transfer between the phases as this increases efficiency by reducing the C term in equation (4.46). The
most efficient capillary columns are those with the narrowest bore (0.1 mm) and the thinnest liquid
coating (0.1 μm) but they have a very low sample capacity ( 0.1 μl) which necessitates the use of a
sample inlet stream splitter, cold trap or special on-column injection system (p. 95). Sample capacity
can be progressively increased at the expense of reduced efficiency by increasing both column diameter
and liquid phase thickness. Wide-bore capillary columns (>0.5 mm i.d.) with a relatively thick layer (1–
5 μm) have sample capacities approaching those of packed columns so they can be used with a simple
packed column injection system. Furthermore, although their efficiencies are much lower than those of
the narrowest bore columns, they are nevertheless considerably more efficient than packed columns and
have become popular replacements for them. Very rapid separations can be achieved on short wide-bore
capillary columns (5–10 metres), with efficiencies at least as good as those of packed columns.
Capillary columns are much more expensive than packed columns but their working life can be
extended and performance improved by chemically bonding the stationary phase to the wall of the
tubing. This greatly reduces 'column bleed', especially at high operating temperatures, and is
particularly advantageous in minimizing the contamination of detectors and GC-MS systems. Bonded-
phase columns, which can also be washed with solvents to remove strongly retained material
accumulating from samples in the first few metres over a period of time, are becoming the most popular
type of capillary column for routine work. Stationary phases of various polarities are available and the
range of applications includes petrochemicals, essential oils and biomedical samples. For gas-solid
chromatography (GSC) porous-layer open-tubular (PLOT) columns are used. These have a thin porous
layer of a finely divided solid, usually alumina or molecular sieve, deposited on the inside wall of the
tube. They are used to separate mixtures of low RMM hydrocarbons and the permanent gases. A
capillary column separation of organic acids in human urine is shown in Figure 4.24.
(4)—
Detectors
The purpose of a detector is to monitor the carrier gas as it emerges from the column and respond to
changes in its composition as solutes are eluted. Ideally a detector should have the following
characteristics: rapid response to the presence of a solute; a wide range of linear response; high
sensitivity; stability of operation.
Most detectors are of the differential type, that is their response is proportional to the concentration or
mass flow rate of the eluted component. They depend on changes in some physical property of the gas
stream, e.g. thermal conductivity, density, flame ionization, electrolytic conductivity, β-ray ionization,
in the presence of a sample component. The signal from the detector is fed to a chart recorder,
computing integrator or