A Three-Column, Two-Detector Gas Chromatographic Method for the Simultaneous Analysis of a Mixture of Fixed Gases and Hydrocarbons SIR: In a study of the photochemical decom1)osition of several azo compounds a novel gas chromatographic technique was devised for t’he analysis of all the resulting deconiposition products. This technique makes use of three columns and two thermal conductivity detectors connected in series. Previous investigators have utilized multicolunin methods which incorporate sample splitting devices (4, 6 ) , backwashing of columns ( I ) , valve switching (3,5 , 8),and series arrangements (7’). The analytical method described in this note differs from those previously described in that it is possible to display on a single recorder the analysis of a mixture containing c2-c6 hydrocarbons plus nitrogen and oxygen and to obtain quantitative analytical results with automatic int,egration of peak areas. Higher boiling compounds are resolved in short times without drastic temperature changes during the analysis and without sacrificing resolution or analytical accuracy. The technique usee a commercially available gas chromatograph to which two additional columns and a detector are connected in series. The principle of operation is that the dual column chromatograph performs in its usual manner with a column chosen for its abilit,y to separate the high boiling components within a reasonable time. The unresolved low boiling constituents and fixed gases pass into a second column which will separate the low molecular weight hydrocarbons efficiently a t room temperature. The fixed gases are then passed into an adsorption column, maintained a t room temperature, in which they are separated and detected by the reference side of the second detector. A block diagram of the apparatus used appears in Figure 1. EXPERIMENTAL
A Perkin-Elmer Model 820 gas chromatograph with flow controller was modified by the installation of a sampling valve equipped with a sample loop for the injection of gaseous samples, and a temperature controller for the thermal conductivity detectors. The combination of a precise temperature controller with dual column operation produces a stable base line during temperature programming. Detector 2 was a Burrell K2 katharometer enclosed in an insulated oven, the temperature of the oven being controlled by a rheostat. Power wab supplied to the W2 filaments in this detector b y a Gow hZac Model 9999 regulated power supply. The
Columns 1.4, 2, and 3 in series were 01,erated with a flow rate of 42 cc./minute of helium, detector 1 was maintained at 120’ C. and detector 2 a t 175’ C. Columns 2 and 3 were operated a t room temperature while column 1 was held initially a t room temperature and programed ballistically to 60’ C. after the emergence of butane from column 2.
COLUMN 18
RESULTS A N D DISCUSSION
COLUMN 3
COLUMN 2
t Figure 1 . Schematic representation of multicolumn multidetector gas chromatography system
output of each detector was brought to a switching network by which the proper detector and polarity could be chosen for observing a given component. Columns 1.4 and 1B consisted of 25 feet of 0.19-inch i.d. copper tubing filled with 259;b w./w. Apiezon L on 40-60 mesh Chromosorb P. Column 2 consisted of 30 feet of 0.19-inch i.d. copper tubing filled with 30y0 w./w. syualarie on 40-60 mesh Chromosorb P, and column 3 consisted of 15 feet of 0.19-inch i.d. copper tubing filled with 40-60 mesh 5A molecular sieve. Squalane was chosen for the analysis of lower hydrocarbons in preference to other possible substrate materials such as carbitol, hexadecane, dimethyl formamide, or dimethyl sulfolane (2) because column bleeding is much more of a problem with the latter materials. Severe bleeding from column 2 would be expected to contaminate the molecular sieve column in a short time and reduce its separation efficiency. Because it has a higher capacity for contaminants, molecular sieve 13X has been recommended for a series column application ( 7 ) . However, in the present study no difficulties were encountered using molecular sieve 5d.
Table I.
CzHa
0 0 0 0
142 141 143 143
CZ&
0 142 0 142 0 144 0 141
Figure 2 illustrates typical chromatograms obtained with the combined column system and shows the separat,ion achieved by each of the columns. Figure 2A was obtained when detector 1 was used to monitor the effluent of the Apiezon “L” column in the combined system. Figures 2B and 2C illustrate the signal produced by the measuring and reference side of detector 2. Figure 2 0 presents the final chromatogram obtained by switching the recorder from detector 1 to detector 2 and by reversing polarity at the appropriate points. The final chromatogram illustrates t’he quality of results and demonstrates that it is readily possible to achieve, in a reasonable time, a complete resolution of the components contained in this fairly complex mixture. The analysis may be displayed on a single recorder equipped with a mechanical integrator, which is used to obtain a quantitative analysis of this mixture. Table I illustrates the reproducibility and precision of an analysis achieved with the present system for a mixture containing equal concentrations of C 2 4 6 hydrocarbons. To ensure that the retention times of each compound are different in the final system, some exact combination of partitioning agent., column length, and temperature must be determined empirically for the analysis in question. Because this arrangement permits separate control of temperature in each column and because there are only modest synergistic effects, it is not as difficult to achieve an optimum configuration as might a t first be supposed.
Reproducibility of Analysis of Synthetic Mixture
Mole Fraction of Compound CiH6 CaH, ~ E C ~ H I Z nC4Hi0
0 0 0 0
142 143 144 144
0 143
0 143 0 142 0 142
0 141 0 144 0 146 N 1)
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0 144 0 144
0 145
0 142 0 141
0 139 0 144
9, AUGUST 1965
s I)
1165
II A
Y
B
C
D n W
I
50
30
I
TIME I N MINUTES c--
Figure 2.
The system greatly reduces the time required for analysis of a complex mixture and displays the chromatogram of all components on a single strip chart, eliminating the need for repeated injections of representative aliquots of the mixture into various chromatographic columns with the attendant possibility of fractionation or contamination. The method is readily extended to other problems whenever it is necessary to analyze a mixture of high and low boiling components plus fixed gases. The present arrangement of the first column to provide adequate resolution of high boiling components within a reasonable time, of a second column to resolve the low boiling constituents incompletely separated by the first column, and a molecular sieve or other adsorption column for the resolution of fixed gases is thought to be the optimum arrangement for a multicolumn system. A useful variation of this method which may be suggested is the incorporation of a flow reversal valve following detector 2 so that the direction of helium flow in the system may be reversed overnight. The use of such a flow reversal valve with moderate heating of the molecular sieve column would have the effect of eluting high boiling components from the molecular sieve and squalane columns by backwashing, so that it should not be necessary with such an arrangement to recondition the column. Although we have used the present arrangement for some months without the necessity of reconditioning the columns, it is certain that the simple system which we have used will ultimately require a conditioning treatment. h variation of this procedure, investigated in the course of developing the technique, was the incorporation of a four-way flow switching valve between column I A , 1B and 2. After air and the lower hydrocarbons had entered the second column, this valve was switched to vent the high boiling components after they had passed through the first column. To eliminate base line instability as a result of flow changes, a restriction was placed in the vent line to match empirically the back pressure produced by the other two columns. This modification proved to be quite satisfactory for qualitative analysis. However, the relative calibration factors are quite sensitive to flow rate in the detector at the time that a particular component is emerging, and the intermediate flow switching procedure proved unsatisfactory for quantitative work.
Typical chromatogams obtained wih combined column system ACKNOWLEDGMENT
A i s produced by monitoring effluent from column I A , detector l j B is effluent from Column 21 and C is the signal produced on the reference slde of detector 2 by effluent from Column 3. Figure 2D Is the final chromatogram produced by selection of the proper detector and signal polarity. The temperature of Columns 1 A and 1 B is ballistically programmed from room temperature to 60' C. after butane Is eluted from Column 2
1 166 *
ANALYTICAL CHEMISTRY
The authors gratefully acknowledge the assistance of C. D. Miller in devising this technique. Mr. Miller de-
signed and constructed the temperature control system and power supplies for the detectors used in the analysis, and assisted in the design of the flow system. LITERATURE CITED
(1) Cvejanovich, G. J., ANAL. CHEM.34, 654 (1962). (2) Dal Kogare, S., Juvet, R. S.,Jr.,
“Gas Liquid Chromatography,” p. 126, Interscience, New York, 1962. ( 3 ) Madison, J. J., ANAL.CHEM.30, 1859 (1958). (4) Merritt, C., Jr., Walsh, J. T., Ibid., 34. 108 (1962). (5) $fitzner, B.‘ M., Gitoneas, P., Ibid., 34, 589 (1962). (6) Mitzner, B. M., Jones, W. V., Ibid., 37, 447 (1965). (7) Ottenstein, D. M., “Analysis of Fixed Gases, Hydrocarbons and Related Com-
pounds by Gas Chromatography,” presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1962. (8) Simmons, M. C., Snyder, L. R., ANAL.CHEM.30, 32 (1958). J. 0. TERRY J. H. FVTRELL Chemistry Research Laboratory Aerospace Research Laboratories Wright-Patterson Air Force Base, Ohio
Purification of Hexamethylbenzene by Zone Refining and Determination of Its Melting Point SIR: For heat capacity and vapor pressure measurements, it has been necessary to prepare as pure a sample of hexamethylbenzene as possible ( I ) . We have prepared a sample of hexamethylbenzene with 0.1 mole % impurity by zone melting and have determined its melting point by modern procedures. An apparatus has been developed which allows purity determinations to be made by the melting point method of Mair, Glasgow, and Rossini (6) at temperatures above 150’ C. with a precision roughly comparable to that obtained a t room temperature. With this apparatus the purity of the starting material and the purified sample was determined. We were thus able to estimate the melting point of the ideally pure compound. Melting points cited elsewhere, along with the method of purification, are : 164’ C., fractional crystallization from ethanol (4); 165’ C., recrystallization from benzene ( 8 ) ; 164.8 + 0.1’ C., recrystallization from benzene after recrystallization from chloroform and ethanol three times ( 5 ) ; and 165.5’ C., fractional crystallization from ethanol (g), as compared with our lowest initial freezing point of 165.644’ C. for the purified compound. The discrepancies in the values, along with the difficulties in purifying this compound and in determining its melting point, make it desirable to report our results a t this time. The apparatus used for the purification was similar to the one described by Herrington, Handley, and Cook (2). Because of an increase in specific volume on melting, as well as a t the second-order transition a t 110’ C., the specific volume of liquid hexamethylbenzene a t the melting point is about 23 % greater (8) than that of the solid a t room temperature. For this reason the usual procedure of zone refining resulted in breakage of the glass tube. The purification was therefore begun with hexamethylbenzene packed tightly in a borosilicate glass tube, rather than in
the form of a continuous ingot. The zone purification process was carried out four times in a nitrogen atmosphere with a zone travel rate of 60 mm. per hour. At this rate of travel the contents of the tube separated into well defined regions during the first pass with a gap of about 20 mm. between each, thus allowing room for expansion in the second pass. After the fourth pass these sections had united so that a fifth pass was not attempted. Because hexamethylbenzene has a vapor pressure of about 50 mm. H g at the melting point (5), the freezing point determinations must be done in a closed system. The upper part of the inner wall of the double-walled borosilicate glass vessel which contained the sample was ground, to provide a means for closing the apparatus with a mating ground joint which formed part of the cap. The cap itself was provided with a ground joint, to provide entry for the thermometer, and a pumping exit. The stirrer shaft entered the apparatus through a stuffing box, containing two Teflon washers, which was soldered on to a Kovar seal whose glass end formed part of the cap. This gastight seal permitted free reciprocal motion of the shaft, with the aid of a constant speed motor and suitable cams. Teflon sleeves over the male half were used instead of grease to seal both ground joints.
Table 1.
Sample (detn. no.) Unpurified (1) Purified (1) Purified (2) Purified (3)
Initial freezing point, tl (“C.)
165 165 165 165
576 644 693 659
The space containing the sample could be evacuated to a dynamic vacuum of about 1 mm. Hg. A positive pressure of about 6 cm. H g of purified nitrogen was maintained over the sample during all runs. The whole apparatus was immersed in a stirred thermostat whose temperature could be controlled to +0.5’ C. at any desired temperature below that of the melting range. Temperatures were measured with a platinum resistance thermometer previously calibrated in this laboratory at the ice, steam, and sulfur points as discussed by Moessen ( 7 ) . The equation of Hoge and Brickwedde (3) was used to compensate for deviations of 0.01” and 0.02’ C., respectively, in the values of the ice and steam points from those of the original calibration. The sample for the purity determinations of the purified material, weighed 42.4 grams (about 25% of the total charge) and was a mixture of two sections of the column of “pure” material symmetrically located with respect to the middle, so that it would give a value representative of the entire pure sample. The results (Table I) show that the unpurified hexamethylbenzene contained 0.26 mole % impurity, and that the impurity present in the product was 0.1 mole yo. The mole fraction of impurity was calculated as outlined b y
Purity Data for Hexamethylbenzene
Mole 70impurity‘ 0 264 f 0 020 0 120 + 0 009 0 080 f 0 007 0 093 f. 0 030
AP
0 0 0 0
toC
(“(2.)
205 093 062 072
165 781 f 0 016 165 757 f 0 007 165 755 f 0 005 165 731 f 0 023 Av. 165 75 f 0 02 0 Average of 6 values calculated from fraction frozen. R.m.s. error given for each. * Lowering of initial freezing point due to calculated amount of impurity. c Freezing point of absolutely pure compound, given by t l At. R.m.s. error given for each, and for average.
+
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