consuming. Another advantage is the possibility to exclude air during the whole operation. LITERATURE CITED
(1) Bayer, E.,Hupe, K. P., Witsch, H. G., Angew. Chem. 73, 525 (1961). (2) Bayer, E.,Wahl, G., R’itsch, H. G., 2.Anal. Chem. 181,384 (1961). (3) Bayer, E.,Witsch, H. G., Zbid., 170, 278 (1959). (4) Camag, A. G., Muttenz, Switzerland, un ublished data. (5) arle, D. W., Johns, T., ZSA (Instr.
8
SOC. Am.) Proc. Nut. Symp. Instr. Analysis, 1958. (6) Deemter, J. J. van, Zuiderweg, F. J., Klinkenberg, A., Chem. Eng. Set. 5,
271 (1956).
( 7 ) Frisone, G. J., J. Chromatog. 6, 97 (1961). (81-Giddings, J. C., Fuller, E. N., Ibid., 7, 255 (1962). (9) Golay, M.J. E., ZSA (Instr.SOC.Am.) Proc. Anal. Znstr. Diu. dnd Int. Gas Chromatography Symp., June 1959, preprints 2, 5-11. (10) Hupe, K. P., Chem. Zng. Tech., in press. (11) Huyten, F. H.,Beersum, W. van, Rijnders, G. W. A., “Gas Chromatography 1960,” ed. R. P. W. Scott, p. 224, Butterworths, London, 1960. (12) Keulemans, A. I. M., discussion GdCh-Tagung Analytische Chemie, Freiburg 1958, Pvlunchen 1960. (13) Keulemans, A. I. M., “Vapour Phase Chromatography,” ed. D. H. Desty, p. 15, Butterworths, London, 1957. (14) Purnell, J. H.,Bohemen, J., “Gas
Chromatography,” ed. D. H. Desty, p. 6, Butterworths, London, 1958. (15) P q k e r , J., “Gas Chromatography 1960, ed. R. P. W. Scott, p. 240, Butterworths, London, 1960. (16) Schroter, M.,Leibnitz, E., “Gas Chromatographie 1961,” Deutsceh Akademie der Wissenschaften, Berlin, 1961.
.
(isso).
’
RECEIVED for review December 3, 1962. Accepted February 7,1963. Presented a t the International Symposium on Advances in Gas Chromatography, University of Houston, Houston, Texas, January 21-24, 1963.
Sol-Coated Capillary Adsorption Columns for Gas Chromatography R. D. SCHWARTZ, D. J. BRASSEAUX, and G. R. SHOEMAKE1 Explorufion and Production Research Division, Shell Development Co. ( A Division of Shell Oil Company), Housfon, Texas
b Capillary adsorption columns can b e prepared by wetting the insides of capillary tubes with a colloidal sol containing an adsorbent in a volatile liquid and then evaporating the liquid. This type of capillary column appears to b e particularly suitable for hydrocarbon separations utilizing ultrasensitive detectors, because difficulties due to bleeding of liquid phases are eliminated.
C
COLUMNS for gas chromatography, which were invented by Golay ( 3 ) ,represent one of the major advances in this field. I n a petroleum analytical laboratory, these columns offer distinct advantages for the separation of complex hydrocarbon mixtures. Several papers ( I , ‘7) have demonstrated the excellent resolution of hydrocarbons which can be obtained with suitable capillary columns. Nevertheless, a number of difficulties in column preparation and usage have delayed the more widespread use of capillary columns. The coating of capillary columns with partition liquids is sometimes difficult. A variety of coating techniques have been proposed to overcome this problem. In practice, it is not always possible to prepare well-coated, efficient columns which provide good resolution of multicomponent samples. Metal columns, often stainless steel, have APILLARY
1 Present address, University of Houston, Houston, Texas.
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ANALYTICAL CHEMISTRY
become popular for the preparation of coated capillaries. We have found that a given grade of stainless steel obtained from different sources may yield columns of varying resolution and hydrocarbon selectivity even when coated in an identical manner with a given liquid substrate. Further, the bleeding of liquid substrate which sometimes occurs and the resultant noise present a serious problem, particularly when sensitive ionization detectors are employed, or when the column temperature is programmed during an analysis. Because most of the problems mentioned are caused, in one way or another, by the liquid substrate, we considered the possibility of preparing capillary columns which would contain a film of a solid adsorbent rather than a liquid film. This paper deals with the preparation and properties of capillary adsorption columns, demonstrates their use for the separation of Pome typical hydrocarbon mixtures, and suggests some possibilities for future research. OTHER WORK WITH CAPILLARY ADSORPTION COLUMNS
Golay’s patent (3) on capillary columns includes a number of claims which pertain to capillary adsorption columns, but the specific directions for column preparation which are disclosed, and the illustrative chromatograms, refer to capillary partition columns. Likewise, during the past few
years. the Perkin-Elmer Corp. has extensively advertised capillary partition columns and has published papers demonstrating their advantages, but it has never mentioned the preparation or the use of a capillary adsorption column. After the xork described in this paper had been completed, two papers which dealt with capillary adsorption chromatography were presented at the 1962 Pittsburgh Analytical Conference. Although these papers have not yet been published, it is apparent from the abstracts that the techniques of column preparation are quite different from those developed here. Zlatkis and Walker (8) discussed the results obtained with capillaries containing a metal-plated inner surface. Petitjean and Leftault (6) described the development and use of aluminum capillary tubing which contains an inner surface of active aluminum oxide. More recently, Halasz and Heine (4) have reported the preparation of “packed adsorption capillary columns.” These columns are prepared by embedding adsorbent particles in the walls of glass tubing, leaving an open space in the middle of the column. .4t approximately the same time, hfohnke and Saffert (5) presented a paper a t the Fourth International Gas Chromatography Symposium, held in Hamburg, June 13 to 16, 1962, describing capillary adsorption columns prepared by corrosion of the inner a-alls of glass capillary columns. I n the introduction to their
I
2
I
Figure 1.
3
4
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6 MINUTES
Separation of C s C 6 Hydrocarbons
Argon detector.
Carrier gas, argon 3 p.s.i.
paper they mention “several unsuccessful attempts to coat the inner wall of a capillary with an adsorptive layer” prior to their discovery of the corrosion procedure. Mohnke and Saffert utilized their adsorption column a t 77.6” K. for the separation of hydrogen isotopes and indicated the possibility of solving other difficult separation problems by capillary adsorption chromatography. EXPERIMENTAL
Equipment. d Barber-Colman Model 20 chromatograph, with an argon ionization detector and a hydrogen flame ionization detector, and a Shell-Hallikainen chromatograph, with a small volume Gow-Mac thermal conductivity cell, were used in this investigation. The recorder ranges employed were 50 mv. for the argon detector, 10 mv. for the flame detector, 1 mv. for the thermal conductivity detector. Reagents. An eight-component hydrocarbon mixture was prepared from the following Phillips pure grade hydrocarbons: isopentane, normal pentane. cyclopentane. 2,2-dimethylbutane, 2,3-dirn~thylbutane, 2niethylpentane, 3-methylpentane. and normal hexane. The C1 to Cg and the CI to Cy hydrocarbon mixtures were prepared by adding natural gas to appropriate petroleum distillation fractions. The mixture of cyclohexane, 2 , 4 dimethylprntane, and benzene was prepared from Phillips pure grade hydrocarbons. The Cs to Ci0 normal paraffins were Phillips pure grade hydrocarbons. Xalcoag 1022 T V R ~a sample from the Xalco Chemical Co., Houston 25, Texas. CAB-0-SIL was a sample from the Godfrey Cabot Corporation, Boston, hIass. Sylon and Delrin capillary tubing nas obtained from Garlock Inc., Houston, Texas; copper capillary tubing, from The American Brass Co., Houston, Texas; and stainless steel capillary tubing, from hletal Goods Corp., Houston 21, Texas.
1
1
I5
24
Figure 2.
Separation of
33 MINUTES
Cl-Co Hydrocarbons
Detector, small volume Gow-Mac thermal conductivity cell. helium 10 ml. per minute
Sample size. Samples of 0.1 to 0.2 p1. were injected when the argon detector was employed, using a split ratio of 100 to 1 or 200 to 1. Samples of 1 to 2 pl., with no split, were injected when the thermal conductivity detector was employed. Temperature. All of the chromatograms shown in this paper were obtained a t room temperalure. Preparation and Properties of Capillary Adsorption Columns. A. Colloidal silica coatings on plastic columns. The first portion of our research dealt with the evaluation of techniques for the deposition of a thin layer of adsorbent material upon the inside walls of capillary tubing. Attempts were made to coat capillary columns, which had been etched, treated with adhesive materials, or annealed under various conditions, with fine particles of adsorbents such as silica gel, alumina, or carbon. I n most cases these columns would wholly or partially close during coating. The results obtained, therefore, were not very satisfactory. One method of reducing the restriction of flow with this type of column is to employ very fine particles of adsorbent. Fortunately, adsorbents are commercially available in colloidal sizes both as solids and as sols. Some of these colloidal materials were evaluated. Typical materials employed we1e Nalcoag 1022 and CAR-0-SIL. We noted that a typical colloidal silica sol, Nalcoag 1022, provides a thin adherent film of silica when allowed to evaporate on plastic and metal surfaces. Nalcoag 1022 is a 227, sol of colloidal silica (approximately 20 mp in particle diameter) in a water-isopropanol mixture. A Delrin capillary, 200 feet long and 0.02-inch i.d., was coated with Nalcoag 1022 and then purged with dry argon to remove the solvent. This column provided good separation of an eight-component blend of pentane and hexane isomers when tested a t room temperature. A typical chromatogram obtained with this column is shown as Figure 1. These initial results mere encouraging, because
Carrier
packed columns of adsorbents such as silica gel are not usually suitable for the separation of pentanes and hexanes a t room temperature. Further, the peaks obtained were sharp and did not show the excessive tailing which is characteristic of most packed adsorption columns. We prepared other columns with Salcoag 1022 using the same coating procedure. Figure 2 is the chromatogram of a C1 to Cshydrocarbon mixture obtained with a 1000-foot length of 0.034-inch i.d. nylon column coated n-ith Salcoag 1022. Because of the relatively high flow rate utilized Fvith this column, it mas possible to monitor the efAuent with a thermal conductivity cell. A sample containing C1 to C? hydrocarbons was also separated with this column. The resulting chromatogram is shown as Figure 3. I t should be noted that through the Cli’s this type of column provides a carbon-number separation ( 2 ) of saturated hydrocarbons; that is, the naphthenes and paraffins of a given carbon number emerge together. This is different from t’he results obtained with partition columns, where naphthenes often emerge with paraffins containing one more carbon atom. As an example, on partition columns cyclopentane is usually eluted after 2,2-dimethylbutane, and cyclohexane is eluted after 2,4-dimethyIpentane. Eggertsen and Knight ( 2 ) have utilized the ratio of the retention time for a naphthene ( t X ) , cyclohesnne, to that for a parafin of the same boiling point (P), 2,4-dimethylpentalie, as a measure of the hydrocarbon-type selectivity of gas chromatographic columns. In general, partition columns yield t:\-/tP values greater than 1, and adsorption columns show t,V/tP values less than 1. The 200-foot column of Delrin coated with Kalcoag 1022 yielded a tS,!tP value of 0.84. This value was determined from the retention times, measured from time of injection, of cyclohexane, and of 2,4-dimethylpentane. With nylon or Delrin columns, it is difficult to obtain symmetrical peaks and quantitative elution of aroVOL. 35, NO. 4, APRIL 1963
497
T 5
E
v) 0
W W
z E
a
W 0
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8 W
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15
30
Figure 3.
Separation of
C&
Detector, small volume Gow-Mac thermal conductivity cell.
I-
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z0
45 MINUTES
P
Hydrocarbons
z
0 k
Carrier, helium 10 ml. per minute
W u 7
I
matic hydrocarbons. Therefore, these plastic columns have been used here for the separation of saturated hydrocarbons. The columns described above have been evaluated with three types of detectors. With thermal conductivity cells and argon ionization detectors, the noise level was lower than that obtained with typical capillary partition columns. Erratic results were sometimes obtained with the hydrogen flame ionization detector. Soise did occur with the flame detector for some silica columns, but the exact cause of the noise could not be determined. It may have been due to inorganic particles such as sodium salts which are present as impurities in certain silica sols, to dust particles of silica, to organic silicates, or to other materials. We did not notice any siliceous deposits on the surfaces of any of the detectors, so presumably the material causing the noise is combustible or is swept out of the detector. If it is necessary to use capillary adsorption columns containing silica with the hydrogen flame ionization detector, careful selection and testing of the silica as well as the tubing is necessary. As an alternative. a filter placed between the column and the detector may eliminate the noise. In our tests, filters
did not completely eliminate noise. Because of this problem the remainder of our tests were performed with thermal conductivity or argon ionization detectors. Other capillary columns of nylon or Delrin were coated with sols prepared by mixing colloidal silica such as Godfrey Cabot Corp. CAB-0-SIL with water or cyclohexane or benzene. The results obtained with these materials were almost identical with those obtained with the commercially available sols. The columns prepared from nonaqueous sols could be used after less conditioning than those prepared from aqueous sols. A further advantage of using nonaqueous sols is the minimization of sensitivity reduction by water, which can occur with the argon ionization detector. To test the applicability of the coating technique described above to metal columns, a series of tests were performed with copper and stainless steel tubing. Typical results obtained are issued below. B. Colloidal silica coatings on metal columns. A 100-foot length of 0.0175-inch i.d. copper tubing was coated with Nalcoag 1022 by the procedure developed for coating plastic columns. A chromatogram of the eight-component pentane and hexane
Figure 5. Separation of Cyclohexane, 2,4-Dimethylpentane, and Benzene Argon detector.
Carrier, argon 0.5 p.s.1.
blend of isomers obtained with this column is shown as Figure 4. The peak-broadening and tailing observed are due to the surface of the copper tubing. Capillary partition columns prepared from this batch of copper tubing showed similar peak-broadening and tailing of hydrocarbon peaks.
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m
>
> E 0 v) W (1
z a a W K
8 W K
m (3
z a
0
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E0 P
E E 50 go a 0 V W
a
z
2 Iu W
7
z
Figure 6. Separation of mal Paraffins Argon detector.
498
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ANALYTICAL CHEMISTRY
CE,-CUJ Nor-
Carrier, argon 2 p.8.i.
A 10-foot section of 0.02-inch i.d. stainless steel tubing was coated with Nalcoag 1022. A three-component mixture (cyclohexane, 2,4dimethylpentane and benzene) was separated a t room temperature. The chromatogram (Figure 5 ) indicates that the tN/tP ratio (2) is less than 1 and that symmetrical peaks are obtained. The separation of the CS to CIOnormal paraffins with this column a t room temperature is shown as Figure 6. Metal columns such as these have been used a t temperatures of 150” to 250” C. and have exhibited good stability. The results obtained with temperature programming were also satisfactory in this temperature range. DISCUSSION
The separations described in A and B above result from the adsorption and desorption of hydrocarbons on one hydrophilic adsorbent, silica. It should prove interesting to extend this work to include other colloidal hydrophilic adsorbents such as alumina or molecular sieves and to test the properties of colloidal hydrophobic adsorbents. The preparation of capillary adsorption columns from colloidal sols of adsorbents is readily achieved, and the columns which we have evaluated provide sharp,
symmetrical peaks for saturated hydrocarbons. The ability to use these columns over a wide range of temperature, both below and above room temperature, with freedom from column bleeding appears to be one of their chief advantages. This is particularly true when the highly sensitive argon triode detector is employed. The fact that these columns provide a different hydrocarbon selectivity than that of partition capillaries may be an advantage in certain cases. A limited number of tests indicate that liquid phases such as silicone gums and Apiezon greases which are difficult to coat on stainless steel capillaries can be applied to these columns after the coating of colloidal silica has been applied. When a small amount of a partition liquid is mixed with the adsorbent before coating or is applied after the adsorbent, the separations obtained are those of a liquid-modified adsorption column. An alternative way of achieving this same sort of selectivity is to connect a capillary adsorption column in series with a capillary partition column. ACKNOWLEDGMENT
The authors thank the Nalco Chemical Company and the Cabot Corpora-
tion for providing samples of colloidal silica. LITERATURE CITED
(1) Desty, D. H., Goldup, A,, Whyman, B. H. F., J . Inst. Petroleum, 45, 287-98 (1959). ( 2 ) Eggertsen, F. T., Knight, H. S.,
AXAL.CHEM.,30, 15-20 (1958). (3) Golay, M. J. E., U. S. Patent 2,920,478, January 12, 1960. (4) Halasz, I., Heine, E., Nature, 194, 971-3. ( 5 ) Mohnke, M., Saffert, W., Preprints of 4th International Gas Chromatography Symposium in Hamburg, Germany, June 13-16, 1962, p. 214-19. (6) Petitjean, D. L., Leftault, C. J., Jr., Paper presented a t Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 5 . 1962. ( 7 ) Polgar,’ A., Holst, J. J., Groennings, S., ANAL. CHEM.,34, 1226-34 (1962). (S) Zlatkis, A., Walker, J. Q., Paper presented at Pittsburgh Conference-on Analytical Chemistry and Applied Spectroscopy in Pittsburgh, Pa., March 5, 1962.
RECEIVED for review December 3, 1962. Accepted February 7, 1963. Presented at the International Symposium on Advances in Gas Chromatography, University of Houston, Houston, Texas, January 21-24, 1963.
Open Tube Columns with Impregnated Thin Layer Support for Gas Chromatography ISTVAN HALASZ and CSABA HORVATH lnstitut fcr Physikalische Chemie der Universitaf Frankfurt am Main, Germany
,It will b e shown that open tube columns with impregnated thin layer support can b e produced, and that they are superior to conventional capillary columns for certain separation problems. Permeability of those columns is similar to, and their charging capacity is greater than, that of the well known open tube columns. A variety of stationary phases can be produced with active supports.
R
we published some results concerning the production and use of thin layer adsorption capillary columns ( 7 ) . An adsorbent adhered to the wall of the capillary there, and the results received were associated with the field of gas solid chromatography (GSC). Most recently we have succeeded in getting solids impregnated with a liquid to adhere to the inner wall of the capillary; thus the thin layer capillary columns may also be used in ECENTLY
the field of gas liquid chromatography (GLC). Golay (6) and Purnell ( l a ) already pointed out the advantages of such columns in their theoretical considerations. PRODUCTION
OF COLUMNS
For making such columns a suspension of a finely dispersed support and a liquid phase in a dispersing agent is produced; a capillary tube is filled with this suspension, care being taken that no air bubbles remain. After closing one end of the capillary it is introduced slowly and with constant speed through a heated slit into an air thermostat, beginning with the open end. Here the solvent evaporates and leaves the capillary through the open end. On the inner wall of the capillary 9 thin layer of support impregnated with the liquid phase remains. In the following we describe two columns: Active ferric oxide is the impregnated solid on the inner wall of the first capillary, and the second bears
a layer of impregnated “inactive” Sterchamol (a commercial support for gas chromatography). Column A. Twenty grams of highly dispersed ferric oxide [p-FepOa,Type E 7 from DEGUSSA, Rheinfelden (Baden), Germany; specific surface area: 3; sq. meters per gram \\ere suspended in 150 ml. of trifluorotrichloro ethane and 20 ml. of chloroform n ith the aid of a high-speed stirrer (Ultra-Turrex from Yanke & Kunkel K.G., Gtaufen i.Br., Germany; 8000 r.p.m.). Afterwards 2 grams of triethylene glycol dissolved in 20 ml. of chloroform nere added t o this suspension under stirring. The suspension v-as then pressed through a sieve (mesh size 60 microns). The inner mall of a copper capillary (Schoeller Werk K. G., Hellcnthal/Eifel, Germany) 28.5 meters in length and nith an inner diameter of 0.25 mm. was silvered over (“Sudversilberungsbad” : E. Durrwachter-DODUCO-K.G., Pforzheim, Germany). Then the suspension was pressed into the capillary tube by the aid of nitrogen under high VOL. 35, NO. 4, APRIL 1963
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