Technique for Coating Gas-Liquid Chromatography Capillary Columns Using Long Chain Quaternary Ammonium Compounds L. D. Metcalfe and R. J. Martin Armour Industrial Chemical Co., McCook, Ill. 60525
GASCHROMATOGRAPHY capillary columns are capable of great efficiencies. Separations that are very difficult a t best with packed columns can readily be made with Golay columns. Cis and trans unsaturation, positional isomers, branched chained compounds are readily separated on capillary columns. In general, most capillary columns are difficult t o coat with various liquids and some are virtually impossible t o coat. Because of this difficulty in preparing capillary columns, they are probably not used as widely as they should be. A number of workers concerned themselves with the problem of improving capillary columns. Mohnke and Saffert (I) chemically etched or roughened the interior of the glass capillaries in order t o get more liquid phase into the column. Halasz and Horvath (2) succeeded in getting solids impregnated with liquid t o adhere to open tubular column walls. They obtained columns superior to conventional capillary columns in this way. Zlatkis and Walker (3) improved capillaries by plating the inner surface with gold and silver. A number of workers (4-6) have proposed the use of wetting agents or surfactants as additives t o liquid phases to improve the properties of capillary columns. Averill ( 4 ) used Span 80 and Atpet 80. Teranishi (7) used small amounts of Igepal and other nonionic surfactants in capillary columns. Long chain quaternary ammonium compounds will form monomolecular layers o n metal and other surfaces. Use of this property is made in their application as antistatic agents and corrosion inhibitors. I t was reasoned that a long chain quaternary ammonium compound added to the liquid phase would plate out on the tubing surface, An organic monolayer would then be formed t o which the liquid phase could bond itself, o r a t least adhere to. If such a combination of organic layers could be achieved, a larger amount of liquid phase would be more evenly coated o n the column wall than without the quaternary. Since the liquid phase would be held by both mechanical and electrostatic forces, the bond between metal and liquid should be stronger. Such a column should be more efficient and should maintain its efficiency for a much longer time. A series of experiments were performed using metal and glass columns which were coated with and without the addition of a quaternary ammonium compound. The quaternary chosen for this work was trioctadecylmethylammonium bromide. This was the highest molecular weight quaternary available in our laboratory. I t promised t o be reasonably (1) M. Mohnke and W. Saffert, “Gas Chromatography 1962,” Symposium, Hamburg, Germany, June 13-16, 1962, M. Van Swaay, Ed., Butterworths, London, 1963, p. 216. (2) I. Halasz and C. Horvath, ANAL.CHEM.,35, 499 (1963). (3) A. Zlatkis and J. Q. Walker, ANAL.CHEM.,35, 1359 (1963). (4) W. Averill, “Gas Chromatography,” N. Brenner, J. E. Callen, and M. D. Weiss, Eds., Academic Press, New York, 1962, pp.
1-6. ( 5 ) T. R.
Mon, R. R. Forrey, and R. Teranishi, J . Gas Chromatog.,
4, 176 (1966).
(6) F. Farre-Ruis, J. Henniker, and G. Guiochon, Nature, 196, 63 (1962). (7) T. R . Mon, R. R. Forrey, and R. Teranishi, 5th National Meeting, Society of Applied Spectroscopy, Chicago, Ill., June 1966. 1204 *
ANALYTICAL CHEMISTRY
heat-stable and provide more available organic surface for the liquid phase t o adhere to. The addition of this quaternary t o the coating solution did improve the column performance considerably. Significant increases in column efficiency and life were observed. EXPERIMENTAL
Reagents. Trioctadecylmethylammonium (available under the name Gas-Quat-L from Lachat Chemical Co., 20200 Ashland Ave., Chicago Heights, Ill. 60411) bromide was prepared in the Armour Industrial Chemical Co. laboratories. Reagent grade chloroform was filtered through a hard filter paper and then stored in a glass stoppered bottle. Column Coating Apparatus. The coating apparatus used consists of a 3-foot, ‘/?-inch 0.d. Teflon tubing to which l/r-inch Swagelok fittings had been attached. This Teflon tube is attached to the capillary column by appropriate reducing fittings. The coating solution is added to the Teflon tube through the upper end. This end is then attached t o a nitrogen cylinder reducing valve. Enough nitrogen pressure (usually 5-10 psi) is applied so that the solution is forced into the capillary tube a t about ml per minute. The advantage of the Teflon tube is that it allows visibility of the solution without the fragility of glass. The gas pressure is applied to the column overnight. This time is usually sufficient t o allow the liquid t o be forced through the column and to evaporate the solvent remaining in the capillary. Procedure for Making Quaternary Modified Columns. One hundred mg of the trioctadecylmethyl ammonium bromide are added to 50 ml of filtered chloroform. A 20% solution of liquid phase is made up with this chloroform solution. The solution is filtered through a hard filter paper that has been previously washed with a few milliliters of quaternary treated chloroform. If the liquid phases being used cannot be made u p to 20% concentration, the most concentrated solution that can easily be handled is used. To coat a 100-200 foot 0.01-0.02-inch i.d. capillary, 5 ml of this solution is placed in the coating apparatus tube. The procedure described in the preceding section is then followed. The solution that comes through the tube may be collected in a graduate. To estimate how much liquid phase remained o n the column, the solvent is evaporated and the solute measured. The column is then conditioned for several hours at 10-20” C above the proposed operating temperature. The column temperature is lowered to the desired temperature and it is ready for use. DISCUSSION
A 110-foot, 0.01-inch i.d. stainless steel column that had resisted many coating attempts was used in the first experiment. This particular column was coated with SF-96 silicone oil using the described modified procedure. Figure 1 shows the chromatograms with the same high molecular weight amine sample obtained using this column before and after the quaternary treatment. The upper curve was the best that was obtained with this column without using the quaternary ammonium compound in the coating solution. The lower curve was the first chromatogram obtained after the modified coating tech-
a
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WITHOUT QUATERNARY
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Figure 1. Capillary column gas chromatograms of a complex amine mixture before and after using quaternary modified coating technique nique was used. There was a striking increase in separating ability of this column. The column showed better than a 10fold increase in efficiency. This chromatogram is shown for historical reasons and does not represent the best chromatograms that were eventually obtained in excess of 100,000 theoretical plates. This improvement in efficiency was general for all peaks and in many types of samples including amines, esters, alcohols, and hydrocarbons. Some work was done t o determine the optimum percentages of quaternary salt to be used in the coating solution. One per cent of quaternary salt appeared to appreciably affect the nature of the liquid phase. Addition of 0.1-0.2% in the solvent effectively helped t o coat the column without altering the liquid used. This amount was eventually standardized upon in the coating procedure. However, even smaller quantities of quaternary o n the order of 0.05 % also appeared to be effective. No further studies were made concerning quaternary salt concentrations in the coating solution. A number of liquid phases were used to coat columns using the modified procedure. These included Apiezon L, polyphenyl ether, and Carbowax 20 M. The Apiezon and polyphenyl ether columns were operated as high as 300°C without difficulty.
Desty and coworkers (8) successfully made and used glass capillary columns for gas chromatography. Other workers (4, 9) modified the glass columns in attempts to improve their performance. Commercial glass capillaries that were available some years ago were extremely difficult t o coat. We attempted to coat one of these commercial glass columns with a silicone oil. A chromatogram of a methyl ester mixture was then obtained with this column. The column was washed out with solvent and coated using the described modified procedure. Again the same methyl ester mixture was chromatographed. The quaternary treated column showed much greater separating ability than the untreated column, although in each case the chromatograms obtained were not ideal GLC curves, We are not sure the technique we used is the best one for coating glass columns. I t is possible that it would be better to precoat the glass with the quaternary solution and then coat with treated liquid phase solution. In our laboratory a 200-foot 0.02-inch i.d. SF-96 silicone oilcoated stainless steel capillary is used more than any other column. This particular column is very efficient, showing better than 1000 theoretical plates per foot for many samples. The one disadvantage of this column was that it had to be recoated after 80-100 hours of use, o r about once a month. It was decided to recoat this particular column using the quaternary modification. A slight increase in column efficiency was immediately observed. The rate of column bleed decreased enormously. The base line stability improved. Previously when a very large sample was injected, the peaks often overrode a n apparent liquid phase peak which appeared as a long drawn out hump. This phenomenon was n o longer observed. After 18 months and several thousand hours of use a t temperatures u p to 210"C, this column was still operating efficiently without any apparent loss of separating ability. Whenever the quaternary modified coating technique was used to coat a capillary column, the separating ability of the column increased markedly for many types of samples including esters, tertiary amines, hydrocarbons, cis and trans unsaturation. Only one very high molecular weight long chain quaternary ammonium compound has been tested with capillary columns. However, many long chain quaternaries are available commercially and undoubtedly many of these would also effectively aid the coating of capillary columns. RECEIVED for review March 10, 1967. Accepted May 15, 1967. (8) D. H. Desty, J. N. Haresnape, and B. H. Whyrnan, ANAL. CHEM., 32, 302 (1960). (9) F. A. Bruner and G. P. Cartoni, ANAL.CHEM., 36,1522 (1964).
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