The Modern Student Laboratory: Chromatography The Preparation and Testing of a Fused-Silica Gas Chromatography Capillary Column An Experiment for a Senior Undergraduate Laboratory Module R. John Kominar Wilfrid Laurier University, Waterloo, ON, N2L 3C5, Canada This article describes the preparation and testing of a fused-silica GC ca~illarvcolumn. A simple procedure is presented that allows coating, immobilizing, conditioning, and testing of a capillary column that can be completed in two to three labor&ry periods. Introduction The capillary columns used today ingas chromatographic analyses provide qualitative and quantitative information better than packed columns. Both glass and fused silica may be used to prepare high-performance capillary colurns. The introduction of polyimide-coated fused-silica capillaries in 1979 has increased the popularity of capillary gas chromatography and facilitated the shift from packed-column to capillary-column gas chromatography due to the ease of handling and manipulating coated fused-silica capillaries as compared to glass capillaries. Stainless steel capillary columns introduced in the 1960's enjoyed only limited success due to the activity ofmetal surfaces towards many substances commonly analyzed by gas chromatogaphy. The production ol'the empty fused-sllica tubing Itself && ex~editedbv the advan& made in laser-controlled drawin;: machines used for the production of fibre optics. Both glass and fused silica provide a siliceous surface of alternating silicon and oxygen atoms with varying concentrations of surface hydroxyl (silanol) groups. Glass also contains metal ions such as boron and sodium, in addition to surface hydroxyl groups. The -OH groups can act as Lewis acid sites, and the metal ions can interact with many compounds containing nitrogen, oxygen, and sulfur (i.e., electron-donor analytes). These active sites cause problems with surface adsorption and must be eliminated for good chromatography The synthetic fused silica used for capillary columns contains less than 1 ppm metallic impurities. Consequently, its activity is due primarily to the presence of surface silanol ZTOUDS.
he discovery of high-temperature silylation techniques bv Welsch (11 . . and their subseouent develooment (2.31 . . . omvided a good approach to silanol deactivation and overcame excessive surface activity. The deactivated tubing could then be wated with a stationary phase. Coating Methods and Materials The Two methods exist for coating ., the stationarv" phase. . static coating technique ( 4 )lnvolvex several steps: preparinga solutionof the stationary phase in a volatiledegassed
solvent; filling the column; sealing one end; and evaporating the solvent from the column by applying a vacuum to the open end. This deposits the stationary phase on the walls of the column. The column is immersed in a constanttemperature bath during this procedure. The thickness of the stationary phase can be calculated from the concentration of the coating solution and the column diameter. The dynamic coating technique involves partially filling the column with stationary phase, either neat or dissolved in a solvent, and using a 1-2-cm plug of mercury pressurized by carrier gas to sweep the liquid through the column (5).The concentration of the coating solution and the rate a t which the mercury plug sweeps i t through the column determine the thickness of the stationary layer leR behind on the column wall. The coated column is then heat-treated toimmobilize the stationary phase to render i t resistant to solvent washout and high-temperature bleeding. Most stationary phases are methylpolysiloxanes, substior methvl~olvsiloxaneand subtuted meth~l~ol~siloxanes. .. . stituted polysiloxane c o p & n e r s ;hat -haw been \inylmodilied. The immobilization occurs tbrouch cross-linking of the vinyl groups. The entire process of m&inga capillary column-deactivating, coating, and immobilizing-is quite complicated. The detailed procedure for both of the above methods involves a number of steps that require special training and technique. The experimental setup is also quite complex. If the polysiloxane used for the stationary phase is silanol-terminated rather than vinyl-modified, the terminal -OH groups may undergo condensation polymerization among each other or with residual silanol surface groups on the fused silica. This process also results in an immobilized stationary phaseand has beenused to deactivate and subsequently coat capillary columns (6). The deactivation of the fused-silica surface, followed by static coating with the stationary phase, and then immobilization, reportedly yields the best quality fused-silica capillary columns. This process as indicated, however, is quite complicated. Recently a modification of the dynamic coating method was published (7)in which the mercury plug was omitted and a high gas pressure (30-100 times higher than in traditional dynamic coating) was used to force a plug of the stationary-phase solution through the column. This high pressure was then maintained for a period of time after the solution was expelled from the column, "smoothing" the stationary phase to a uniform thickness on the column wall. A uniform, immobilized layer of stationary phase on the (Continued on page A250) A249
Volume 68 Number 10 October 1991
The Modern Student laboratory: Chromatography inside wall of a finished column is a critical requirement for good column performance. ;1 the proced&e presented here this latter dynamic method of coating i s modified and simplified to allow both deactivation androatingofa fused-silica column in a single step using silanol-terminated polydimethylsiloxane. w i t h a-short piece of fused silica the head pressure needed for the coating process i s kept low enough to be safe for student use and t;,'require onl; the standkrd pressure regulators normally used in gas chrornntomaph.y. The short corumn still provides very adequate resoiution for testing a multicomponent mixture. It also allows short retention times, so many samples can be chmmatographed in a single laboratory period. Equipment, Materials, and Supplies Below is a list of equipment, materials, and supplies t h a t can be used. A eapillary-column gas chromatograph with a flame-ionization detector, a strip chart recorder or computingintegrator, and helium carrier gas for conditioning and testing capillary columns Fused-silica capillary tubing; 0.25-mm i.d. (Supelco, Hewlett-Packard) Column baskets (Hewlett-Packard; part no. 146&1821) on which to coil fused-silicatubingCapillary-scaringtool for cutting fused silica (Supelco) . A 10-20-X magnifier to inspect fused-ailica ends Glassed (Suoelco)capillary-column connectors . . Solvents: GC-grade hexane, methylene chloride.
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Stationary phase: p~lydimeth~lsiloxane, silanol-terminated (PS 348.1; Petrareh Systems Ine., Bartram Rd, Bristol, PA 19007) .Coating setup: a cylinder of inert, high-purity, carrier gas with pressure regulator and regulating and shutoff valve (e.g., Whitey); 118-in. copper refrigeration line; l@-1116-in. Swagelock reducing union; 114-V8-in. Swagelock reducing union; one foot of 1116-in. stainless steel line; coating reservoir(e.r.. .. .-2-5-mL Pierce reactivial. J&W column-rinse kit. or SGE filling rerervwr Hewlett-Packurd, 10-JL, e c. . , Hamiltnn X1N . . fixed-needle CC svrinaes . Test mix components are the CI0-Cl4 normal hydrocarbons, n-actanol, 2,Bdimethylphenol, 2,6-dimethylaniline,dicyelohexylarnine, methyl decanoate,methyl undecanoate,methyldodecanoate (Fluka). Fluka also offers a ready-to-use "test mixture far capillary columns according to Gmb". Supelco also offers test mixtures for capillary columns. 'A magnifying micrometer to measure the peak widths accurately. ~
Experimental Column Preparation A7-m piece of fused silica is wound onto a column basket. The empty column i s rinsed with 2 mL of methylene chloride, using a setup similar to t h a t i n Figure l a , and blown dry with high-purity, dry, inert carrier gas for 1h or longer. Alternately, a longer piece of fused silica may be rinsed, blown dry overnight, and its ends flame-sealed. The shorter (7-m) pieces may be taken fmm this longer section a s
required, and the students may proceed directly to the column-coating step. The stationary phase is prepared by making 5 mL of 3% WIVsolution of silanol-terminated polydimethylsilox-
VALVE COATING SOLUTION
GLASS
u-u CONNECTOR
REGULATING
