Application of sol-gel method in unpolar polysiloxanes coating to multicapillary columns

Abstract

Due to their chemical inertness, wide operating temperature range, and low volatility, non-polar dimethyl-
and methylphenyl polysiloxanes are some of the most frequently used stationary phases in gas chromatographic practice. Commercial multicapillary columns with polydimethylsiloxanes SE-30 (or OV-1) and phenyldimethylsiloxanes SE-54 (or OV-5) are well known, but their upper operating limit is 250°C, which is significantly lower than the stated thermal stability of the stationary phases themselves. The high-efficient
multicapillary column preparation method using the polyethylene glycol stationary phase and the sol-gel
technology was proposed earlier. This method allows to increase the thermal stability and to reduce the column bleed. The possibility of coating the multicapillary columns with non-polar silicone stationary phases
using the sol-gel method was examined in order to prepare the columns with improved performance characteristics.
In the current study, straight multichannel hexagonal tubes with an apothem of 2.2 mm and a capillary
diameter of ~40 μm were used as the source material. The precursor used for the application of polydimethylsiloxane was prepared from OH-terminated polysiloxane PS 347.5 and polymethylhydrosiloxane, with trimethoxymethylsilane as a crosslinking agent, and trifluoroacetic acid as a catalyst. The precursor composed of the stationary phase SE-54, trimethoxymethylsilane and trifluoroacetic acid and methylene chloride was used for the applying of SE-54. After adding all the components, the mixture was kept for a while for the sol-gel to be ready.
The static low-pressure application of the PS 347.5 stationary phase from a solution obtained by diluting
of the corresponding precursor with hexane or chloroform led to an unsatisfactory result, due to capillary
blockage during the solvent removal. Under similar conditions, the use of the SE-54 stationary phase
based sol-gel provided for the smooth coating procedure, however the obtained columns lacked efficiency.
The stationary phase immobilization degree for various columns ranged within 49-77%, which is less than is
desirable for the stable work of the column. Since the static coating method did not allow to produce multicapillary columns with satisfactory characteristics, attempts were made to use the dynamic method of applying stationary phases to multicapillary columns.
It is theoretically shown that dynamic coating performed by moving a short plug of a stationary
phase solution along a multicapillary is correlated, that is, the thickness of the resulting film in the capillary
depends on its cross-section, and a thicker film of the stationary phase forms in the wider capillary.
It was established experimentally that with the speed of the undiluted precursor plug within
14-20 and 37-45 mm/s for the stationary phases PS 347.5 and SE-54, respectively, after dynamic coating and
subsequent conditioning, multicapillary columns with high efficiency were obtained (3200-3500 theoretical
plates on dodecane). In both cases, the process suffered from insufficient reproducibility, often inefficient
columns were obtained, and the shape of chromatographic peaks also frequently differed from the Gaussian
curve. A comparison of the commercial SE-30 column with one of the columns prepared with the PS 347.5
stationary phase showed the closeness of their chromatographic characteristics. However, the latter had
slightly higher polarity.
Using a freshly prepared sol-gel, the immobilization degree of the stationary phase for the studied
columns was 86-92 and 90-97% for silicones PS347.5 and SE-54, respectively. The prepared columns
showed a slight activity to alcohols. This may be due to the presence of free silanol groups in the products of
the hydrolysis and polycondensation of trimethoxymethylsilane.
It was shown that columns with both types of stationary phases withstand heating in an inert gas
flow for several hours at 300°C without significant loss of efficiency. It was established that for the sol-gel
multicapillary columns at 300°C the bleed level (caused by stationary phase breaking down) is about an order
of magnitude lower than that of commercial columns with similar stationary phases. The increased thermal
stability and lower bleed of the columns can be explained by the formation of a stable spatial organicinorganic copolymer of polysilicic acid and polysiloxanes on the capillary surface.