A Detector Tube for Determination of Aromatics in Gasoline P. V. Peurifoy, L. A. Woods, and G. A. Martin’ Shell Oil Co., Houston Research Laboratory, P . 0. Box 100, Deer Park, Texas 77536
OFTEN ITWOULD be advantageous to have a rapid, simple, and inexpensive test for total aromatics in gasoline and similar products for use in the field. Such a test might be carried out with a suitable detector tube. Although detector tubes are used principally for gas samples, they should be adaptable to use with liquid samples. This test could be useful for making spot checks on samples during blending operations or on process streams during plant operation. In addition, the detector could be used for the rapid determination of aromatics in mineral spirits, unfinished white oils, and reformer feeds and products. Detector tubes are available commercially that can detect the vapors of certain monoaromatic hydrocarbons, but these tubes also respond to other compounds present in gasoline. An example is the Kitagawa No. 118 detector tube for benzene in air, which was found to respond to olefins as well as aromatics. There are a number of compounds described in the literature which could be candidates for an aromatics detector tube. Among these compounds are the following: piperonal chloride, benzal chloride (I), antimony pentachloride (2), 1,2dinitrobenzene (3), tetrachlorophthalic anhydride (4), 2,4,7trinitrofluorenone (TNF) ( 4 ) , tetraethyl ammonium hydroxide (9,and sulfuric acid-formaldehyde (6). The present report describes the results obtained with 2,4,7-trinitrofluorenone, the most promising of this group. EXPERIMENTAL Apparatus. A detector tube support is required which may consist of a rubber stopper with a hole of proper size in the axial direction and a lateral hole or slot for pressure relief. A short length of 3/s-inch 0.d. copper tubing or a pair of pliers may be used to open a sealed detector tube. The sample is introduced into the detector tube with a I-ml syringe or with a 53/&1ch Pasteur pipet. A felt marking-pen with a fine tip is suitable for marking the location of color and solvent fronts. The lengths of the color and solvent fronts may be measured with a plastic ruler against a white or green background. Present address, Shell Chemical Co., Petrochemicals Division, P. 0. Box 2099, Houston, Texas 77001 1
(1) E. Sawicki, R. Miller, T. Stanley, and T. Hauser, ANAL.CHEM., 30, 1130 (1958). (2) E. D. Bergmann and T. J. Gruenwald, J. Appl. Chem., 7 , 15 (1957). (3) E. Sawicki, T. W. Stanley, and J. Noe, ANAL.CHEM., 32, 816, (1960).
(4) L.F. Fieser and M. Fieser, “Organic Chemistry,” D. C. Heath & Co., Boston, 1950, p. 623. ( 5 ) E. Sawicki, Chemist-Analyst, 53, 60 (1964). (6) A. L. LeRosen, R. T. Moravek, and J. K. Carlton, ANAL. CHEM.,24, 1335 (1952). 1002
ANALYTICAL CHEMISTRY
Preparation of Treated Gel. Prepare a solution of 2,4,7trinitrofluorenone in acetone containing 1.25 grams/100 ml. Prepare a solution of methylene blue in water containing 0.10 gram/25 ml. Using U. S. Standard Sieves, prepare a 200-325-mesh fraction of Davison Grade 922 silica gel. Place 20 grams of the silica gel in a 250-ml beaker and dry in a vacuum oven at 55” C for 2 hours. Add 16 ml of the TNF solution and 0.1 ml of methylene blue solution to 20 grams of the dried silica gel and mix well. Dry in a vacuum oven at 55 O C for 2 hours. Preparation of Detector Tube. Cut a 30.5-cm length of 3-mm i.d. glass tubing. Place a 5-mm cotton plug about 3.5 cm from one end. Fill the tube with TNF-treated silica gel to a depth of 16 cm when well packed. Place a I-mm cotton plug on top of the packed gel. Mark the tube with thin line at 135 mm from the top of the gel bed. This line is used later as a reference point in measuring the length of the solvent front. Seal both ends of the tube with a flame. Procedure. Break open both ends of a detector tube. Place the detector tube in the tube support in a vertical position. The end of the tube with the largest unfilled portion should be up. Load the syringe with the sample and charge 0.8 ml into the top of the detector tube. When the liquid front reaches the first mark (at 135 mm from the top of the bed), make a dot on the tube with the pen at the boundary between the green and blue areas of the tube. When all of sample has passed into the top cotton plug, mark with dots, in line, both the liquid front and the green boundary. Remove the detector tube from the tube holder and lay it down on the white background. With the ruler, measure in millimeters the distance from the top of the bed to the first dot. Divide this number by the corresponding length of the liquid front (135 mm). This ratio is known as the R, value or retention factor. In similar fashion, determine the R / value for the last pair of dots placed on the tube. Determine the average of the two R f values. Using the average R, value, read the aromatics content (in wt) from the calibration curve. A calibration curve is prepared by carrying out the above procedure on a series of samples of known aromatics content and plotting wt aromatics us. R f values. RESULTS AND DISCUSSION Several polynitro compounds form molecular complexes with aromatic hydrocarbons. Polycyclic hydrocarbons form more stable complexes than monocyclics, but 2,4,7-trinitrofluorenone forms a complex with gasoline aromatics that is stable enough for detector tube use. Silica gel treated with T N F is white and turns light yellow on contact with aromatic hydrocarbons. Better color distinction is obtained if a small amount of methylene blue is added to the TNF solution used to impregnate the silica gel. In this case, the presence of aromatics changes the color from blue to green. Three different sizes of tubes were examined; namely, 1-, 2-, and 3-mm diameter. The most repeatable results were obtained with the 3-mm i.d. tube, and no additional work was undertaken with the smaller sizes.
100
I
I
I
I
I
I
DAVISON GRADE 922 SILICA GEL 200-325 MESH
100
-
90
-
IO
Table I. Aromatics Detector Tube Repeatability Data Silica gel, mesh size 2CO-325"
- 30 ~
-
10
- 9
- 8
-7 - 6 - 5
1 2 3 4
A
Av.
20
3
1 2
ui
Av.
P -
Run
Tube identiiicationb
3
2 < d
E e
B
Aromatics,
Rf value
x wt
de
0.746 0.745 0.745 0.748 0.746 0.754 0.757 0.756
26.4 26.4 26.4 27.0 26.6 28.0 28.7 28.4
0.2 0.2 0.2 0.4 0.3 0.4 0.3 0.4
Davison grade 922 silica gel. Tube letter indicates the particular 3-mm i.d. tube used. Between runs, the tube was emptied and refilled. c Deviation from the average observed for a particular tube and a particular silica-gel mesh size. a
b
- 4
Table II. Comparison of Aromatics Detector Tube with Other Techniques
-
Sample
I
0.300
0.400
0.500
I
I
I
0.600
0.700
0.800
I
0.900
I 1.000
Rt VALUE
Figure 1. Aromatics detector calibration curve 0
Curve A. arithmetic
0 Curve B. semilogarithmic
Aromatics, wt Detector tube Other techniques
Gasoline Sample A
43.0
Gasoline Sample B
45.3
Reformate A Reformate B Mineral spirits Naphtha
60.0 41 .O 13.2 10.6
42. la 39.1b 46.60 45.46 60.8~ 42.9 12.7b 9.3s
Capillary gas-liquid chromatography. Fluorescent indicator adsorption method. c Preparative gas-liquid Chromatography. a
b
Detector tubes, made with several mesh sizes of silica gel, were investigated. A fine mesh, 200-325, gave the best line of demarcation at the color-reaction front and the most repeatable values. Silica gel prepared specifically for thin-layer chromatographic (TLC) work was found to be too fine for use in this application as the testing time for tubes made with TLC gel was too long and channeling often occurred. Use of 200-325 mesh silica gel, as contrasted with 60-200 mesh, increases the testing time from 4-5 minutes to 10-12 minutes but this increase is not considered intolerable. Alumina could not be used as a support for T N F because the intensity of the color change was greatly reduced. A calibration curve, relating R, value and aromatic content, was prepared with the data obtained from a series of samples of known total aromatics content. The calibration blends were prepared from regular gasoline, platformate, and alkylate to give a wide range of aromatics content. The aromatic content of the blending components was determined by gasliquid chromatography (GLC). Typical calibration curves are shown in Figure 1. Curve A is an arithmetic plot, and curve B is a semilog plot which is nearly a straight line. If different batches of silica gel of a given mesh size are compared, it appears that the calibration curve is sometimes shifted to the right or left, but the slope is generally the same. Each time a new batch of TNF-treated silica gel is prepared, it would be desirable to relate it to the standard curve with one or two standard samples. The effect of normal variations in detector tube diameters and of variations in gel mesh size on the results was examined. Examination of 3-mm i.d. glass tube diameters revealed a diameter variation of 1 0 . 1 mm. To test the repeatability /using a single diameter, a particular tube was emptied and refilled between runs with the same sample (a gasoline contain-
ing 26.6% wt aromatics). The data given in Table I compare the single tube data with those obtained with another randomly selected tube. Random variations in the diameters of 3-mm i.d. glass tubing appear to introduce errors in aromatic content of about 11 for the detector tube, using 200-325 mesh gel. The average deviation for a single tube with 200325 mesh gel is only A0.3 Zwt. By use of a calibration curve similar to that shown in Figure 1, curve B, the aromatic content of two gasoline samples was determined and compared with results from other analytical techniques as shown in Table 11. For purposes of illustration, the results obtained with other types of samples are shown also in Table 11. From the data given above, it can be concluded that the analytical results from an aromatics detector tube compare favorably with more conventional analysis techniques, and are obtained much more simply and rapidly. This statement holds not only for typical gasoline samples, but also for samples in the gasoline boiling range with much lower or higher aromatics content. An unused detector tube is stable for at least 12 months and the compound types present in gasoline do not appear to interfere with the test. Olefins such as octene-1 do not interfere even if the sample is entirely olefinic. The detector can be used with a variety of products, and it is probable that the range can be extended beyond that indicated by the calibration curve. The detector tubes were briefly investigated for the determination of aromatics in fractions boiling above the gasoline range (kerosenes, furnace oils, and gas oils). It was found VOL. 40, NO. 6, MAY 1968
1003
that when the gasoline-range calibration curves were used, low aromatics values were generally obtained. Also, it was observed that these samples of higher boiling range produced two colored zones, rather than a single green zone. The upper zone was brown, and the lower zone was the usual green. Tests with pure compounds indicated that benzene and naphthalene derivatives produced green colors while phenanthrene,
anthracene, and higher aromatics produced brown colors. This suggest the possibility of differentiating aromatics with three or more condensed rings from aromatics with one or two rings. RECEIVED for review August 17, 1967. Accepted October 6, 1967.
Perfluorokerosene: A Useful Internal Reference for Negative Ion Mass Spectrometry R. S . Gohlke and L, H. Thompson Spectroscopy Laboratory, Analytical Department, Dow Corning Corp., Midland, Mich. 48640 WE WISH TO REPORT that perfluorokerosene (PFK) provides a negative ion spectrum rich in ions of useful abundance over a mass range extending from m/e 19 to at least mje 940. Because the ions occur at m/e values which are not commonly found in nonperfluorinated compounds, the PFK is used as an internal m/e reference by admitting it t o the mass spectrometer with the compound under investigation. As can be ~~
~~
~~~~
Intensity m/e
Neg.
19 31 38 39 43 50 59 62 69 70 72 74 81 85 93
100.0 0.9 2.1 0.5
0.2 0.2 0.2 5.4 0.4 0.2 0.4 0.6 0.3 2.7
94
100 101 105 112 117 119 120 124 131 132 143 144 150 155 162 167 169 170 174 181 182 186 193
1004
0.4 3.1 1.1 0.4 0.3 7.2 0.3 1.3 1.1
~~~
seen from Table I, in the m/e range 220 to 800, any unknown ion peak is always bracketed by a pair of PFK peaks, one of which is never more remote than 6 mass units. Two fractions of PFK are available from Columbia Organic Chemicals Co., Inc., Columbia, S. C.-high boiling and low boiling. We find the former more useful because of its wider mass range.
~~
Table I. Positive and Negative Ion Mass Spectra of Perfluorokerosene Intensity Intensity Pos. m/e Neg. Pos. m/e Neg. 219 8.6 4.5 369 5.5 1.7 220 0.3 370 0.4 224 0.4 374 1.4 229 0.2 379 0.4 231 11.4 5.0 381 21.4 0.7 232 0.6 0.4 382 1.8 236 0.7 386 0.9 241 0.2 393 9.8 100.0 243 3.7 2.6 394 1.o 1.1 244 0.2 0.3 398 0.3 250 0.7 400 0.8 255 1.1 1.4 405 5.1 256 0.2 406 0.7 262 2.2 0.3 410 0.2 2.2 267 0.4 0.8 412 3.3 0.2 269 8.5 2.1 413 0.3 4.5 270 0.2 417 1.4 1.0 274 0.7 0.3 418 0.3 0.2 28 1 26.8 3.3 419 4.1 0.8 282 1.8 0.3 420 0.4 286 0.4 424 1.7 18.0 291 0.2 425 0.3 0.6 293 10.2 2.2 429 0.5 0.7 294 0.8 0.3 431 11.5 300 0.9 432 1.3 15.4 0.9 305 1.5 0.9 436 0.9 1.5 312 4.9 441 0.3 0.3 313 0.4 443 12.4 1.2 317 0.7 0.5 444 1.3 1.4 319 1.3 448 0.4 1.9 320 4.7 0.1 450 0.4 0.3 321 0.3 455 4.6 10.3 324 1.5 0.2 456 0.7 0.5 329 0.3 0.2 462 2.0 0.4 331 30.9 2.2 467 1.9 10.1 332 0.6 0.2 468 0.3 0.6 336 0.4 469 2.4 0.7 343 9.8 1.8 470 0.2 1.8 344 0.9 0.2 474 1.8 (Continued)
ANALYTICAL CHEMISTRY
~
Pos.
0.8 0.2
1.9 0.2 1.7 0.2
1.2
0.4
0.4
1.8 0.2
1.6 0.2
1.5 0.2
0.3
0.4
