corresponds to the reversed effect; the heavier isotopic species is the more volatile and less retained by the chromatographic column. I n the temperature range where the normal effect occurs, as is observed for the CHI-CD~ systems, the retention and 0216021* volume ratio depends upon the sum of two terms of opposite signs, one of which is a function of l / T 2 and the other a function of 1/T. The large deviations observed from the vapor pressure measurements are an indication of the magnitude of the interactions of the isotopic systems with the chromatographic column. I n Table I1 are reported the differences of enthalpy and entropy changes related to the gas chromatographic separation of the isotopic molecules investigated.
NOMENCLATURE
c
= term
of resistance to mass transfer in gas and liquid phase L = column length K = partition coefficient k’ capacity ratio n = number of theoretical plates R = peak resolution s = relative volatility (vapor pressure ratio) t = retention time (measured from start) tl, t z = corrected retention time of a sample to = retention time of an inert component @ , =average linear gas velocity vc = volume of gas phase in column VL = volume of liquid phase in column w = peak width a = separation factor (relative retention time)
ACKNOWLEDGMENT LITERATURE CITED
The authors thank the Consiglio Naaionale delle Ricerche for providing funds to support this investigation.
( I ) Armstrong, G. T., Brickwedde, F. G., Scott, R. B., J . Chem. Phys. 21, 1297 (1953).
(2) Bruner, F. A., Cartoni, G. P., ANAL, CHEM.36, 1522 (1964). (3) Cvetanovic, R. J., Duncan, F. J., Falconer, W. E., Can. J. Chem. 41, 2095 (1963). (4) Desty, D. H., Haresnape, J. N., Whyman, B. H. F., ANAL.CHEM.32,302 (19601. \ - - - - I
(5) Gant, P. L., Yang, K., J . Am. Chem. Soc. 86, 5063 (1964).
(6) Johns, T. F., “Proceedings of International Symposium Isotopic Separations,” p. 90, North Holland Publishing Co., Amsterdam, 1958. (7) Liberti, A., Cartoni, G. P., Bryer, F., “Gas Chromatography 1964, A. Goldup, ed., p. 301, Institute of Petroleum, London, New York, 1964. (8) Liberti, A., Cartoni, G. P., Bruner, F., J . Chromatog. 12, 8 (1963). (9) Purnell, H., “Gas Chromatography,” p. 113, Wiley, New York, 1962. (10) Scott, R. P. W., J . Znst. Petrol. 47, 284 (1961). (11) Stern, M. J., Van Hook, W. A., Wolfsberg, M., J . Chem. Phys. 39, 3179 (1963). (12) Van Hook, W. A., Kelly, 31. E., ANAL.CHEM.37, 508 (1965). RECEIVED for review July 2, 1965. Accepted Xovember 8, 1965, Third International Symposium on Advances in Gas Chromatography, Houston, Tex., October 1965.
High-Resolutio n Capillary Adsorption Columns for Gas Chromatography R. D. SCHWARTZ, D. J. BRASSEAUX, and R. G. MATHEWS Exploration and Production Reseorch Division, Shell Development Co., Houston, Texas High-resolution capillary adsorption columns, which appear useful for the separation of hydrocarbons, have been prepared by coating stainless steel tubing with colloidal sols containing hydrophobic silica. These columns provide a carbon number separation of saturated hydrocarbons. Under optimum conditions, the resolution obtainable with these columns i s comparable to that achieved with capillary partition columns.
G
ADSORPTION chromatography has not enjoyed the same degree of popularity as gas-liquid partition chromatography. There are a number of reasons for this situation. When packed columns are employed, retention times tend to be long for adsorption separations. Further, peak tailing is often a problem. Lately, as more effort is being expended upon research studies of adsorption separations, some of these difficulties are being diminished. I n a previous paper (S), we discussed the preparation and properties of solcoat.ed capillary adsorption columns for gas chromatography. With these AS-SOLID
columns, prepared from hydrophilic sols, we were able to elute saturated hydrocarbons as symmetrical peaks in reasonable retention periods. This work has now been extended to include the preparation of similar capillary columns containing a film of a hydrophobic colloidal adsorbent. The columns prepared with a hydrophobic adsorbent provide high-resolution separations of complex hydrocarbon mixtures. The retention times and the peak shapes are comparable to those obtained with capillary partition columns. The results obtained in this study indicate that t,hese capillary adsorption columns can provide separations useful for the analysis of hydrocarbon samples. EXPERIMENTAL
Equipment. A Barber-Colman Model 20 chromatograph, equipped with a hydrogen flame ionization detector or an argon triode detector, was used in this investigation. Reagents. The standard hydrocarbon samples were prepared from Phillips pure grade and API standard hydrocarbons. Nalco CD-100, a hydrophobic
colloidal silica was a sample from the Nalco Chemical Co., Houston, Texas. Stainless steel capillary tubing was obtained from Metal Goods Corp., Houston, Texas. Delrin capillary tubing was obtained from Garlock, Inc., Houston, Texas. Sample Size. Samples of 0.1 pl. were injected with a split ratio of from 50 to 200 to 1. Temperature. The chromatograms shown in this paper were obtained a t room temperature or a t approximately 0” C. Nalco CD-100. Some time ago Nalco Chemical Co., Chicago, Ill., announced the availabilitv of a colloidal hydrophobic silica (h’alco CD-100) (21. Preliminaiy tests indicated that CD100 was miscible with hydrocarbon solvents such as benzene, cyclohexane, and toluene. The resulting mixtures are stable for periods of a t least six months. However, we noted that the solid CD100 tends to decompose on standing. The decomposed material does not disperse well. Therefore, it is advisable to prepare CD-100 sols from fresh CD-100, and to use these sols for the preparation of capillary columns. The exact composition of CD-100 is not given in the literature provided by Nalco. The decomposition of CD-100, upon standing, VOL. 38, NO. 2, FEBRUARY 1966
303
may be due to hydrolysis - of ester or othkr linkages. Preparation of CaDillarv Columns. The most efficient adsorption columns were prepared by coating a special grade of stainless steel with a coating of CD-100. Type 316 stainless steel, fully annealed to a bright finish, with a 15 t o 20 micro finish was utilized. Lengths of 100 or 200 feet of 0.01-inch i.d. by 0.062-inch 0.d. tubing were coated with CD-100 employing the standard technique for the preparation of capillary partition columns (4). A few experiments were also performed with 0.02-inch i.d. Delrin tubing which was coated with CD-100 by the same technique. Evaluation of Capillary Columns (Stainless Steel). -4200-foot length of 0.01-inch i.d. stainless steel was coated with CD-100 from a solution containing 10% CD-100 in cyclohexane. A hydrocarbon mixture containing normal hexane, cyclohexane, benzene, 2,4-dimethylpentane, and normal heptane was separated with this column a t room temperature using a n argon pressure of 15 p.s.i. The chromatogram obtained is shown as Figure 1. The retention times for these hydrocarbons are considerably less than those obtained with the same length of a capillary partition column, coated with a lOyosolution of a liquid, operated under similar conditions. The elution of cyclohexane before 2,bdimethylpentane indicates that the naphthene-paraffin selectivity ( 1 ) tX/tP, is less than one, and that adsorption effects predominate in the separation. However, the elution of benzene prior to 2,4-dimethylpentane is the reverse of the separation which we obtained with a capillary column coated with hydrophilic silica. This indicates that the chemical treatment of the silica, and/or the dispersing agent present in CD-100 has reduced the affinity of the silica for aromatic hydrocarbons. The resolution (4) between normal hexane and normal heptane obtained in this separation is approximately 10. I n order to effectively separate more complex hydrocarbon mixtures, which occur in crude petroleum or in refinery streams, it is necessary to prepare capillary columns which have a resolution of about 50 to 60. There are a number of ways to increase the resolution of capillary columns including increasing the column length, altering the thickness of the coating, changing the temperature or the carrier gas velocity. Because
I ; iNJECTlON TIME
8
16 MINUTES
Figure 1. Separation of carbon mixture'
lower temperatures are often advantageous for chromatographic separations, some experiments were performed with the CD-100-coated column in a bath of ice water. A sample containing 39 standard hydrocarbons, which boil between 28' and 114' C., was separated a t the reduced temperature employing argon at 15 p s i . as the carrier gas. The resulting chromatogram is shown as Figure 2. The separation of this complex hydrocarbon mixture, a t reduced temperature, is superior to that which is obtained at room temperature. The c6-c7 resolution is 57, which is about as good as can be obtained with 200 feet of 0.01-inch capillary partition columns. The peaks for thc saturated hydrocarbons are symmetrical, while those of benzene and toluene tail. This column, operated in an ice-water bath, provides an excellent separation of the saturated hydrocarbons which occur in the 28' to 114' C. portion of petroleum. It should be noted that the separation of these saturated hydrocarbons is by carbon number; that is, the C5's elute before the Cs)s and so forth. The tN/tP value (1) for this separation was 0.78, and the tA/tP value (4) was 0.90. The components in this chromatogram were identified by comparison of the retention times with those of the pure compounds. As a further comparison, the 5 component hydrocarbon mixture which had been separated at room temperature was ' C. also analyzed a t approximately 0 The CsC7 resolution was increased to 64. It can be noted that the retention
five-component hydro-
time for normal heptane increased by a factor of about 3 while the resolution increased by a factor of over 6. Thus, the overall resolution and time-requirement factor is more favorable for this column at the reduced temperature. This column was tested, a t room temperature, for the separation of the C8 to CS aromatic hydrocarbons. The chromatogram obtained with 20 p.s.i. argon as carrier is shown as Figure 3. It is interesting that an adsorption capillary column permits the separation of the c6-cS aromatic hydrocarbons at room temperature while most selective liquidcoated capillary columns suggested for this type of sample are operated a t a n elevated temperature. The CD-100coated column provides a much longer useful life than the liquid-coated columns. Evaluation of Capillary Columns (Delrin). Plastic capillary columns have been utilized, a t low temperatures, for the separation of saturated hydrocarbons. CD-100 was evaluated as a coating on a 200-foot length of 0.02-inch i.d. Delrin tubing. The coating was applied from a 30% solution of CD-100 in toluene. An eightcomponent hydrocarbon standard containing the pentane and hexane isomers which boil between 28' and 69" C., was analyzed with this column. At room temperature, with 5 p.s.i. of hydrogen as carrier gas, there is a partial separation of these eight components. The time of analysis under these conditions is short, and the peaks are sharp. I n an ice-water bath, with 5
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304
ANALYTICAL CHEMISTRY
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Separation of thirty-nine-component hydrocarbon mixture
72
80 MINUTES
I
'INJECTION
30 MINUTES
IS
TIME
Figure 3.
Separation of
Ceto Csaromatic hydrocarbons
p.s.i. of hydrogen as carrier, complete separation of these components is achieved in approximately 6 minutes (Figure 4). The effect of pressure variation upon the separation of these components was studied. At room temperature, when the hydrogen pressure was reduced to 2 p.s.i. the separation was not much better than a t 5 p.s.i. and the time of analysis was increased from approximately 4 minut,es to 9 minutes (Figure 5 ) . The effect of reduced temperature upon this separation at 2 p.s.i. is shown in Figure 6. These results indicate that the operating temperature is the more important factor for obtaining good separations.
I
INJ ECTlON TIME Figure 5. Separation of mixture
c 6
and
DISCUSSION
The results obtained in this study indicate that capillary columns prepared by coating stainless steel or Delrin tubing with a sol containing a commercially available hydrophobic silica adsorbent provide efficient and rapid separations of a variety of hydrocarbon mixtures. These columns are stable and no noise difficulties were encountered when they were employed with the argon or the hydrogen-flame ionization detection systems. It is particularly important to note that the elution of hydrocarbons from these columns is more rapid than from
I
I
8
IO
4 6 MINUTES INJECTION TIME Figure 4. Separation of c6 and Cshydrocarbon mixture
partition columns of the same dimensions. Thus, it is possible, and indeed better, to operate the adsorption columns a t lower temperatures than those utilized for comparable partition columns. This results in long column life and freedom from the problems caused by the deterioration and volatilization of liquid phases. The resolution obtainable with these columns, a t the proper temperature, is comparable to that obtained with the most efficient partition columns. The hydrocarbon selectivity which the adsorption columns show is quite different from any of the partition columns. This selectivity can offer advantages for the identification of unknown hydrocarbon components, since the separation is by carbon number (molecular weight) rather than by boiling point. The objectionable features of adsorption columns, mentioned in the introduction to this paper, have been circumvented by the construction of these hydrophobic adsorption capillary col-
CS hydrocarbon Figure 6.
Separation of Cg and
Ce hydrocarbon mixture
VOL. 38, NO. 2, FEBRUARY 1966
305
umns. Because of the variety of other hydrophobic colloidal materials which are available from various companies, or which may be prepared in the laboratory, it should be possible to extend this work to permit the rapid and efficient analysis of other complex samples. It is, of course, possible to use CD-100 or other hydrophobic sols as a coating for packed columns. We performed a few tests with columns containing glass beads, or diatomaceous earth supports coated with CD-100. The resolution was not as good as with the capillary columns, but the packed
columns are useful for prep-scale separations. Since CD-100 is ordinarily applied to capillaries from organic solvents, it is feasible to add a partition liquid to the sol in order to modify the selectivity of a given column. This is particularly useful for elastomers, or other complex coating liquids, which are ordinarily difficult to coat on capillaries. ACKNOWLEDGMENT
The authors thank the Nalco Chemical CO. for providing samples of CD100.
LITERATURE CITED
(1) Eggertsen, F. T., Knight, H. S., ANAL. CHEM. 30, 15-20 (1958). (2) Nalco Chemical CO., Chicago, Ill., technical data sheet, Nalco CD-100 (1962). (3) Schwartz, R. D., Brasseaux, D. J., Shoemake,G. R,, ANAL. CHEM. 35, 496499(1963). (4) Schwartz, R. D., Brasseaux, D. J., Ibzd., pp. 1374-82.
RECEIVED for review July 15, 1965. AcCePted November 5, 1965. Third International Sym osium, Advances in Gas Chromatograpiy, Houston, Texas, October 1965.
Microanalysis of Titanium by Gas Chromatography ROBERT E. SIEVERS Aerospace Research Laboratories, ARC, Wright-Patterson Air Force Buse, Ohio GUTHRIE WHEELER, Jr., and WILLIAM 0. ROSS Monsanto Research Corp., Dayton laborafory, Dayton, Ohio Gas chromatography of volatile metal complexes offers a rapid and highly sensitive method for the analysis of mixtures of metals. A simple, sensitive, and selective method has been developed for the quantitative microanalysis of titanium in oxide mixtures. The sample is converted to titanium tetrachloride by reaction with carbon tetrachloride at elevated temperature followed by gas chromatographic analysis. Problems of atmospheric hydrolysis prevalent in earlier studies have been eliminated by the use of a microreactor technique in which the reactants are sealed in a small capillary, the chlorination is effected, and the reaction products are introduced directly into the carrier gas stream. Validity of the technique was verified by analysis of a standard NBS sample of bauxite (certified found percentage TiOz 2.78%,
2.75%).
D
few years workers in several laboratories have been studying the application of gas chromatography to metal analysis. This concept requires that the metal constituents of a sample be converted to volatile compounds that can be analyzed by gas chromatography. It was reasoned that the technique might afford an extremely sensitive, selective, and rapid method for analyzing metals. The sensitivity of the technique has exceeded early expectations; quantities as small as lo-'* gram of some metals can be detected by electron capture 306
URING THE PAST
ANALYTICAL CHEMISTRY
45407
detectors (1, 14-16). A recent monograph by RIoshier and Sievers (13) summarizes the advances made in this field; the reader is referred to this work for a bibliography and historical description. Other pertinent studies that have been reported since the monograph was written are those of Morie and Sweet (ZO), Moshier and Schwarberg (11), and Scribner, et al. (17, 18). Very recently Eisentraut and Sievers (5) successfully chromatographed complexes of fifteen rare earths. Compounds containing more than half the elements in the periodic table have now been chromatographed. The separation of metal chlorides by gas-liquid chromatography (GLC) was investigated as early as 1959 by Freiser (6) and by Juvet and Wachi (7, 62). Other investigators (8, 19-61) have improved and extended the separations of metal chlorides using this technique. However, all previous studies have yielded only qualitative data. Atmospheric hydrolysis of the moisture-sensitive halides has prevented the earlier workers from making quantitative measurements. The present paper describes the successful quantitative determination of titanium tetrachloride and demonstrates the application of the method to a practical analytical problem. We have developed a method for the quantitative determination of titanium dioxide whereby the titanium in metal oxide mixtures, such as bauxite, is quantitatively converted to titanium tetrachloride. The chloride is then analyzed by gas chromatography. The
problem of atmospheric hydrolysis has been circumvented by devising a simple microreactor technique that completely avoids contact of the sample with the atmosphere. Camboulives (4) demonstrated that carbon tetrachloride reacts with several metal oxides a t elevated temperatures, converting them to metal chlorides. Conducting the reaction in sealed glass capillaries, we have coupled this chlorination technique with the analysis of the reaction products by GLC to yield a rapid, sensitive, and selective method for the analysis of titanium. EXPERIMENTAL
Instrumentation. An F and M Model 700 Temperature Programmed Gas Chromatograph equipped with a Barber-Colman 0-1 mv. recorder was used in these studies. This chromatograph was equipped with a bayonet type F and M Solid Sample Injector SI-4 system which permitted sealed capsules made from melting point capillaries to be inserted into the carrier gas stream, then crushed to release the reaction products directly into the elution system of the chromatograph without coming in contact with the atmosphere. The device consists of a hollow metal tube in which the capsule is placed to permit insertion into the injection port, and a plunger that slips inside the metal tube and is used to break the glass capsule once it is inserted. The only modification made on the device was to replace the metal spacer holding the silicone rubber septums in place with a spacer machined from Teflon. The detector block was equipped
