overloading causes peaks to tail and resolution to deteriorate. Special injec-
tion systems are therefore required, as described in the previous section, to
reduce the amount of sample injected with a conventional microsyringe. The
widest bore columns (megabore), which exceed 0.5 mm internal diameter,
are the least efficient, but have the highest sample capacities, so special injec-
tion techniques are not needed.
● Packed columnsare much shorter than capillary columns, rarely exceeding
2 m, the length being limited by the back-pressure generated by the gas
flowing through a packed bed. The stainless steel or glass tubing has an
internal diameter of 2 or 3 mm and is filled with a granular material that acts
as a solid supportfor a thin coating of a liquid stationary phase for GLC, or as
an adsorbent for GSC. Solid supports are inert, porous silaceous materials such
as diatomaceous earths (kieselguhrs) with a large surface area. Their particle
sizes vary between 0.125 mm (US sieve mesh 120) and 0.25 mm (US sieve mesh
40), individual columns being packed with particles having a narrow range
between these limits to improve packing characteristics and chromatographic
efficiency. The smaller the particle size, and the thinner the coating of
stationary phase, the less solute bands spread by the multiple path and mass
transfer effects (Topic D2). Packed columns are much cheaper than capillary
columns but their overall efficiencies and resolving power are limited. They are
best suited to the separation of mixtures of up to ten or twenty components.Examples of separations on capillary and packed columns are shown in Figure 4.GLC stationary phasesare thin coatings of very high boiling liquids, oils or
waxes, some with a polymeric structure, e.g. polysiloxanes and polyethylene
glycol. Those for GSC are solid adsorbents and polymers. They can be classified
according to their polarity, varying from nonpolar hydrocarbons to polar poly-
esters, cyanopropyl silicones and alumina. There are special phases that show
particular selectivities for specific types of solute such as fatty acids, bases and
enantiomers, and high-temperature phases based on silicone-carborane co-poly-
mers. Hundreds of stationary phases have been investigated, many having very
similar characteristics, but most laboratories use only a few for routine work.
The most important features of stationary phases are:● They should be nonvolatile, chemically and thermally stable over a wide
temperature range, and nonreactive towards the separating solutes;
● Most liquids have a recommended operating temperature range; beneath the
lower limit, peak shapes become badly distorted because the phases solidify.
At temperatures close to or exceeding the upper limit, the liquid gradually
bleedsfrom the column and/or degrades, which changes the chromato-
graphic characteristics and leads to an unstable detector signal;
● Liquids chemically bondedto the walls of capillary columns bleed much
less, and the columns can be washed through with solvents to remove
strongly retained sample residues contaminating the stationary phase;
● The thinnest coatings of stationary phase give the highest efficiencies and
resolving power, but the lowest sample capacities;
● The choice of stationary phase is determined by the sample; generally, they
should have similar polarities otherwise peaks may be distorted, but
compromise choices must be made where solutes in a mixture have a wide
range of polarities;
● Elution order can be altered by changing the stationary phase where thereD4 – Gas chromatography: principles and instrumentation 143