2048
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
tubes while giving excellent reproducibilities. This should make metal carbide-coated tubes particularly suitable for routine operation or for use on automated systems. Copper sensitivity is appreciably increased when the coating consists of tantalum carbide. This may indicate a different atomization process from the inert impermeable carbide layer. A study of sensitivities obtainable for analytes normally forming stable carbides in graphite tubes is at present being undertaken by the authors to determine whether reactivity is less in carbide layer tubes. The fact that samples may be introduced into the carbide layer tubes in a perchloric acid medium, normally considered inadvisable in graphite tubes, opens up some interesting possibilities. It may be possible to destroy organic matter directly before atomization and to prevent the deposition of carbon filaments which shield off radiation. The possibility of coating the tubes externally is also being investigated as this could further extend the lifetimes of tubes. The possible coatings are of course not limited to the carbides of tungsten and tantalum as any material which is
conductive may in principle be sputtered.
LITERATURE CITED (1) M. K. Chooi, J. K. Todd, and N. D. Boyd, Clin. Cbem. ( Winston-Salem, N.C.). 21. 632 (1975). (2) C. W: Fuller, At: Absorpt. Newsi., 18, 106 (1977). (3) G.Volhnd, G.Kolblin. P. Tsct-d~el.and G. Tob. Fresenius' 2.Anal. Chem.. 284, 1 (1977). (4) D. C. Manning, F. J. Fernandez, and G. E. Peterson, Ind. Res., 19(2), 82 (1977) I . - .. I .
(5) RT E. Sturgeon and C. L Chakrabarti, Anal. Cbem., 49, 90 (1977). (6) J. C. Bokros, U S . Patent 3969 130 (1976). (7) R . Cioni, A. Mazzucotelli, and G. Ottonello. Anal. Cbim. Acta, 82, 415 (1976). (8) I. A. Kuzovlev, Y. N. Kuznetsov, and 0. A. Sverdlina. Zavodsk. Lab.. 39, 428 (1973). (9) H. M. Ortner and E. Kantuscher, Talanta, 22, 581 (1975). (IO) J. H. Runnels, R. Merryfield, and H. B. Fisher, Anal. Cbem., 47, 1258 (1975) (11) T. Stiefel, K. Schulze, G. Tolg, and H. Zorn, Anal. Cbim. Acta, 87, 67 (1976). (12)V. J. Zatka, Anal. Cbem., 50, 538 (1978). (13) K. Julshamn, A t . Absorpt. News/.. 16, 149 (1977)
RECEIVED for review April 24, 1979. Accepted June 18, 1979.
Determination of C8 and Heavier Molecular Weight Alkylbenzenes in Petroleum Naphthas by Gas Chromatography Joseph J. Pesek" Department o f Chemistry, San Jose Stafe University, San Jose, California 9 5 792
Bruce A. Blair Safety-Kleen Corporation, Elgin, Illinois 60 120
ASTM D1319 ( I ) , fluorescent indicator absorption (FIA), has been utilized as the standard procedure in the petroleum industry for the determination of aromatic content in petroleum hydrocarbon fractions having a distillation end point below 316 O C . This method consists of a special glass adsorption column packed with activated silica gel and a mixture of fluorescent dyes. The petroleum distillate sample to be treated is introduced into the column with alcohol and is separated into distinct hydrocarbon groups according to polarity. Under ultraviolet light the various hydrocarbon groups can be determined quantitatively by the length of each zone in the column. Unfortunately, the time required for the analysis of the aromatic content of various petroleum fractions ranges from 1 to 4 h and C5 and lighter hydrocarbons must be removed from the sample. Sautoni, Garber, and Davis ( 2 ) have developed a high pressure liquid chromatographic method for the determination of saturates, olefins, and aromatics in hydrocarbon fractions having a distillation range of 60-271 "C. Their technique employs a low polarity perfluorocarbon mobile phase and a small particle silica column. After resolution of the saturates and olefins, a backflush valve is operated and the aromatics are eluted as one peak. The total analysis time is about 10 min, results are reasonably accurate and precise, and the C5 and lighter hydrocarbons do not have to be removed from the sample. Several gas chromatographic methods have been developed for the determination of hydrocarbon group types in petroleum fractions. Boer and Van Arkel (3)have described an automatic analyzer system using several columns in conjunction with flow switching and hold-up fractions. This can be used for petroleum fractions with distillation end points up to 200 OC and it gives the aromatic and saturate content at each carbon number. However, the analysis time is 2 h. Stuckey ( 4 ) has 0003-2700/79/0351-2048$01 .OO/O
developed a method using an open tubular column coated with 1,2,3-tris(2-cyanoethoxy)propane.This method is not useful for a wide range of petroleum samples because benzene is eluted after n-Cll and the analysis time is 40 min. ASTM D2267-68 (5)is used for aviation gasolines with a boiling point below 177 "C and other petroleum products with a boiling point below 149 "C. Any one of five stationary phases can be chosen and the analysis time is only 10 min, but the aromatic constituents begin eluting after n-Clo. Another ASTM method, D3606-77 (6),uses N,N-bis(2-cyanoethyl)formamide (CEF) as a stationary phase. Using this column with a rotary backflush valve and a 3-m length of 0.04-mm capillary tubing results in benzene being eluted after n-CI5. In this method the saturated hydrocarbons are eluted first followed by benzene and toluene. Then the backflush valve is activated and the remainder of the aromatic constituents are eluted as one peak. The total analysis time is about 15 min, but the method requires a reproducible sample volume. This leads to relatively poor agreement between laboratories on the same sample with overall accuracy about *3%. If the latter problems could be overcome, a CEF column in conjunction with a backflush valve and a flow restrictor could be used for petroleum naphthas because they contain no benzene, very little toluene, and saturated hydrocarbons no heavier than n-C15.
EXPERIMENTAL Apparatus. The gas chromatograph used was a Perkin-Elmer Model 910 equipped with a flame ionization detector and a Sargent Model DSRG recorder. The He flow rate was maintained at about 26 mL/min. The injection port and detector were operated at 225 "C and the oven temperature was maintained at 145 OC. The CEF column was 2.44 m X 0.31 cm i.d. stainless steel. The stainless steel flow restrictor column was 3.05 m X 0.040 cm i.d. The system was equipped with a Velco Instruments Company
C 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, WO. 12, OCTOBER 1979
that the column operating parameters were correct and the flow restrictor was the proper length. A response factor was determined for each of the C8 to Clo molecular weight aromatic hydrocarbons relative to o-xylene. The relative response factor for each of the aromatic hydrocarbons was determined experimentally by comparing the respective areas of a 50/50 mixture by volume of o-xylene and the aromatic species. Ethylbenzene's relative response was determined indirectly utilizing benzene as the common intermediate with o-xylene. Durene was dissolved in o-xylene so that it was 25% by volume. The relative response factors for the C8 to C,, alkylbenzenes are shown in Table I. A mixture of normal saturated hydrocarbons was prepared to approximate the make-up of a typical petroleum naphtha hydrocarbon fraction. The mixture consisted of (by volume) 5% n-Cg, 30% n-Clo,35% n-Cil, 20% n-Clz, 8% n-Cls, and 2% n-C1& o-Xylene was then added to the hydrocarbon mixture to make a series of samples containing 2-16% by volume of the aromatic component. Next 1.5 pL of each sample was injected into the chromatograph and after 250 s, the rotary backflush valve was activated. The area under the hydrocarbon peak is compared to the area under the o-xylene peak by the following equation: area of aromatic peak = v, (1) area of aromatic peak + area of alkane peak
Table I. Relative Response Factors for AIkylbenzeRes Contained in Petroleum Naphtha volume aromatic hydrocarbon C, o-xylene C, ethylbenzene C, 1,2,4-trimethylbenzene C, cumene C,, durene C,, 1-butyl-3-methylbenzene
%a
1 }
response factorb 1.000
lo
1.149 0.933
40
1.158
40 10
0.518 0.700 weighted av 0.803
a From mass spectroscopic analysis. response factor (vol/area).
Experimental
(Houston, Texas) Model V-4-HP rotary backflush valve. A Hamilton Model 710s 5-pL syringe was used throughout. Reagents. The CEF stationary phase was obtained from Alltech Associates (Arlington Heights, Ill.) and was coated to 35% by weight on the solid support which consisted of a mixture of 50% by weight 80-100 mesh and 50% by weight 4540 mesh acid washed Chromosorb P (Anspec, Ann Arbor, Mich.). The mixed mesh solid support was chosen to minimize the pressure drop in the column while maximizing the surface area and efficiency. The alkylbenzenes and normal saturated hydrocarbons (Aldrich Chemical Co., Milwaukee, Wis., or Sargent-Welch, Chicago, Ill.) used in the standard samples had a minimum purity of 98%. Procedure. It is assumed that the samples dealt with in this study contain hydrocarbons no larger than n-C15and alkylbenzenes of C7 or larger. A 50/50 mixture by volume of toluene and n-C15 was injected into the gas chromatograph and it was experimentally determined that the backflush valve should be operated after 250 s to achieve an acceptable separation. A mixture of n-C15,toluene, o-xylene, ethylbenzene, cumene, 1,2,4-trimethylbenzene, and durene was injected into the system and the backflush valve was activated after 250 s. The chromatogram showed one peak for n-CI5and a single peak for all the aromatic components indicating
A plot of V , vs. percent volume of o-xylene is linear. When determining the aromatic content of a typical petroleum naphtha, the average relative response factor (Table I) must be incorporated into the relative area calculation as shown below: area of aromatic peak X 0.8 = v,, (2) area of aromatic peak X 0.8 + area of alkane peak
A plot of V,, vs. percent by volume of aromatic components is also linear with a correlation coefficient of 0.98.
RESULTS AND DISCUSSION For samples with an aromatic volume in the 2 to 16% range, the volume percent of C8 and heavier alkylbenzenes can be determined to within *0.9% at the 95% confidence level using
Table 11. Recycled Petroleum Naphtha GC method t 0.9
sample
aromatic content, % by volume FIA method aromatic t 3.0 GC-FIA methods
11.3 10.8 11.6 9.1 10.9
12.2 12.7 12.6 9.9 12.5
-0.9 -1.9 -1.0 -0.8 -1.6
6
11.7
11.5
+ 0.2
7 8
12.4
11.7
11.1
11.2
12.1 12.1
13.1
10
12.6
+0.7 -0.1 -1.0 -0.5
11
8.1
12
8.7 7.4 8.4 8.2
9.5 7.8 7.4 7.6 8.6
+0.9 + 0.0 10.8 -0.4
1
2 3 4 5
Clayton"
Denton"
9
13
Elgin"
14 15 16 17 19 20
12.1 11.6 11.6 10.0 9.7
21 22 23 24 25
2.1 1.9 2.1 1.I 1.9
18
Lexington"
Reedley"
-1.4
14.1 13.5
-2.0
13.6
-2.0 -1.2 -0.4
-1.9
11.2
10.1 3.2
-1.0
3.8
-1.9 -1.9 -1.9 -1.9
4.0 3.6 3.8 av
a
2049
Hydrocarbon composition varies between these samples.
1.1
2050
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
the V,, plot. This is significantly better than the *3% by volume for ASTM 1319 (FIA). Although slight aberrations in the base line occurred when the backflush valve was operated, no corrections were made for this deviation. T h e aromatic content of 25 recycled petroleum naphtha hydrocarbon fractions was determined by the gas chromatographic method. The results are given in Table I1 and compared to the aromatic content determined by the FIA method. The greatest difference between the two methods was no larger than 2% with the average difference being 1%. Only the samples from the Reedly recycling center gave consistently large differences from the FIA method. However, these samples contained percentages of aromatic hydrocarbons which are well below the maximum allowed by law. This gas chromatographic method could be utilized to determine the aromatic content of lower boiling petroleum naphtha fractions. Fractions having a boiling point similar to benzene would contain light molecular weight saturated hydr~carbons-c~and lower. Because benzene is eluted after n-CI5,the benzene content and the content of heavier molecular weight alkylbenzenes present in the light petroleum naphtha fraction could be determined by operating the
backflush valve after the elution of benzene. The nonaromatic hydrocarbons would be eluted first followed by benzene which in turn would be followed by the c8 and heavier alkylbenzenes. Such a procedure could be used to determine the aromatic content of gasoline. The gas chromatographic procedure described here offers a quick and reasonably accurate determination of C8 and heavier molecular weight alkylbenzenes contained in petroleum naphtha. Where the time required for the analysis of aromatic content must be kept to a minimum, the gas chromatographic method is far superior to the FIA method. L I T E R A T U R E CITED (1) "Hydrocarbon Types in Liquid Petdeum Products by Fluorescent Indicator Absorption", ASTM Stand., D1319-77, Part 23, 693 (1978). (2) J. C. Sautoni, H.R. Garber, and 8. E. Davis, J . Chromatogr. Sci., 13, 367 (1975). (3) H. Boer and P. Van Arkel, Hydrocarbon Process., 51, 80 (1972). (4) C. L. Stuckey, J . Chromatogr. Sci., 7 , 177 (1969). (5) "Aromatics in Light Napthas and Aviation Gasolines by Gas Chromatography", ASTM Stand., D2267-66, Part 24, 251 (1978). (6) "Benzene and Toluene in Finished Motor and Aviation Gasolines by Gas Chromatography", ASTM Stand., D3606-77, Part 25,371 (1978).
RECEIVED for review April 23,1979. Accepted June 18,1979.
Calibration of Methanol and Ethylene Glycol Nuclear Magnetic Resonance Thermometers David S. Raiford, Cherie L. Fisk, and Edwin D. Becker' National Institutes of Health, Bethesda, Maryland 20205
In high resolution proton NMR the temperature of the sample is frequently determined by measurement of the chemical shifts of methanol at low temperature and ethylene glycol a t high temperature. As the temperature rises, the amount of hydrogen bonding diminishes, and the OH proton resonance moves upfield toward the CH3 or CH2 resonance. The relation between chemical shift and temperature has been reported in several calibration curves (1-4), the most extensive and reliable of which are generally accepted as those of Van Geet (3, 4 ) . Van Geet reported a quadratic equation describing the difference in temperature of a methanol sample as a function of the separation (in Hz) between the OH proton and the CH3 protons ( 3 ) ,and a linear relationship between temperature and separation of the OH protons and CH, protons of ethylene glycol ( 4 ) . Both of these studies were performed a t 60 MHz. With the advent of higher field spectrometers, it has been necessary to scale up the results, with possible attendant magnification of any errors. We report below the results of measurements a t 220 MHz, which are entirely compatible with Van Geet's and indicate that the errors in his measurements are probably even smaller than stated in his papers. EXPERIMENTAL A Varian HR-220 spectrometer with pulse Fourier transform capability was used. A calibrated copper-constantan thermocouple was held in place in a spinning NMR tube at a uniform depth to eliminate temperature gradient effects. Methanol or glycol (reagent grade) was used in an open sample tube for thermocouple measurements. The thermocouple was secured by a Plexiglas adaptor which screwed on top of the probe. Into the adaptor a 1-mm open-ended capillary tube was inserted, through which the thermocouple,attached to a Doric Trendicator 400 digital display, extended into the solution. After careful equilibration at a specific temperature (1.5-2.5 h), the thermocouple was removed and a standard Varian 5-mm sealed temperature calibration sample was
Table I. Separation (in H z ) of CH,(CH,) and OH Proton Lines of Methanol and Glycol a t 220 MHz as a Function of Temperature ( K ) T(K)
238.8 257.9 267.2 278.4 292.2 293.0 308.8
methanol A u (Hz) 459.7 424.8 405.2 383.8, 383.5a 356.4 356.2 320.8
glycol
T(K)
303.4 314.4 327.6 339.9 352.8 372.4
Av
(Hz)
353.3, 353.2b 330.0 300.1, 300.0b 279.0 246.8 202.5
a An unsealed sample tube containing methanol, into which a capillary tube was subsequently inserted as during temperature calibration, was used for these two measurements to eliminate the possibility that the apparatus itself Measurements on changed the sample temperature. two Varian glycol samples.
introduced for the NMR measurements and allowed to equilibrate. Once equilibrated, the probe temperature was stable to zk0.14.2 K over a period of several hours in the temperature range below 320 K. Above that temperature, stability was good only to f1.0 K over several hours. The frequency differences between the hydroxyl resonance and either the CH3 or CH2resonance were measured with a digital resolution of 0.18 Hz. All samples were spinning during measurement. Experimentaldata were fitted to either two- or three-parameter curves by a least-squares regression using the NIH DEC-10 MLAB program ( 5 ) . RESULTS Table I gives the observed data. For methanol the least squares fit gives the equation
T ( K ) = 429.2
-
0.283 JAvl - 2.862 X
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
(Av)'
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