StationaryPhases
41
Wall-coated open-tubular (WCOT)
columns contain the stationaryphase as a filmdepositedon
the internal surfaceof the tubewall. The formationof a thin anduniform film along the total
lengthof the column represents thekey to capillary columns’ efficiency.
Porous-LayerOpenTubular (PLOT)
To increase the sample capacityof capillary columns anddecrease the film thickness, porous
layers for the insidewalls of the column tubingwere used. The stationary phase amount is
directly related to the efficiencyof capillary columns andgenerates efficiency increasingdue
toporous layers.
The method presented here used capillary columns of glass and quartz, the most used
today. Glass capillaries are drawn from a device consisting of an electric furnace inwhich
glassmelts, two traction rollerswhichpull the column, a curvedmetallic tube for the spiral
of the tube electrically heated at glass melting temperature. Capillary dimensions are set
from the ratio of the two rolls’ traction speed. Silica (quartz) tubes require temperatures of
1800-2000 ° C. After pulling, the capillary tube is coveredwith a polymer film in order to
increase the tensile strength. Polyamide or polyimide polymers are used, which are stable
up to temperatures of 350-400 °C.
For the glass preparation, silicamixedwith somemetal oxides (Na
2
O, CaO,MgO, B
2
O
3
, Al
2
O
3
etc.) areused that break covalent bonds. Ingeneral, glass is chemically inert; however, because
of impurities and the superficial structure, capillaries present catalytic activity and adsorption
of the sample component. Metal oxides will act as Lewis acid. Molecules of analyzed
compounds containing
π
electrons or non-participating electrons will react with Lewis acids.
Adsorption and catalytic properties of glass and silica are due to silanol groups and siloxane
bridges on the surface of the capillary tube. Also, water in the atmosphere adsorbs to the
surface by hydrogen bonds. Heating the tube, water can be removed in order to form stable
siloxane bridges. For a capillary tube to become a chromatographic column, the inner surface
of the tubemust be coveredwithauniformandhomogenous filmof stationaryphase.
By silanizationof silanol groups, the critical surface tension (one that is establishedbetween
the solid surface and stationary phase) is significantly reduced, but has the same value for
glass and quartz. Silanizated surfaces have a surface tension so small that only non-polar
stationaryphases (OV-101) can “wet” them. Polar stationaryphases (Carbowax 20M) donot
give uniform films. The basic elements of glass capillary tubes’ chemistry are alkalinemetal
ions, silanol groups and siloxane bridges. These are active centres that catalyze at high
temperatures thedecomposition reactions of sample components aswell as of the stationary
phase. For these reasons these active centreshave tobedeactivated.
Capillary tube surface deactivation involves removing the metal ions and blocking out the
silanol groups with no catalytic activity groups. Removal of metal ions of silica and glass
capillary tubes is achieved by acidwashing in a static or dynamic regime. An acidic wash
causes not only metal removing, but also increases the number of silanol groups, by
breaking the siloxane bonds on the surface of the tube. In the past, tubes’ surface
deactivation was with surfactants, but they have low thermal stability and are no longer
used. Current methods use disilazane, cyclic siloxanes and polysiloxanes. In all cases
siloxane groups turn into silicate esters. Methods to deactivate with silazane realize the
blocking of silanol groups by reaction with hexamethyldisilazane, di-fenil-mehyl-
disalazane, dibutiltetramethyldisilazane, dihexyltetramethyldisilazane etc.
