bonds of the fatty esters. This finding is in harmony with the well known fact that certain polar substrates, such as diethylene glycol succinate, have an affinity for multiple bonds (or ?r electrons) and, therefore, delay the passage or lengthen the retention times of compounds with such groups. For a single double bond we may picture the T electrons as being above and below this one bond; but for two double bonds that are conjugated, the ?r electrons extend over not only the two bonds but over the intermediate bond as well. The extra length of T-electron cloud per double bond may account for the increased retention time. A compound with a double bond conjugated with a carbonyl group also has a lengthened *-electron cloud and may be similarly delayed in passage. Several anomalies were encountered after developing the foregoing reasoning. One was with the diethyl esters of fumaric, maleic, and succinic acids. The fumarate, even though its double bond is conjugated with two carbonyls, has a lesser retention time than its saturated analog, the succinate; the maleate, with a similiar structure, has a separation factor of only 1.39. The compound 2,4,7,9-tetramethyl-5-decyn4,7-diol has a lesser retention time than Note that the its saturated analog. anomalous behavior is associated with unsaturates having rigid structures, and it is possible that contact of the substrate with the molecule's T electrons is inhibited by steric interference of neighboring groups.
It has been shown that separation factors do change some with tempers ture (1, 6) and possibly slightly with concentration. Although these variations may introduce minor differences in results, major variables are the substrate and the chromatogrsphic procedure, Although it is recognized that separation factors on different substrates can provide additional specificity for identification (6),it would be most helpful to be able to set up one or two standardized substrates and procedures so that separation factors that are determined may become more generally useful. For this purpose, a dead-volume correction should be applied. Also, separation factors in a hydrogen carrier gas cannot be expected to agree precisely with those determined in another carrier gas. C.4TALYST AND COLUMN AT DIFFERENT TEMPERATURES. Analyses were made with the injection-port hydrogenator at one temperature and the analytical column a t another. Separation factors obtained from several of the runs, which were made with the tubular bypass arrangement, are given in Table IV. Catalyst temperature does not appear to be crucial; it was varied between 140' and 250" C. without seriously affecting the determination of separation factors. Analysis of Sensitive Compounds. Some compounds are too sensitive to be analyzed quantitatively by gas chromatography. If the lability is due t o multiple bonds, analysis with the present hydrogenator may be possible
and is worth trying because of the ease of installing the apparatus, Sequential or Concomitant Andy-
The hydrogenator can be placed a t any point in the chromatographic pathway, on one side of a split stream, or after a nondestructive detector to allow the hydrogenated compound to pass through a second column. Caution. Because many gas chromatographic ovens have exposed heating elements, it is extremely important to avoid leakage of hydrogen through faulty connections into the oven area. As a safety precaution, the oven area may be purged with nitrogen to displace its oxygen. sis.
LITERATURE CITED
(1) Ackman, R. G., J. Am. Oil Chemists SOC.40, 564 (1963). (2) Beroza, M. Sarmiento, R., ANAL. CHEM.35, 13: 13 (1963). (3) p t t o n , _ H . J., Mounts, T. L., J . Cataiyszs J, 303 (1964). (4:) Ettre, L. S., ANAL.CHEM.36 (8), 31A
(1964). 5') Franc, J., Kolouskova, V., J. Chromatog. 17, 2:2 1 (1965). (6) Mounts T. L., Dutton, H. J., ANAL. CHEM.37, 641. (1965). (7) Smith, B., Ohlson, R., Acta Chem. 1 7Iiafin\ Scand. 14, 131, , ),
ZV"",.
RECEIVED Dec. 10, 1965. Accepted April 18, 1966. Presented at Two-Day Research Conference on Progress in Gas Chromatography, University of California, Los Angeles, Calif., Feb. !, 1966. Mention of proprietary names IS for identification purposes only and does not constitute endorsement by the U. s. Department of Agriculture.
Automobile Exhaust Hydrocarbon Ana lysis by Gas Chromatography DAVID J. McEWEN Fuels and lubricants Department, Research Laboratories, General Motors Corp., Warren,
b A new gas chromatographic method has been developed to analyze the complete range of hydrocarbons in both raw and highly diluted automobile exhaust gas. A commercial gas chromatograph was modified to include a separate oven for thermostating a gas sampling valve and a flow switching valve, a subtractor column for removing the unsaturated hydrocarbons, and an adsorption column in dual arrangement with a capillary column. Sampling of exhaust gas and hydrocarbon calibration mixtures was investigated. The capabilities of the method are demonstrated with examples of analyses of exhaust gas from different engine operating modes.
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4
EXTENSIVE program by both industry and government is currently under way to study and alleviate the automotive contribution to air pollution. Included in this program is a study of the influence of automobile engine operating conditions and modifications on exhaust hydrocarbon emissions. To more fully evaluate the results of this study, detailed hydrocarbon analyses are required. Gas chromatography has become the most useful technique for analyzing the hydrocarbon portion of exhaust gas, and partial analyses have been frequently reported (6-9, 11, 16, 22). However, Hurn and coworkers (4, 7) are the only group to report reasonably complete analyses. None of these pub-
Mich. lished methods, however, was considered satisfactory for our work, in which much more complete and detailed hydrocarbon analyses were required, The present paper describes a new method that utilizes a commercial gas chromatograph. As shown in Figure 1 the basic apparatus consists of a gas sampling valve, three columns (adsorption, capillary, and subtractor), a flow-switching valve, and a single flame ionization detector. With the flowswitching valve in the position depicted by the solid lines, the carrier gas bypasses the subtractor column and flows only through the adsorption and capillary columns. I n the other valve position (broken lines), the carrier gas flows through all three columns in the order: VOL. 38, NO. 8, JULY 1966
1047
SUBTRACTOR COLUMN FLOW SWITCHING
COLUMN GAS SAMPLING VALVE ADSORPTION COLUMN Figure 1.
IONIZATION DETECTOR
I
Basic apparatus
subtractor, adsorption, and finally cap& lary. Thus, in analyzing a hydrocarhon sample, the initial position of the flowswitching valve determines whether the sample is first passed through the subtractor column to remove unsaturated hydrocarbons and then into the adsorption column, or is passed directly into the adsorption column. In the adsorption column the hydrocarbons are trapped and concentrated, while most of the inorganic gases in the sample continue through the columns and detector to vent. The CI and C2 hydrocarbons are not strongly held in the adsorption column and emerge h e fore switching the valve and hackflushing the remaining hydrocarbons into the capillary column, where they are separated. Thus, two runs are required for a complete analysis; the first for a complete sample, the second
Figure 2.
-200
7 Temperature
programmer hold ot
for only the saturated hydrocarbons, and the unsaturated hydrocarbons are calculated by difference. APPARATUS
Gas Chromatoeraoh. A PerkinElmer Model 800ugas chromatograph was used for most of this work.
[Some method development work and exhaust gas analyses were performed with an F and M Model 609 gas chrcmatograph (fd).] Figure 2 shows a
1 2 Tronrito cover for detector 13 Instrument
