Capillary Chromatography - Capillary Tube Connections 3
Figure 14. Different Types of Small Bore Columns
The first small bore column to be explored was the packed column. The mass sensitivity of a detector-column combination is a function of the I.D. of the column and so the reduction of the columns diameter is highly desirable for trace analysis (mass sensitivity is the minimum detectable mass, as opposed to the minimum detectable concentration and depends not only on the detector sensitivity but also on the dimensions of the column). The I.D. of small bore packed columns is considered to be 1 mm or less but GC columns having an I.D of less than 0.5 mm are extremely difficult to pack. Small bore packed columns, however, are popular and extensively used in LC. The wall coated open tubular column (WCOTT), the impressive term given to capillary columns, have I.D.s lying between 100 mm and 550 mm and carry a film of stationary phase on the wall surface that ranges between 5 mm to 50 mm thick.
The third type of small bore column is the porous layer open tubular column (PLOT) which also has and I.D. lying between 100 mm and 550 mm but, in this case, carries a layer of adsorbent particles on the wall surface that can range between 5 mm to 50 mm thick and which holds the stationary phase. The PLOT column was designed to increase the loading capacity of the column but is far more difficult to construct than the simple capillary column and is, consequently, less commonly used.