StationaryPhases
33
Type 2
Uniformly porous adsorbents with wide pores. Silica gel with pore diameters
between 10 and 200 nm (Porasil, Spherasil) and some other forms of silica gel chemical
bonded (Durapak a.o.) andother polymers styrenedivinylbenzene typewith largepores (20
–400nm).
Type 3
includes adsorbents with a uniform pores system, but small pores. In this category
should bementionedmolecular sieves (zeolites), carbonmolecular sieves, porous glass and
porous polymers. Poresdiameter is around 10nm.
Type 4
presents adsorbentswith non-uniform pores. Amongmaterials of this type there are
activated carbon and alumina. Because of geometrical and chemical heterogeneity, having
pores in the range 2-20 nm (mezopores) up to 200 nm, these adsorbents are not suitable for
GCS. Classification is based on pores size. The difference between porous adsorbents and
non-porous adsorbents consists in the free spaces form developed by the porous system.
This system is quantitatively characterizedby the followingparameters (Rotzsche 1991):
-
Specific surface areaS
A
(geometric sizeof theporewall area/gramof adsorbent).
-
Specificporevolume
V,
(total porevolume/gramof adsorbent).
-
Meanporediameter
d
So
(averagediameter of 50%of thepores).
-
Pore sizedistribution.
These parameters could be measured by gas chromatography and other methods, such are
mercury porosimetry and reversed size exclusion chromatography. The ratio of pore
diameters for porous adsorbents to the molecules diameters of the adsorbate is a significant
parameter because thehigher ratio is (themoleculediameters are smaller comparedwithpore
diameters), the faster the adsorption equilibrium is reached. For a similar size of pore
diametersandmoleculesdiameters theadsorption ratedepends equallyon thepore shapeand
the adsorbate molecules’ size. For narrow pores the adsorbedmolecule on surface atoms of
onepore could interactwithother pore surface atoms and in this case the adsorbedmolecules
are trapped, and the transferofmoleculesbetweenadsorbent andmobilephase is stopped.
The most widely used chromatography supports are Kieselguhr (diatomaceous earth =
light-coloured porous rock composed of the shells of diatoms) based, containing polysilicic
acid in the hydrated amorphous silica form having a porous structure and containing
varying amounts ofmetal oxides of Fe, Al,Mg, Ca, Na, K. These supports are knownunder
different commercial terms, the most commonly used type being
Chromosorb
. They are
found in several varieties, ofwhich themost important are:
-
Chromosorb P
(pink) is reddishpink andhas a surface area situatedbetween 4 -6m
2
/g.
It has the advantage that it canbe loadedwith a large amount of liquid stationaryphase
(up to 25 30%) and has highmechanical strength. It is not sufficiently chemically inert
and for this reason it isusedmainly to separatenon-polar substances.
-
Chromosorb W
(white) is white and has a smaller surface area than Chromosorb P, 1-2
m
2
/ g. Therefore it can be loadedwith a smaller amount of liquid stationaryphase, up
to 15%. It is chemically inert, but is friable, having a lowmechanical strength. It has
general usability, so it can be used to separate all classes of compounds, bothpolar and
non-polar.
-
Chromosorb G
surface area is even smaller than ChromosorbW, of only 0.5m
2
/ g and
can upload up to 5% stationary phase. But it has the advantage of combining the
chemical inactionof typeWwith themechanical strengthof typeP.
