● Capillary(open tubular) columns have become the most widely-used in
recent years. They consist of long, narrow bore, high-purity quartz tubing
with a very thin layer of a liquid or solid stationary phase (vide infra) coated
or chemically bonded to the surface of the inner wall. This minimizes band
spreading because of rapid mass transfer(Topic D2). Also, as there is an
unrestricted flow of carrier gas through the center of the column generating
little back-pressure, very long lengths (up to 100 m) can be used, resulting in
extremely high efficiencies and resolving power for the separation of
complex mixtures.
Capillary columns are available with a range of internal diameters and
thicknesses of stationary phase. The narrower the bore and the thinner the
coating, the greater the efficiency, but the lower the sample capacity before142 Section D – Separation techniques
Table 2. A comparison of capillary and packed GC columns
Capillary columns (open tubular) Packed columns
Tubing Tubing
Fused quartz (SiO 2 ) Stainless steel or glass
Very high purity (<1 ppm metals)
Length 10–100 m, coiled Length 1–3 m, coiled
Internal diameter 0.1–0.7 mm Internal diameter 2–3 mm
External protective coating of a polyimide or aluminum
Packing
Granular (0.125–0.25 mm), inert, silaceous solid
support for liquid stationary phase or porous solid
adsorbent
Stationary phase Stationary phase
Very thin liquid layer (0.1–5mm) Thin, 1–10 percent w/w coating of liquid for GLC;
Coated or chemically bonded to inside wall (WCOT) must cover solid support completely
For GLC, bonded phases reduce column-bleed Porous solid adsorbent or polymer for GSC
considerably
Very thin finely-divided porous solid (PLOT) for GSC
Injection systems Injection systems
Split, splitless or on-column Flash-vaporization or on-column
Sample capacity Sample capacity
Narrow bore <<0.1ml 0.1–20ml
Megabore 0.1 to 10ml
Preferred carrier gas Preferred carrier gas
Helium or hydrogen Nitrogen
Performance (Fig. 4) Performance (Fig. 4)
Very high efficiencies and resolving power for complex Limited efficiency and resolving power for up to
mixtures of up to 100 or more components, especially about 20 components
for narrow bore columns, but sample capacities limited;
solutes elute at lower temperatures than with
corresponding packed column
Price and source Price and source
Expensive, from specialist supplier Inexpensive, can be packed in the laboratory