Page 267 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

APPARATUS 9.2

copolymers although modified with different, mainly polar, monomers. An
entirely different type of porous polymer is Tenax GC which is based on
2,6-diphenylphenylene oxide; a special feature of this column packing is its high
maximum operating temperature of 400 OC. Tenax GC has been used for
concentrating and determining trace volatile organic constituents in gases and
biological fl~ids.~~
The selection of the most suitable liquid phase for a particular separation is
crucial. Liquid phases can be broadly classified as follows:
1. Non-polar hydrocarbon-type liquid phases, e.g. paraffin oil (Nujol), squalane,
Apiezon L grease and silicone-gum rubber; the latter is used for high-
temperature work (upper limit -400 OC).
2. Compounds of intermediate polarity which possess a polar or polarisable
group attached to a large non-polar skeleton, e.g. esters of high-molecular-
weight alcohols such as dinonyl phthalate.
3. Polar compounds containing a relatively large proportion of polar groups,
e.g. the carbowaxes (polyglycols).
4. Hydrogen-bonding class, i.e. polar liquid phases such as glycol, glycerol,
hydroxyacids, etc., which possess an appreciable number of hydrogen atoms
available for hydrogen bonding.

The column packing is prepared by adding the correct amount of liquid
phase dissolved in a suitable solvent (e.g. acetone or dichloromethane) to a
weighed quantity of the solid support in a suitable dish. The volatile solvent is
removed either by spontaneous evaporation or by careful heating, the mixture
being gently agitated to ensure a uniform distribution of the liquid phase in the
support. Final traces of the solvent may be removed under vacuum and the
column packing re-sieved to remove any fines produced during the preparation.
The relative amount of stationary liquid phase in the column packing is usually
expressed on the basis of the percentage by weight of liquid phase present,
e.g. 15 per cent loading indicates that 100g column packing contains 15 g of
liquid phase on 85 g of inert support. The solid should remain free flowing after
being coated with the liquid phase.
Micropacked columns, sometimes referred to as packed capillary columns,
have been used in gas chromatography, e.g. for the determination of trace
components in complex mixtures. These columns are characterised by small
interna1 diameters (id. < 1.0 mm) and packing densities comparable with
conventional packed columns. In general, the column packing technique requires
higher pressures and constant vibration (e.g. ultrasonic) to achieve the necessary
packing density. Micropacked columns give high efficiency but practical
problems, especially sample injection at high back-pressures, have limited their
use.
(6) Open tubular columns. These capillary columns (id. < 1 mm) are increasingly
used in GLC because of their superior resolving power for complex mixtures.
This results from the high theoretical plate numbers which can be attained with
long columns of this type for a relatively small pressure drop. In these capillary
columns the stationary phase is coated on the inner wall of the tube, two basic
types of capillary column being available:
1. wall-coated open tubular (WCOT), in which the stationary phase is coated
directly on to the inner wall of the tubing;