StationaryPhases
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The advantages of gas chromatography gas-solid type from gas-liquid-solid can be
summarized as follows: 1) stabilityof stationaryphase on awide range of temperatures; 2) a
low detector noise limit; 3) lower values of HETP (height equivalent to a theoretical plate)
than for gas-liquid chromatography (the adsorption-desorption process can be faster than
the corresponding diffusion process in the liquid phase); 4) increased capacity of structural
selectivity in separating geometrical isomers (e.g. using molecular sieves or graphitized
carbon black); 5) high chemical selectivitywhen complexing agents are used as adsorbents;
6) high adsorption capacity allows the separation of gaseous or vapour compounds at room
temperature; 7) increased chemical stability of adsorbents provides analysis of aggressive
compounds; 8) important techniques have been developed in solids photochemistry and
therefore there is better knowledge of adsorption; 9) numerous studies of heterogeneous
catalysisweredeveloped.
The limitations of this technique can be summarized as follows: 1) risk of asymmetric area
due to the nonlinear adsorption isotherm of some analyzed compounds; 2) low
reproducibility of chromatographic characteristics because adsorbents’ properties are not as
easily standardized as liquid substances; 3)more loss of the analyzed compounds as a result
of irreversible adsorption or catalytic conversion of the separationprocess; 4) limited access
to different commercial adsorbents for gas chromatography; 5) strong dependence of
retention time of the sample size is common due to nonlinear adsorption, which combines
the main advantage of gas-solid chromatography to the efficiency of capillary
chromatography.
Adsorbents’ selectivity is independent of the type of column used (packed or capillary
column). Gas chromatography of adsorption has applications not only in the separation of
gasmixtureswith low boilingpoints, but also in the separation of hydrocarbonmixtures, as
well as someorganic compounds and some aromatic separation.
In the recent past therewas adistinct tendency towards solid-gas chromatography. This can
be attributed to two factors: first, new achievements in the field of adsorbents and second,
using the improved logistics of gas chromatography (better trace analysis, more
reproducible techniques for temperatures programming and high coupling techniques).
Both solid and liquid stationary phases must have extraordinary properties, among which
solid stationary phases are designed to solvemany critical issues involved (Rotzsche, 1991).
Adsorbents can be classified according to following criteria: chemical structure and
geometric structure.
According to the chemical nature classification different interaction types of molecules’
samples can take place. Kiselev & Yashin (1985) proposed grouping adsorbates into four
groups (A-D), and in threegroups (I-III) of adsorbents.
Adsorbents of type I
have no functional groups or ions on the surface and thus they are not
able to interact specifically with adsorbates. Interaction with all types of molecules A-D
occurs non-specifically. Adsorbents are saturated hydrocarbons, graphite or rare gas
crystals. The most important representatives of this type is graphitized carbon black
(thermal graphitized carbon black) whose properties are close to ideal non-specific
adsorbents. Similar to graphite are some inorganic compounds, such as graphite-like boron
nitride (BN) or sulphides ofmetals (i.e.MoS) (Avgul et al., 1975).
