LITERATURE CITED
(1) Browning, L. C., Watts, J. O., ANAL. CHEM.Z9,24(1957). (2) Deemter, J. J. van, Zuiderweg, F. J., Klinkenberg, A,, Chem. Eng. Sci., 5, 271 (1956). (3) Dimhat, M., Porter, P. E., StrOES, F. H., ANAL. CHEM. 28, 290 (1956). (4) Eggertsen, F. T., Knight, H. S., Ibid., 30, 15 (1958).
(5) Eggertsen, F. T., Knight, H. S., Groennings, S.. Ibid., 28, 303
(1956). (6)
Fredericks, E. M., Brooks, F. R.,
Ibid., 28, 297 (1956). (7) Knight, E. S., Groennings, S., Ibid., 28, 1949 (1956). (8) Martin, A. E., Smart, J., Natwe 175, 422 (1955). (9) Martin, C. C., Knrta, S. S., Jr.,
others, ANAL. CHEM. 28,
490 (1956). (10) Rossini, F. D., Li, Xunn, Science 122, 513 (1955). (11) sax,K, J,, stress, F, H., A ~ CHEM.29, 1700 (1957).
RECEIVED for review April 4, 1957. Accepted July 22, 1957. Division of Refining, 22nd Meeting, Amencan Petroleum Institute, Philadelphia, Pa., May 1957.
Exploratory Studies of High Tempercri Gas-Liquid Ghromccltography JAMES L. OGILVIE, M. C. SIMMONS, aind G. P. HINDS, Jr. IHouston Research Laborc!tory, Shell Oil CcL, P. 0. Box 2527, Houston I , Tex. L A ----?:-,,:A rcJu~-,,yv,urhr-mmatography system has been designed which is capable of operoting up to about 400" C. The preheater, separation column, and detector are all cast in an aluminum block and heated electrically. The stationary liquid employed is petroleum-derived asphaltenes, which a r e thermally stable and 'sufficiently nonvolatile to operate in the range from 300' to 400' C. This technique has been used for the determination of n-paraffin distribution in waxes. The results obtained compare favorably with independent moss spectrometric analyses, and indicate that gas-liquid chromatography can be a useful technique for the analysis of higher boiling petroleum fractions.
APPARATUS
Figure 1 shows a schematic flow diagram of the apparatus. The helium enters the preheater coil after passing through a series of regulating devices, proceeds through one passage of the double-pass thermal conductivity cell, and enters the column through an intermediate tee which also serves a s a sample inlet. The exit end of the chromatographic column is attached t o the other passage of the conductivity cell. From this cell passage the gas is vented to the atmosphere.
G
is a useful and growing field in both organic and andytical chemistry (1 4 ). However, in consideration of the upper operating temperatures of most stationary liquid phases and detector systems, the limit imposed by most investigators has been placed a t compounds boiling approximately at 250' C. The possibility of extending the range of gasliquid chromatography t o substances of higher molecular weight has been intriguing. High temperature gas-liquid chromatography would be of interest both as a complete analytical procedure in itself and as a separation tool for other analytical methods. Higher molecular weight separations have been made possible in this laborstory by the use of asphaltenes as the stationary liquid phase and bare-wire thermal conductivity filaments in the detector cell. The apparatus is actually of more or less conventional design, with specific pains being taken t o a 5 sure satisfactory operation at elevated temperature. As-LIQmn CHROMATOGRAPHY
final form of a cylinder 10 inches in diameter and 12 inches high. The top of the conductivity cell was flush with the top of the cylinder and one end of the sample inlet tee protruded from the block. Resistance wire was evenly wound around the circumference of the block to provide beat to the unit. The temperature was controlled by means of a Powerstat. The operating temperature of the instrument was measured by means of a thermocouple placed in a hole drilled next to the conductivity cell. The finished assembly is illustrated in Figure 2. Because of the requirements of low volatility and thermal stability at high temperatures, the choice of a stationary phase was limited. It seemed that a petroleum-based fraction of high molecular weight would satisfy these requirements. The fraction decided upon was derived from the propane deasphalting of West Texas residue. The asphalt from this separation was pentane deoiled, and only the ether-soluble portion of the resulting asphaltenes was used. Although this fraction was chosen arbitrarily, it is assumed that other asphalt fractions would give similar separations.
Figure 1. Gas flow diagram of high temperature gos-liquid chromatographic apparatus
The chromatographic column was prepared by packing 23 feet of stainless steel tubing s/8 inch in outside diameter with 20- t o 40-mesh C-22 firebrick (Johns-Manville). The tubing was vibrated during the introduction of the firebrick, which had been previously screened, water-washed, and dried. The packed column was then helically coiled; one end was attached to the thermal conductivity cell and the other end t o the sample inlet tee. The preheater coil was then fastened in position and these parts of the apparatus were cast in an aluminum block which had the
Figure 2. assembly
Top of
oluminum block
VOL. 30. NO. I , JANUARY 1958
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Because of the high temperature involved in the casting of aluminum, the stationary liquid was placed on the firebrick after the column was cast. The asphaltenes were dissolved in just enough diethyl ether to fill the free volume of the column, and this solution was blown into the column by nitrogen pressure. The ether was allowed to evaporate, leaving the asphaltenes on the firebrick (and column walls). The unit was then heated slowly with helium passing through the column, and repeated injections of small amounts of pentane and ether were made in order to facilitate the removal of light material which may have been trapped by the asphaltenes. The column prepared in this manner contained approximately 10% by weight of asphaltenes. The detector system was based on a Gow-Mac Model 30-S thermal conductivity cell connected in a conventional Wheatstone bridge circuit. The output from the bridge circuit is recorded by a standard Brown recorder with a span of 0 to 5 mv,, a chart speed of 12 inches per hour, and a pen response time of 4.5 seconds. Preliminary work in the range of 100' to 280' C. mas carried out with glass-coated thermistors (KO.32 PB 1, Thermistor Corp. of America) having a 2-inch probe sealed in copper adapters which allowed them to be placed in the Cow-Mac thermal conductivity cell. The tip of the thermistor was placed in the helium stream with the normal diffusion space of the cell being occupied by the copper adapter. As the response of these thermistors falls off with increasing temperature (no detectable response a t 300' C.), bare-wire filaments were employed a t the higher temperatures studied. These filaments (Cow-Mac Type 9225) consisted of a ceramic-filled metal grommet through which the two lead wires were placed. A copper gasket with a beveled edge was placed under the grommet to provide a seat for the holding nut in the cell. These filaments have been operated a t a temperature of 350' C. with no apparent instability. Little
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ANALYTICAL CHEMISTRY
I
work has been done a t higher teniperatures because of the increased likelihood of thermal decomposition. At temperatures of about 150' C., these thermistors have approximately the same response as the filaments and are theoretically more sensitive a t room temperature or lower. PROCEDURE
The sample (0.01 to 0.05 ml.) was placed on the column by injection from a hypodermic syringe through a rubber serum cap. This cap was on a short length of glass tubing which was sealed to a stainless steel tubing sleeve by means of a graphite-on-asbestos string and Sauereisen insulating cement. A tubing nut which fits over the sleeve was screwed in place on the end of the sample inlet tee. The glass tubing acts somewhat as thermal insulation to reduce deterioration of the serum cap by heat. A Kovar-glass seal can also be used for this purpose, with the Kovar silversoldered to the tubing sleeve. Even with these arrangements, the serum cap must be replaced daily when the apparatus is operating a t temperatures in the range of 300' to 350" C. The samples for which high temperature gas-liquid chromatography is used are usually either solid or estremely viscous liquids. To introduce such samples to the column, both the L-l
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1
1
sample and the syringe used for injection were heated by an infrared lamp until the sample became fluid enough to be drawn into the needle. The injection through the rubber serum cap was made immediately, while the sample was still liquid, An alternative method, which consists of dissolving the sample in a solvent of lower molecular weight, has also proved satisfactory, although more cumbersome. This method could also provide an internal marker for quantitative estimation of components. In either case a long enough needle is used so that injection occurs in the hot portion of the sample inlet tee. RESULTS AND DISCUSSION
Kith the unit operating a t 282" C. a separation of the n-paraffin components of an East Texas 100 distillate wax was obtained. This wax has a n-paraffin distribution ranging from Czo to about Cm. Figure 3 illustrates the separation achieved with the thermistors in the detection cell. In this case the flow rate is measured a t the inlet of the column. Figure 4 shows the same separation a t a slightly different flow rate (measured a t the column exit), and illustrates the higher response obtained with the tungsten bare-!Tire thermal conductivity filaments a t this temperature. The identification of the peaks in the I
1
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Figure 3. East Texas 100 distillate wax with thermistors in detection cell Temperature, 282' C.; helium inlet rate, 42 ml. per minute I
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-
-
1
cliromatograms of the wax samples is based on pure compound data a t lower tcmperatures. The retention time of woctadecane was obtained from a pure w n p l e and compared to the retention times of the peaks of a Chromatogram of :i gas oil wax run under identical conditions. The n-octadccane peak in the gas oil \\ax was thus identified, and on tlie premise that all of the peaks in the \$ax chromatogram are due t o n-paraffins the distribution of n-paraffins in this \\:I\ was obtainrd.
,
1
3 -
Figure 6.
1
1
1
1
1
East Texas 250 distillate wax with bare-wire filaments Temperature, 320' C.; helium exit rate, 50 rnl. per minute
factor was then applied to the remaining areas. Table I shows good agreement between mass spectrometric and gasliquid chromatographic results.
Table II. Comparison of n-Paraffin Distributions of East Texas 250 Distillate W a x by Mass Spectrometry and Gas-Liquid Chromatography
% Volunxn-Paraffins
RI S
G-LC
CX C?,
0.4 0 4 0.7 1 1 3 0 5.6 7.1 7 7 7 9 7 7 6 . 00 4 9 3 4 2 6 58 5
...
Table I. Comparison of n-Paraffin Distributions of East Texas 100 Distillate W a x by Mass Spectrometry and Gas-liquid Chromatography
CW .. CW
yo Volume n-Paraffins
LIS
G-LC
0.5
CW
0.2
Figure 5. Relationship between carbon number and log retention time
C31
Cd? C33
The premise that all the peaks are due to n-paraffins was borne out by plotting the logarithm of the retention time against the carbon number assigned to the peaks. A straight line was obtained (Figure 5 ) . If any deviation from linearity had been observed, the assignment of n-paraffin carbon numbers would have been less certain. This technique n a s first suggested by James and Martin ( 5 ) . The n-paraffin components of the other wax samples were then identified by reference to this sample through comparison of peak retention times measured under identical conditions. Further confirmation of the peak identification was obtained by mass spectrometric analysis of these samples. Table I sho\\-s a comparison of mass spectrometric ('7) and gas-liquid chromatographic analyses of the East Texas 100 distillate wax. Because some of chromatographic peaks were broad and diffuse. it was not possible to calculate percentages of the components directly. The area of each measurable peak was obtained from the triangle which best approximated the peak contour. The area of the peak for tetracosane (n-C,) was multiplied by a factor which made it equal to the mass spectrometric percentage for n- Ca. This proportionality
C",-" C2i
Total
~
10.7 5 7 81.1
10.4 7.9 82.6
~
Total
0.2 0.8 0.5 1.8 4.3 7.1 8.0 7.7 7.3 5 . 55 5 0 2 7 2 1 53 0
~
~
The high temperature gas-liquid chromatography unit has also been used to separate a wax with a higher molecular weight distribution: a fully refined paraffin wax from East Texas 250 distillate with carbon numbers ranging from 20 to 38. Figure 6 shows a chromatogram obtained a t 320' C. on this sample. The carbon number distribution of the n-paraffins was determined by the same method used for the 100 distillate wax, hexacosane (n-C26) being the reference component. A comparison of these results with mass spectrometric data for the same sample (Table 11) shows good agreement. These wax samples are, of course, not composed solely of n-paraffin components. There are also isoparaffins and cycloparaffins in varying concentrations, ranging as high as 4QY0in the case of the 250 distillate wax. These components appear as a smeared background under the n-paraffin peaks, and the individual components are not separated into discrete peaks. The technique described should be useful in solving many problems con-
cerned with materials having a molecular weight as high as 500. ACKNOWLEDGMENT
Preliminary experiments by L. R. Snyder were helpful in this work. The authors also wish to express their appreciation to M. J. O'Neal for many valuable suggestions and criticisms, and to Isaac Dvoretaky for his critical examination of the manuscript. LITERATURE CITED
(I) Cropper, F. R., Heywood, A., NatUTe 172, 1101 (1953). (2) Ibid.,174, 1063 (1954). (3) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEM.28, 290 (1956). ( 4 ) Fredericks. E. RI.. Brooks. F. R.. Ibid.. 28, 297 (1956). ( 5 ) James, A. T., Martin, A. J. P., Biochem. J . 50, 679 (1952). (6) James, A. T., Martin, A. J. P., Smith, G. H., Ibid.,5 2 , 238 (1952). (7) O'Neal, M. J., Jr., Wier, T. P., Jr., AXAL. CHEV.23, 830 (1951).
RECEIVED for review May 22, 1.957. Accepted September 12, 1957. Division of Petroleum Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
vot.
30, NO. 1, JANUARY 1958
27
