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, East Lansing, Michigan, 1962. (6) Lovelock, J. E., Zlatkis, A., Becker, R. S., i?'ature 193, 540 (1962). " I ,-Y"",.
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
1853
Table 1.
Repeatability Test with Mariotte Bottle
Length Sample of flow Period colored Min- Sec- zone, Tube Utes onds in. 1 2 3
14 8 7
4
8
10 43 13 13
5 6
6
37
6
is
Av.
0.60 0.60 0.65 0.70 0.70 0.70 0.66
Deviation, in. -0,06 -0.06 -0.01 +0.04 +0.04 +0.04 f0.04
Table II. Repeatability Test with Rubber Balloon Approximate vol. = 2 liters
Tube
utes
onde
in.
Deviatjon, In.
1 2 3 4 5
30 29 20 10 20 8 17 11 12
0 0 0 40 45 0 20 30 0
1.58 1.84 1.68 1.60 1.75 1.70 1.73 1.68 1.64 1.69
-0.11 + O . 15 -0.01 -0.09 +0.06 +0.01 +0.04 -0.01 -0.05 zt0.06
6 7 8 9
Av.
80-mesh silica gel (Davison Grade 950) in a 50-ml. beaker. Add slowly, with stirring, 4 ml. of the PdCl, solution to the silica gel, Stir the mixture until uniform, then place the wetted gel in a vacuum oven and heat at 50' to 55" C. for 5 hours. Stir occasionally to prevent darkening of the outer layer of gel. Store the gel in a dessicator.
size 2 rubber stopper. Break off the tips of a detector tube by inserting them into the end of the beaker and giving a quick snap. Squeeze all the air out of the balloon and insert the indented end of the detector tube into the rubber stopper-balloon assembly. Prepare a rubber stopper to fit the detector tube on one side and the vaporizer tube on the other. Insert the free end of the detector tube into this stopper and connect it to the vaporizer. Figure 2 illustrates the apparatus assembled for a test. The test should be made with the system liquid full up to the needle valve. Open the needle valve slowly until a pressure of about 25 p.s.i.g. is obtained on the pressure gauge. If the valve is opened too wide, the connections may be blown off and the test will have to be started over; furthermore, liquid is a p t to be forced through the valve faster than it can be vaporized in the vaporizer tube. The sample should he passed through the tube slowly enough that no liquid is observed in the detector tube. I t is sometimes expedient to warm the valve with the hand to aid vaporization of the sample. With a pressure of 25 p.s.i.g., about 4 minutes are required to fill the balloon to proper size. Higher flow rates will result in liquid passing the detector tube. Place the balloon in the standardvolume ring and allow it to expand with filling-gas until the balloon and ring make a snug fit. When the balloon fills the ring, it contains 2 liters of gas and the test is complete. Close the needle valve and carefully remove the detector tube and balloon. Observe the number of bright yellow zones formed. Multiply the number of yellow zones (including fractional zones) by 10 to obtain the concentration of ethyl mercaptan in parts per million by weight.
PREPARATION OF DETECTOR TUBE. Cut a &inch length of 4-mm. glass tubing and make three small indentations in the side of the tube about 1.5 inches from one end. Place a 0.10- to 0.15inch cotton plug on the indentations. Fill the tube with alternate layers of treated silica gel (0.0400 gram) and pure silica gel (60 to 80 mesh, 0.0200 gram). Sufficient accuracy may be obtained by measurine; out the gel with calibrated scoops made from short sections of 4-mm. glass tubing. After each layer has been added, the tube should be vibrated with a mechanical vibrator for a few seconds. A completed tube consists of 3 layers of treated silica gel and 2 layers of pure silica gel. Place another cotton plug in the tube and pack down cotton from both ends. Seal both ends of the tube with a flame so that the final length is 5 to 5.5 inches. The adsorbent bed length will be 2 to 2.5 inches. Procedure. Using the required fittings (Figure 2), connect the needle valve to the L P G container or other source. Connect t h e free end of the needle valve via a pipe 5" t o a pressure gauge (0 to 30 1i.s.i.g.) and t o the vaporizer tube. Place the mouth of the balloon over the large end of a 1854
ANALYTICAL CHEMISTRY
RESULTS A N D DISCUSSION
The nature of the support has a pronounced effect on the sensitivity of the reagent. If alumina is used as a support, the unreacted section remains white, but the reacted area is a much lighter yellow than that obtained with a silica gel support. When tubes were prepared with silica gel in a range of mesh sizes 30-60, 60-80, and 60-200, the sensitivity in terms of zone length increased with increasing particle size because of a decrease in adsorptive capacity for mercaptan and a smaller amount of reagent per unit volume of silica gel. Furthermore, the sensitivity depends upon the degree of activation of the silica gel. If the prepared reagent is allowed to stand exposed to the air so that it adsorbs moisture, the length of yellow color observed becomes greater because of loss in adsorptive capacity. The formulation and procedure described in the reagents section were designed to give an optimum length of yellow-colored zone. Testing a number of different formula-
tions showed that if an acid solution of PdClz is deposited on silica gel, a more distinct color change is obtained. The unreacted PdCl,-silica gel ranges from almost colorless to tan, depending upon the amount of PdC1, per unit weight of silica gel. Unodorized LPG produces no appreciable color change immediately, but on standing overnight a light brown color is produced. When 0.02 ml. of a dilute solution of ethyl mercaptan in n-pentane (0.018% volume) was passed into a PdC12 tube, a yellow color was observed similar to that obtained with LPG containing ethyl mercaptan. T o exclude possible interference by olefins and other impurities which might occur in LPG, the PdCl, tube was tested with n-pentane and with heyene-1. No color was formed by 0.05 ml. of n-pentane, even on long standing; 0.05 ml. of hexene-1 gave only a light brown color after standing for several days. Preparation of the detector tubes would be simplified if they were packed with treated silica gel alone. However, with tubes packed in this manner, it is difficult to identify correctly the boundary betmeen reacted zone and nonreacted zone. Packing the tubes in short reagent zones separated by unreactive zones of different color facilitates substantially the visual detection of the color boundary. To test the repeatability of the method, a series of consecutive runs were made, identical in every respect except for rate of sample introduction. One liter of LPG with odor was passed through each tube. The volume of gas passed was accurately measured with a Mariotte bottle and a graduated cylinder. Results for this series of tests as shown in Table I indicate that satisfactory repeatability can be obtained. To meet the needs outlined for this test, a simple, convenient method of measuring 1 to 2 liters of LPG is necessary. The Mariotte bottle and graduated cylinder system is accurate and not expensive, but it is somewhat inconvenient, An extremely simple and ineapensive technique is that of filling a rubber balloon to some definite size. If a round balloon is used and expanded until it touches the inside of a round ring of fixed circumference, fairly repeatable results can be obtained. Table I1 shows the results obtained in a series of runs made with the balloon technique. Incidentally, the tubes used to obtain the data shown in Tables I and I1 were packed to a depth of 2.5 inches with treated gel alone, and did not contain separate layers of untreated silica gel. However, comparable results are obtained with tubes prepared by the standard procedure. Calibration of the detector tubes wac established with an LPG sampls analyzed for ethyl mercaptan by caustie
scrubbing and potentiometric titration with silver nitrate. Tests with detector tubes first revealed a difference in ethyl mercaptan concentration resulting from sampling of the gas phase (top of container) and the liquid phase (bottom of container). The sample analyzed by the potentiometric method showed 8 p.p.m. ethyl mercaptan in the gas phase and 22 p.p.m. in the liquid phase; approximately 1 and 2.2 zones of bright yellow color were observed by a detector tube. I n the preparation of a number of detector tubes as in large-scale production, the treated silica gel unavoidably adsorbs moisture, which changes the sensitivity of the tubes. A large batch of filled but unsealed tubes can be restored to a uniforrn sensitivity by placing them in a vacuum oven at 50’ to 55’ C. for 1 hour. After this drying step, the tubes are temporarily kept in a desiccator while the ends of individual tubes are rapidly sealed. The manner in which the tubes are vibrated during filling affects the bed depth to a considerable extent. If the tube is vibrated continuously during the filling, some zone mixing is a p t to occur; hence it is recommended that the tubes be vibrated only between additions of solid. Each tube contains only 1.90 X lov4 gram of PdCI2 and 0.16 gram of silica gel. Total cost of materials per tube is about 2 cents. I n addition to low cost, the tubes appear to have a long shelf life when sealed from the air. Other tests with other reagents are positive to some degree. A basic solution of sodium nitroprusside (2, 4)
deposited on basic alumina changes from white or light yellow to a dark pink in the presence of a mercaptan. LPG alone does not change the original color of the adsorbent. The pink color is caused by formation of an unknown complex between the sulfur group and the nitroso group of the nitroprusside. This complex formation is reversible, and stability of the color depends upon the ability of the substrate to retain the mercaptan. Since ethyl mercaptan is quite volatile, the color fades in about 5 minutes. For best results, the detector tube should be immersed in a bath of cold water. Another problem associated with the nitroprusside is that base slowly decomposes the reagent; nevertheless, this tube probably would have an acceptable shelf life if a large amount of reagent and no base were used. Commercial doctor soliltion ( 3 ) on basic alumina gave about 1 inch of light yellow color when left standing for a few minutes after passage of 1 liter of LPG with mercaptan odorant. The demarcation of the yellow zone is not as clear as desired. However, if the tube is heated-e.g., over a cigarette lighterthe yellow zone turns black and the odor of ethyl mercaptan in vapor driven from the tube is quite strong. Iodine deposited on alumina changes from tan to white upon contact with ethyl mercaptan in LPG. I‘nfortunately, this tube also reacts with olefins. TJse of a sulfuric acid-silica gel, Cosorbent, or Lusorbent prescrubber tube would allow the iodine tube to show a white zone three to four times longer on LPG with mercaptan than without mercaptan.
p-Aminodimethylaniline (1) gives a positive test, both in solution and on silica gel. The solution used for the liquid test also contains ferric and mercuric chlorides. The color change with added mercaptan is from light brown to wine red. For quantitative data, the resulting solution is percolated through a tube filled with untreated silica gel, with the aid of suction from a rubber bulb. The color retained by the silica gel is stable for about 24 hours. For deposition on silica gel, the solution contains ferric ammonium sulfate, sulfuric acid, and mercuric chloride. The color change with added mercaptan is from violet to pink. One liter of LPG with mercaptan gives about 0.4 inch of color, which darkens on standing. LITERATURE CITED
(I) Brychta, M.,Rudolf, J., Paliva 36,
307 (1956); C . A . 51, 3966a (1957). ( 2 ) Cheronis, S . D., Entrikin. J. B.,
“Semimicro Qualitative Organic Analysis,” p. 140, Thomas Y. Crowell Co., New York, 1947. f3) Zbid.. D. 141 (4) Mapsione, G. E., Petrol. Processing 7, 1655 (1952). (5) Mylius, F., Mazzucchelli, A,, Z. Anorg. Chem. 89, 1 (1914); C.A. 9, 419 (1915). PAUL V. PEURIFOY M. J. O’NEAL,JR. Houston Research Laboratory Shell Oil Co. P. 0. Box 100 Deer Park, Texas 77536 ISAAC DVORETZKY ‘Shell Development Co. Emeryville, Calif. 94608 RECEIVEDfor review March 5 , 1964. Accepted May 19, 1964.
Application of an Analog Computer in Analytical Chromatographic and Electrophoretic Separations SIR: In many practical separations u m g liquid or gas chromatographic tec3hniques and in electrophoretic separationq for analytical purposes, it is difficult and time consuming to find satiifactory substrat e? and conditions N hich provide complete separation of complex mixtures. When two or more components of a milture can be separated sufficiently to provide indication that mnre than m e component is p r e v n t but the resolution is insufficient tor accurate determinations of either retention times (or migration diFtances) or percentage compocition, the system can be resolved by computer techniques. Mathematical treatments of this same protilem and techniqut.i for resolution of tems have been made (1, 3 , 8).
Recently we have utilized a relatively simple and comparatively inexpensive analog computer developed in this laboratory ( 7 ) for this application. OPERATION
The operation of the instrument is of sufficient simplicity to allow technical laboratory personnel to obtain satisfactory results after only a few minutes’ instruction. .A more skilled operator can often resolve as many as 10 incompletely separated componentq in as little a? 10 to 15 minutes. The computer functions and is used in the following manner: a component distribution function can be .et up quickly on each of 10 channels. The function can be deliberately distorted; for example,
in chromatographic applications a skewed gaussian distribution is used which closely (within about 1%) approximates the actual chromatographic response. This is accomplished by plotting the desired curve. A projection oscilloscope (see Figure 1) focuses a distribution function on the plotted curve and a series of adjustments are made until the projected function matches the plotted curve. After initial function set up, three controls are used for each channel. One control adjusts the height of the function (peak), the second, its width, and the third its horizontal position. I n operation only as many channels are initially used as there are apparent components in the incompletely reVOL. 36, NO. 9, AUGUST 1964
* 1855
