AdvancedGasChromatography–Progress inAgricultural, Biomedical and Industrial Applications
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Reaction fordeactivationwithdisilazane is:
Si –OH+NH(SiR
3
)
2
→
Si –O –SiR
3
+NH
2
–SiR
3
Si –OH+NH
2
–SiR
3
→
Si –O–SiR
3
+NH
3
Uniform films of stationary phase can be obtained when the adhesion forces between
stationary phase and capillary column surface are greater than the stationary phase
molecules. Cohesion forces are stronger with increased similarity between organosiloxanic
groups and functional groups of stationary phase. Methyl groups have high thermal
stability, are chemically inert and are used to deactivate capillary columns with non-polar
stationaryphase. Phenyl groups have lower thermal stability thanmethyl groups, but allow
the deposition of medium polarity stationary phase. Cyanopropyl and trifluoropropyl
groups are strong polar and it is difficult to bond on the glass by reactionwith disilazane,
because disilazane have a reduced thermal stability. Another method of blocking the
activity of silanol groups is based on reaction with cyclosiloxanes. Deactivation with
cyclosiloxanes allows bonding some functional groups similar to those of the stationary
phase.
Another method for modifying the surface chemistry of glass and silica capillary tubes is
based on the stationary phase, by blocking the surface active centres, as well as getting a
very stable film of stationary phase. Carbowax 20Mwas the first stationary phase used to
deactivateglass capillary tubes. This step is followedby aheating treatment.
Another method of decomposition of the stationary phase is to use radicalic initiators. The
initiator used for immobilization of polyethylene glycol is dicumilperoxide. Another
possibility of immobilization of Carbowax 20 M is based on the reaction between OH
groups of polyethylene glycol with diisocyanate in the presence of dibutyltindilauril. A
chemically inert and thermal stability capillary column improves also by immobilizing the
stationaryphase polysiloxanes typewith terminal hydroxyl groupsOH, such asOV-17-OH,
OV-31-OH,OV-240-OH.
Changing the surface chemistry by immobilizing the stationary phase can be donewith a
large number of siloxane polymers. A simplemethod of immobilization is polymerization
in a column of silicon monomers consisting in the formation of bonds Si–O–Si between
siloxane polymers and the capillary column wall. Immobilization can be done in the
presence of radicalic initiators. Immobilizationof the stationaryphase canbe initiated also
by gamma radiation at room temperature. Stationary phaseswithout vinyl groups require
a high dose of radiation. Immobilization in the presence of gamma radiation eliminates
the danger of degradation of the stationary phase and allows very precise control of
reaction.
Micro-PackedColumns
Due to their parameters concerningmass capacity, phase ratio, resolution and short analysis
time, micro-packed columns are very attractive. They are micro pore tubes having inner
diameters ranging from 0.3 - 1 mm and at lengths varying from 1 to 15 m, packed with
particles 0.007-0.3 mm in diameter. Table 6 presents geometrical characteristics for packed
columns andmicro-packed columns.
