made of fused silica. The stationary phase is a very thin layer of a non‐volatile liquid coated on the inside wall of the tube to selectively dissolve sample molecules from the sample gas. PGCs rarely use these columns as they are fragile and tend to be unstable in use.The WCOT columns typically use fused‐silica tubing and tend to be too fragile for process use. The PLOT and SCOT columns mostly use steel capillary tubing. Figure 1.6 Three Kinds of Capillary Column.

 A “porous‐layer open‐tubular” or PLOT column uses stainless steel capillary tubing. The stationary phase is a very thin layer of solid material coated on the inside wall of the tube to selectively adsorb sample molecules from the sample gas.

 A “support‐coated open‐tubular” or SCOT column typically uses stainless steel capillarytubing. The stationary phase is a coating on very fine support particles in a uniform layer on the inner wall of the tube. These rugged columns have become quite popular in process gas chromatographs.

Open‐tubular columns have smaller diameters than packed columns and require special operating techniques. As in packed columns, the film thickness is an important variable, but we'll defer discussion on that. While they achieve better separations, the operating conditions of open‐tubular columns can be difficult to sustain in the process environment.

For more information about column types and column liquid phases, refer to the excellent detailed review by Rahman et al. (2015).