Page 266 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV

resulting in a greater number of secondary reactions. An important
advantage of the static-mode system, however, is the usually larger sample
capacity.
(b) Dynamic ($lament) reactors in which the sample is placed on the tip of a
filament or wire igniter (platinum and nichrome wires have been used)
which is then sealed in a reactor chamber; in PGC the latter is typically
the injection port of the gas chromatograph. As the carrier gas passes over
the sample, a d.c. charge is applied, and the sample is heated rapidly to the
pyrolysis temperature. As the sample decomposes, the pyrolysis products
are carried away into a cooler area (reducing the possibility of secondary
reactions) before entering the gas chromatographic column.
3. The column. The actual separation of sample components is effected in the
column where the nature of the solid support, type and amount of liquid phase,
method of packing, length and temperature are important factors in obtaining
the desired resolution.
The column is enclosed in a thermostatically controlled oven so that its
temperature is held constant to within 0.5 OC, thus ensuring reproducible
conditions. The operating temperature may range from ambient to over 400 OC
and for isothermal operation is kept constant during the separation process.
Although many types of column have been developed for gas chromatography,
they may be divided into two major groups:
(a) Packed columns. Conventional analytical columns are usually prepared with
2-6 mm internal diameter glass tubing or 3-10 mm outer diameter metal tubing,
which is normally coiled for compactness. Glass columns must be used if any
of the sample components are decomposed by contact with metal.
The material chosen as the inert support should be of uniform granular size
and have good handling characteristics (i.e. be strong enough not to break down
in handling) and be capable of being packed into a uniform bed in a column.
The surface area of the material should be large so as to promote distribution
of the liquid phase as a film and ensure the rapid attainment of equilibrium
between the stationary and mobile phases. The most commonly used supports
(e.g. Celite) are made from diatomaceous materials which can hold liquid phases
in amounts exceeding 20 per cent without becoming too sticky to flow freely
and can be easily packed.
Commercial preparations of these supports are available in narrow mesh-
range fractions; to obtain particles of uniform size the material should be sieved
to the desired particle size range and repeatedly water floated to remove fine
particles which contribute to excessive pressure drop in the final column. To a
good approximation the height equivalent of a theoretical plate is proportional
to the average particle diameter so that theoretically the smallest possible
particles should be preferred in terms of column efficiency. Decreasing particle
size will, however, rapidly increase the gas pressure necessary to achieve
flow through the column and in practice the best choice is 80/100 mesh for a
3 mm i.d. column. It may be noted here that for effective packing of any column
the internal diameter of the tubing should be at least eight times the diameter
of the solid support particles.
Various types of porous polymers have also been developed as column
packing material for gas chromatography, e.g. the Porapak series (Waters
Associates) and the Chromosorb series (Johns Manville) which are styrene