AdvancedGasChromatography–Progress inAgricultural, Biomedical and Industrial Applications
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3. Typesof stationaryphase for adsorption
The conditions that liquid stationaryphases have tomeet are:
-
To be a good solvent for the components of the sample, but the solubility of these
components has tobedifferentiated;
-
To be practically non-volatile at the temperature of the column (vapour pressure to be
less than 0.1mmHg);
-
Be chemically inert;
-
Have ahigher thermal stability.
The categories of the stationary liquidphase are:
-
Non-polar stationary phases
, which are compounds, such as hydrocarbons (paraffin) or
silicone oils (polysiloxanes) without grafted polar groups. Examples of such stationary
phases are: squalane (C
30
H
62
hydrocarbon), silicone oilsmetylsilicon type (namesOV 1,
SE-30). These phases separate the analyzed compounds in order of increasing boiling
points.
-
Polar stationary phases
, which contain a high proportion of polar groups, i.e. the average
molecular weight polyethylene glycol (Carbowax 20M), silicone oils with cianopropil
groups (OV 225 cianopropyl-methyl-phenyl silicone) diethylene glycol succinate
(DEGS), nitrotereftalic ester of polyethylene glycol (FFAP) etc. They differentiate
between the non-polar andpolar compounds, retainingonly thosewhich arepolar, and
areused especially to separatepolar compounds.
-
Intermediate polarity stationary phase
, containing polar groups in the lower concentration
or polarizable groups grafted onto a non-polar support. Examples of such stationary
phases are phenyl methyl silicone phase (OV 17), dinonyl phthalate, polyethylene
glycols having highmolecularweight. They are universal stationary phases, which can
beused to analyzebothpolar andnon-polar compounds.
-
Specific stationary phases
, which are used in certain cases. They contain compounds that
interact only with certain components of the mixture to be analyzed, for example
AgNO
3
dissolved inpolyethyleneglycolwhich forms adductwitholefins.
-
Chiral stationary phases
, containing chiral compounds interacting with only one optical
isomer of apair of enantiomers. Suchphases are basedon cyclodextrinor certain amino
acids.
-
Given thepolarityof the stationaryphase andpossible interactions, organic compounds
canbegrouped in terms of chromatographic separation in the following five classes:
-
Class I
very polar compounds, able to give hydrogen bonds: water, glycerin, glycols,
hydroxyacids, and amino acids. These compounds are difficult to separate by gas
chromatography, due to high polarity. Excepting water, they derivatized before
separation.
-
Class II
polar compounds, which have active hydrogen atoms: alcohols, carboxylic
acids, phenols, primary and secondary amines, nitro andnitrileswith a hydrogen atom
in
ǂ
position. These compounds are separated inpolar stationaryphases.
-
Class III
intermediate polarity compounds, without active hydrogen: ethers, esters,
aldehydes, ketones, nitro and nitriles without hydrogen atom in
ǂ
position. These
compounds are separatedon stationaryphases of intermediatepolarity.
-
Class IV
compounds with low polarity, but which have active hydrogen: aromatic
hydrocarbons, alkenes, chloroform,methylene chloride, dichloroethane, trichloroethane
