AdvancedGasChromatography–Progress inAgricultural, Biomedical and Industrial Applications
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Adsorbents of type II
develop positive partial charge on the surface adsorbent. Besides these
dispersion forces specific interactions develop leading to the orientation and localization of
adsorbatemolecules on centres having high charges. That concerns the salts having cations
with positive charge with small ray, while negative charges are distributed in a relatively
large volume (i.e. BaSO
4
). Thus, the most significant representatives of this type are
adsorbents with functional groups of protonated acids, such as hydroxylated silicagel,
Lewis acids aprotic centres located on the surface. Molecules A type (saturated, rare gases)
are non-specifically adsorbed and they are dispersion forces only.Molecules B, C andD can
be adsorbed non-specifically. Type B includesmolecules with electron density localized on
some bonds or atoms: type II bonds of unsaturated or aromatic compounds, functional
groups, atoms having electrons couple (ethers, ketones, tertiary amines, pyridines and
nitriles), moleculeswith high quadrupolemoments (N
2
molecules). Interactions between an
adsorbent type II and an adsorbed type B develop between centres with electronic high-
density (molecules in the sample) and positive charges of adsorbent (i.e. acidic proton of
hydroxylated silica gel or a cation of Li, Na, Mg, Ca in acid zeolite or aprotic acids Lewis
type on the surface). Type Cmolecules have a positive charge located on ametal atom and
the excess of electrondensity is distributed on adjacent bonds (organometallic compounds).
Type D are molecules containing peripheral functional groups (OH, NH, etc.) whose
electronicdensity increase thedensityof some atoms (O,N) and reduce thedensity for other
atoms (H). This group includeswater, alcohols andprimary and secondary amines. Specific
interactions of type D adsorbates with Type II adsorbents involves the forces between the
adsorbent centreswithpositive charges and theonly coupleof electrons of the atoms ofOor
Nof the samplemolecules.
Adsorbents type III
. Adsorbents types III are specific having centreswithhighdensity centres
on the surface. In this group are polymers as polyacrylonitrile, copolymers of vinylpyridine
anddivinylbenzene andpolymers groupwith (C=O) and (–O–) on the surface. This group
could include porous polymers based on styrene ethylvinylbenzene cross linked with
divinylbenzene, varyingbyusingdifferent polymerizationpromoters and evennon-specific
dispersion forces. Adsorbents Type III includes crystalline surfaces formed by anions and
chemically modified non-specific adsorbents covered with a monomolecular layer of
adequate substance, creating negatively charged surface centres. Adsorbents type III
interacts non-specificwith adsorbates typeA and specificwith typeB, CorDby forces from
the negative charges of the adsorbent surface and from the positive charges of metallic
atoms (C) or from functional groups (OH, NH) proton type (D) and dipole or induced
dipole type (B).
Classificationaccording togeometric structure
This classification concerns the possibilities to increase the surface. Increasing the surface
meets a series of reserves, such as an increase in surface leads to increased dispersionwith
an increase in heterogeneity. Therefore, increasing contact points between particles will
reduce the pore diameter with Knudsen diffusion disadvantage. Kiselev & Yashin (1985)
have overcome these difficulties in the development of GCS. The adsorbents are
geometrically classified as follows (Rotzsche, 1991).
Type 1
Non-porous adsorbents
Crystalline products with a smooth surface (sodium chloride, graphitized thermal carbon
black, BN,MoS,). S
A
in the rangeof 0.1–12m
2
/g.
