suggested in another paper that a carrier gas of high molecular weight would improve accuracy, but no experimental data were presented. ACKNOWLEDGMENT
The author thanks Alfred Pollara for assistance in making the infrared identifications and W. B. Prescott and
J. T. Woods for their encouragement and many helpful suggestions.
(4) Nerheim, A. G., ANAL. CHEM. 35, 1640 (1963). ( 5 ) Phillips, C. S. G., Timms, P. L., Chromatog. 5 , 131 (1961). JAMESS. PARSONS
LITERATURE CITED
( 1 ) Guillemin, C. I,., Aurcourt, F., J . Gas Chromatog. 1 , 24 ( 1963). ( 2 ) Liberti~ Conti, L., Crescenzi, V., Accad. Nazl. Lincei Rend. 2 0 , 6 2 3 (1956). ( 3 ) Liberti, A , , Conti, L., Crescenzi, V., Nature 178, 1067 (1956).
organicChemicals ~
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i
American Cyanamid Co. Bound Brook, N. J. RECEIVEDfor review March 20, 1964. Accepted May 25, 1964.
Detection of Traces of Polynuclear Aromatics in Hydrocarbons by Gas Chromatography SIR: Traces of polynuclear aromatic hydrocarbons in petroleum products are usually determined by ultraviolet spectrometry (3) or gas chromatography ( 4 ) . Isolation of the polynuclear aromatics and elimination of interferences is time-consuming. With an electron capture detector, however, certain polynuclear aromatics may be determined by gas chromatography with few or no prior separations, because this type of detector is insensitive to most other hydrocarbons (6). An electron capture detector for detecting polynuclear aromatics in heavy oil samples is connected in series with a hydrogen flame ionization detector, which responds to all hydrocarbons, for determining the boiling point distribution of the entire sample. Two parallel detectors have been used t o analyze portions of the effluent from a gas chromatography column ( 1 , 6); Lovelock calls this “twochannel” gas chromatography. To differentiate the two modes of operation, we suggest the terms “parallel detector” and “series detector” gas chromatog,H2-FLAME
raphy. The number of detectors need not be limited to two. One requirement of series operation is that all detectors except the last must be nondestructive. Series detector systems are simpler, more sensitive, and more accurate than parallel systems. The entire column effluent passes through both detectors, giving greater response in both. No correlation of the ratio of the flows through the detectors is necessary; there is no danger that the ratio will change, or that the gases entering the detectors will have different compositions because of fractionation in a stream splitter. Our specific application has an additional advantage-e.g., the hydrogen flame detector establishes that the sample has passed through the electron capture detector, which gives no response if there is no material present that captures electrons. A disadvantage of series operation is that the components separated by the column may be partially scrambled in the first detector. I n our electron capture
detector, which has a volume of about 1 ml., little remixing occurs. The time lag between responses of the two detectors is about 3 seconds; it could be reduced by introducing dilution gas upstream of the first detector. EXPERIMENTAL
A Barber-Colman Series 5000 modular gas chromatograph is used. I t consists of a temperature-programmable column oven, a parallel-electrode electron capture detector and a hydrogen flame ionization detector in an isothermal oven, two electrometers, and a dual-pen 5 m v . recorder. The two detectors are connected by sections of ‘/je-inch stainless steel tubing joined by a short section of Teflon tubing for electrical insulation. The chromatograph was modified to give on-column injection to achieve proper vaporization of high-boiling samples. Auxiliary heat is supplied to the dilution gas line entering the bottom of the electron capture detector to prevent condensation of sample a t that point. A Moore 63-BCL flow controller maintains the eluting gas flow rate constant.
DETECTOR
BENZO(a)PYRENE
BENZ(a)ANTHRACENE
3- METHYLCHOLANTHRENE HOLANTHRENE
I
I 0
I 20
I
IO
I
I
30
MINUTES Figure 1 .
Chromatograms of white oil containing a d d e d polynuclear aromatics
4-ft. X 3-mm. 0.d. gloss column; 0.5% SE-30 on nonacid-worhed 30- to 60-mesh Chromororb W; flow rote, 20 ml. Nn/min.; no dilution go,; 1.0 #I, som ple column temp. progrommed from 130’ to 260’ C. a t 4’ C./min.; 33 volts on electron copture detector; 300 volts on hydrogen Rome detector
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ANALYTICAL CHEMISTRY
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Nitrogen eluting gas is passed through granulated copper a t 500" C. to remove traces of oxygen to avoid oxidation of hydrocarbons in the column. RESULTS
Figure 1 shows this two chromatograms drawn simultaneously during the separation of a Na tional Formulary Grade white oil to which was added 50 p.1i.m. of each of six polynuclear aromatics known to produce a response in the electron capture detector. The white oil had a viscosity of 75 seconds, Saybolt Universal, a t 100' F., and contained no detectable polynuclear aromatics. The component peaks on the electron capture detector chromatogram were identified by separai e gas chromato-
istry and Applied Spectroscopy, March 1963. (2) Green, L. E., Schmauch, L. J., Worman, J. C., 2nd International Symposium on Advances in Gas Chromatography, Houston, Texas, March 23, 1964. (3) Lijinsky, W., ANAL. CHEM.32, 684 (1960). (4) Lijinsky, W., Dornsky, I., Mason, G., Ramahi, H. Y., Safavi, T., Ibid.,
graphic separations of each of the polynuclear aromatics in white oil. With properly chosen operating conditions, the electron capture detector will respond to less than 1 p.p.m. of each of the six polynuclear aromatics. The hydrogen flame chromatogram, after calibration with individual hydrocarbons, can be used to obtain the boiling point distribution of the entire sample ( 2 ) . For this latter analysis, the resolution of individual hydrocarbons does not need to be complete. A future paper will discuss column requirements, detector characteristics, sensitivities, and interferences.
35. QW ( l a m ) ( 5 ) Lovelock, J E., ASTM E-19 Meeting, " I ,-Y"",.
East Lansing, Michigan, 1962. (6) Lovelock, J. E., Zlatkis, A., Becker, R. S., i?'ature 193, 540 (1962). HAROLD J. DAWSON, JR. Research and Development Department American Oil Co. Whiting, Ind.
LITERATURE CITED
(1) Amy, J. W., Dimick, K. P., Pitts-
RECEIVEDfor review April 3, 1964. Accepted May 20, 1964.
burgh Conference on Analytical Chem-
Rapid Color Test for Mercaptan Odorant in Liquefied Petroleum Gas SIR: Determination of small amounts of mercaptan odorant in liquefied petroleum gas (LPG) is easily accomplished in the laboratory. Howw e r , there is a conspicuous lack of a method which can be carried out without laboratory facilities. Obviously, such a method should be simple and inexpensive and require a minimum of equipment and operator skill. This communication describes a method in which a known volume of odorized LPG is passed through a standard sized tube containing acidified F'dCI2 deposited on silica gel. Mercaptans form yellow compounds with PdCI2 ( 5 ) , and the length of the colored zone formed in the tube is an accurate measure of the mercaptan content of the LPG.
Standard-volume ring, 6bj32-inch diameter (Figure I). For convenience, a handle may be fastened to the ring. Tip breaker, 3-inch length of 3/16-inch 0.d. copper tubing, alternatively a 1/8inch hole drilled in the valve handle. Reagents. PREPARATIONO F TREATED GEL. Place 0.0500 gram
of PdClz in a 50-ml. beaker and add 10 ml. of distilled water, 5 ml. of 6 S HCl, and 5 ml. of 6AVH2S04. Warm the solution gently and stir until all the solid is dissolved. Transfer the solution to a 25-ml. volumetric flask and dilute to the mark with distilled water. Weigh out 5.00 grams of 60- to
Pressure Gauge
EXPERlMElrlTAL
Apparatus. Vaporizer tube, 4-fOOt 0.d. aluminum tubing. length of 3/16-in~h Seedle valve, aluminum. such as Dragon S o . 40. Snherical-shaned rubber balloon. having 'a diameter of a t least 6 inches'when expanded. 6-5/32 Inch ID
-P -&
Vaporizer Tube
Rubber Stopper
,
Detector Tube
Rubber Stopper
\r
Balloon Standard-Volume Ring Material: 5/8 Inch Steel Strap Figure 1.
Standard-volume ring
Figure 2.
TI7
Mercaptan detector tube assembly VOL. 36, NO. 9, AUGUST 1964
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