Apiezon L (non-polar) and esters of adipic, succinic and phthalic acids.b-Cyclodextrin-
based phases are available for chiral separations (Section 11.5.5). The most commonly
used support is Celite (diatomaceous silica), which because of the problem of
support–sample interaction is often treated so that the hydroxyl groups that occur in
the Celite are modified. This is normally achieved by silanisation of the support with
such compounds as hexamethyldisilazane. The support particles have a large surface
area and an even size, which, for the majority of practical applications, ranges from
60 80 mesh (0.25mm) to 100120 mesh (0.125 mm) (Section 11.3.1). The smaller
the particle size and the thinner the coating the less band spreading occurs.- Capillary (open tubular) columns: These are made of high-quality fused quartz and are
10 100 m long and 0.11.0 mm internal diameter. They are of two types known as
wall-coated open tubular(WCOT) andsupport-coated open tubular(SCOT), also
known asporous layer open tubular(PLOT) columns, for adsorption work. In WCOT
columns the stationary phase is thinly coated (0.1 5 mm) directly onto the walls of the
capillary whilst in SCOT columns the support matrix is bonded to the walls of the
capillary column and the stationary phase coated onto the support. Commonly used
stationary phases include polyethylene glycol (CP wax and DB wax, very polar) and
methyl and phenyl-polysiloxanes (BP1, non-polar; BP10, medium polar). They are
coated onto the supporting matrix to give a 1% to 25% loading, depending upon the
analysis. The capacity of SCOT columns is considerably higher than that of WCOT
columns.
The operating temperature for all types of column must be compatible with the
stationary phase chosen for use. Too high a temperature results in excessivecolumn
bleedowing to the phase being volatilised off, contaminating the detector and giving
an unstable recorder baseline. The working temperature range is chosen to give a
balance between peak retention time and resolution. Column temperature is controlled
to0.1oC. Analyte partition coefficients are particularly sensitive to temperature so
that analysis times may be regulated by adjustment of the column oven, which can be
operated in one of two modes:- Isothermal analysis: Here a constant temperature is employed.
- Temperature programming: The temperature is gradually increased to facilitate
the separation of compounds of widely differing polarity orMr. This, however,
sometimes results in excessive bleed of the stationary phase as the temperature is
raised, giving rise to baseline variation. Consequently some instruments have two
identical columns and detectors, one set of which is used as a reference. The currents
from the two detectors are opposed, hence, assuming equal bleed from both columns,
the resulting current gives a steady baseline as the column temperature is raised.
The choice of phase for analysis depends on the analytes under investigation and is
best chosen after reference to the literature.
Application of sample
The majority of non- and low-polar compounds are directly amenable to GC, but other
compounds possessing such polar groups asOH,NH 2 andCOOH are generally473 11.9 Gas chromatography