Gas Chromatography: Chromatographic Columns

Chromatographic Columns

A chromatographic column provides a location for physically retaining the station- ary phase. The column’s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be eas- ily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography.

Packed Columns 

A packed column is constructed from glass, stainless steel, copper or aluminum and is typically 2–6 m in length, with an internal diameter of 2–4 mm. The column is filled with a particulate solid support, with particle diam- eters ranging from 37–44 μm to 250–354 μm.

The most widely used particulate support is diatomaceous earth, which is com- posed of the silica skeletons of diatoms. These particles are quite porous, with sur- face areas of 0.5–7.5 m2/g, which provides ample contact between the mobile phase and stationary phase. When hydrolyzed, the surface of a diatomaceous earth con- tains silanol groups (–SiOH), providing active sites that absorb solute molecules in gas–solid chromatography.

In gas–liquid chromatography (GLC), separation is based on the partitioning of solutes between a gaseous mobile phase and a liquid stationary phase coated on the solid packing material. To avoid the adsorption of solute molecules on exposed packing material, which degrades the quality of the separation, surface silanols are deactivated by silanizing with dimethyldichlorosilane and washing with an alcohol (typically methanol) before coating with stationary phase.

More recently, solid supports made from glass beads or fluorocarbon polymers have been introduced. These supports have the advantage of being more inert than di- atomaceous earth.

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1–10 μL are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000–10,000 theoretical plates. Assuming V_max/V_min is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of

Capillary Columns 

Capillary, or open tubular columns are constructed from fused silica coated with a protective polymer. Columns may be up to 100 m in length with an internal diameter of approximately 150–300 μm (Figure 12.17). Larger bore columns of 530 μm, called megabore columns, also are available.

Capillary columns are of two principal types. Wall-coated open tubular columns (WCOT) contain a thin layer of stationary phase, typically 0.25 μm thick, coated on the capillary’s inner wall. In support-coated open tubular columns (SCOT), a thin layer of a solid support, such as a diatomaceous earth, coated with a liquid stationary phase is attached to the capillary’s inner wall.

Capillary columns provide a significant improvement in separation efficiency. The pressure needed to move the mobile phase through a packed column limits its length. The absence of packing material allows a capillary column to be longer than a packed column. Although most capillary columns contain more theoretical plates per meter than a packed column, the more important contribution to their greater efficiency is the ability to fashion longer columns. For example, a 50-m capillary column with 3000 plates/m has 150,000 theoretical plates and, assuming V_max/V_min is approximately 50,3 a peak capacity of almost 380. On the other hand, packed columns can handle larger samples. Due to its smaller diameter, capillary columns require smaller samples; typically less than 10–2 μL.