Anal. Chem. 1003, 65,2493-2496
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Determination of Benzene, Toluene, Ethylbenzene, and Xylenes in Gasolines by Gas Chromatography/Deuterium Isotope Dilution Fourier Transform Infrared Spectroscopy John W. Diehl,' John W. Finkbeiner, and Frank P. DiSanzo Paulsboro Research Laboratory, Mobil Research and Development Corporation, Paulsboro, New Jersey 08066
Benzene, toluene, ethylbenzene, and xylenes (BTEX) in gasolines were determined by isotope dilution GC/FTIR with the perdeuteratedanalogs of the analytes as internal standards (ISTD). Careful selection of frequencies for selective absorbance reconstructions provided good selectivity of the analytes over their respective ISTDs as well as coeluting hydrocarbons such as l-methyl-1-cyclopenteneinthe case of benzene. Detection limits were less than 0.1 wt %, and calibration curves were linear and stable for several months. An average relative standarddeviationof 0.8%and an average percentage accuracy of 0.5 % were found. INTRODUCTION As a result of clean fuel legislation, the measurement of benzene and total aromatic hydrocarbons in reformulated gasolines has become increasingly important to the oil industry. A number of methods have been or are under development by ASTM and regulatory agencies to determine aromatics by gas chromatography (GC)lJ and GC/mass spectrometry (MS).3 These proposed methods use complex multidimensional GC, have potential interferencesfrom other hydrocarbons or oxygenates that are used for gasoline blending, or require frequent calibration. GC/FTIR has been shown to be a good quantitative tool for organic acids5 and various pollutant^.^^^ Recently, we reported the use of GC/FTIR for the accurate and precise determination of ethers and alcohols in blended gasolines.8 Aromatic compounds have distinct infrared absorptions resulting from aryl C-H out-of-plane bending? and the coupling of gas chromatography with Fourier transform infrared spectroscopy (FTIR) should prove a powerful technique to both identify and quantify these compounds. The results of our investigation into the applicability of GC/ FTIR for the accurate and precise measurement of benzene, toluene, ethylbenzene, and m-, p-, and o-xylenes (BTEX) in gasolines are presented below.
EXPERIMENTAL SECTION A Hewlett-Packard 5890 Series I1 GC/5965B IRD FTIR spectrometer was configured as follows: column, a J&W 60 m X (1)ASTM method D4420. (2) CARB SOP 116 GC/PID method.
(3) Proposed EPA GUMS method for reformulated gasoline. (4) Silverstein, R. M.; Bassler, G.C. Spectrometric Identification of Organic Compounds, 2nd ed.;John Wiley & Sons,Inc.: New York, 1967. (5)Olson, E. S.; Diehl, J. W.; Foehlich, M. L. Anal. Chem. 1988,60, 1920-1924. (6) Gurka,D. F.; Pyle, S. M. Enuiron. Sci. Technol. 1988,22,963-967. (7) Gurka, D. F.; Pyle, S. M.; Titus, R. Anal. Chem. 1992,64, 17491754.
(8)Diehl, J. W.; Finkbeiner, J. W.; DiSanzo,F. P. Anal. Chem. 1992, 64, 3202-3205. 0003-2700/93/0365-2493$04.00/0
Table I. Analyte and ISTD Infrared Reconstruction Frequencies' frequencies frequencies (cm-1) ISTD (cm-l) compd benzene toluene ethylbenzene m-xylene p-xylene o-xylene
670-678 724-732 694-702 687-695 790-798 736-744
benzene-de toluene-ds ethylbenzene-d1o p-xylene-dlo p-xylene-dlo o-xylene-dlo
2280-2288 2272-2280 2224-2232 2259-2267 2259-2267 2277-2285
"The analytes' frequencies arose from aryl C-H out-of-plane bending while the ISTDs' frequencies arose from aryl C-D stretching. The MCT detector cutoff was too high to observe the aryl C-D outof-plane bending. 0.32 mm 5.0-pm film DB-1 with Hz carrier at 42 cm/s set at 300 "C; injector, on-column with injector heater turned off. The injector temperature lagged the oven temperature by approximately 5 OC. A 0.5 m X 0.53 mm section of J&W deactivated fused-silica tubing was connected between the injector and the column to allow use of an HP 7673B autosampler. The injection volume was 0.1 pL with a 5-pL syringe and a nanoliter adaptor; oven temperature program, 50 "C for 0 min, 2 OC/min to 100 "C (0 min), 4 OC/min to 300 "C; FTIR spectrometer detector, wideband MCT (4000-550 cm-') (nominal D* = 1 X 1Olo cm Hz 0.5/ W); light pipe temperature, 300 OC; transfer line temperature, 300 O C . The light pipe exit line was connected to a vent outside the GC's oven with a 0.25 m X 0.53 mm i.d. section of J&W deactivated fused-silica tubing. Back pressure was observed in the lightpipe with 0.32-mm4.d. tubing but not with the 0.53mm4.d. tubing; resolution, 8 cm-l; scan rate, six interferograms coadded for 1spectrum/s. Selectiveabsorbance reconstructions, second difference type,BJO derivative function width, 75. Table I contains the frequency ranges used for the analytes and their respective internal standards (ISTDs). Second difference reconstructions provided better (2:l) signal-to-noise ratios over average absorbance and maximum absorbance reconstructions. A reference spectrum for the reconstructions was obtained by averaging the spectra collected from 0.1 to 0.5 min of each chromatogram. No compounds eluted during this period. As part of the selective absorbance chromatogram reconstructions, the software produced an absorbance spectrum for each chromatographic data point by ratioing this reference spectrum against the spectrum stored for each point. The spectrometer was purged with dry Nz gas with an inlet pressure of 40 psi. The restrictors were removed from the spectrometer's purge gas lines to allow a flow of 100 L/min. This provided greater safety since the GC/FTIR was occasionally in use around flammable gasolinesand other hydrocarbon solvents. The high purge rate also eliminated any hazard resulting from the Hz carrier leaking from the light pipe and reaching the hot infrared source. No leak was observedduring 1year of operation. Pure compounds were purchased from Aldrich Chemical Co. and Norell,Inc. Calibrationsolutionswere prepared in n-pentane by pipeting 1-,3-, 5-, 7-, and 10-mL amounts of each analyte (9) de Haseth, J. A.; Isenhour, T. L. Anai. Chem. 1977,49, 1977. (10) Bowater, I. C.; Brown, R. S.; Cooper, J. R.; Wilkins, C. L. Anal. Chem. 1986,58, 2195.
0 1993 American Chemlcai Society
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501
,
I
458 409’ s50
I~m-lzlacu-1 I00 50 I
C----------;;BB 3000 e500 ee0n 1580 I088 Frequmnoy Fburo 2. Infrared vapor-phase spectrum of benzene. The dlstlnct aryl C-H outof-plane bending frequency at quintitation. 4000
except benzene and toluene into respective 100-mL volumetric flasks and accurately recording the weights. In the case of benzene, 1, 2, 3, 4, and 5 mL were added, and in the case of toluene, 5,10,15,20, and 25 mL were added. Two milliliters of benzene-de and toluene-d8and 1mL of each of the other ISTDs were added to each calibration solution, while the weights were accurately recorded. This procedure bracketed the range over which the analytes occurred in gasoline, which was 0 4 % for benzene,5-25 % for toluene, and 1-105’6 for the other compounds. Calibrationcurves were based on the areas of the peaks in the analytes’and internal standards’respective selectiveabsorbance chromatograms. The ratios of the area of the analyte to the area of the ISTD vs the ratios of the weight of the analyte to the weight of the ISTD (Le., AJAi vs WJWi)were plotted to derive the curve for each compound. All calibration routines were performedwith the commercialsoftwareprovided with the FTIR spectrometer.
674 cm-1 can be used for
RESULTS AND DISCUSSION
compounds in the gasolines. Figure 1shows a typical MTBEcontaining gasoline with a Gram-Schmidt (GS)reconstruction which detects all of the major compounds in the gasoline, a 1205-1213-~m-~ reconstruction which is selective for MTBE, and a 600-798-cm-l reconstruction which is selective for the BTEX. The concept of unique selectivities can also be used for q~antitation.”~Just as benzene-de and benzene have distinctly different mass spectra with mtz values at 84 and 78, respectively, they also have distinct vapor-phase infrared spectra.” These are shown in Figures 2 and 3. Note that there is no absorbance at 2284 cm-l (aryl C-D stretching) for benzene and no absorbance a t 674 cm-l (aryl C-H out-ofplane bending) for benzene-de. Selective absorbance chromatogram reconstructions indicated no interferences between the ISTD and the analyte with selectivities greater than 1OOO. This was also the case for the other analytetinternal standard pairs (Table 1). Figure 4 shows a Gram-Schmidt chromato-
Selective infrared absorbance reconstructed chromatograms of unique wavelengths can be used to hightlight target
(11) Pinchas, S.;Laulicht,1. Infrared Spectra ofLabeZZedCompounds; Academic Press: New York, 1971.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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2284 ca-1
Figure 3. Infrared vapor-phase spectrum of benzene-& compound as an Internal standard.
a'
IS
14
Distinct differences from the spectrum of benzene allow the use of the deuterated
La
IS 14"
le
f!4
2s
es
30
(If".)
Figure 4. Gram-Schmldt chromatogram of the anaiytes and their respectbe internal standards. Some palrs such as ethylbenzene and ethyibenzenedloare completely resolved while others such as benzene and benzene-& are not separated at all. Coelution had no adverse effect on quantltatlon.
Table 11. Precision and Accuracy Data. compd actual mean RSD(n- 1) benzene toluene ethylbenzene m-xylene p-xylene o-xylene
4.40 112.0 34.14 34.07 33.89 34.68
4.42 112.1 33.97 34.59 34.08 34.69
0.6
0.3 1.5 0.5 0.6
1.2
% accuracy
0.5 0.1 0.5 1.5 0.6 0.0
Results are in milligrams per milliter; 1.1 mg/mL = -0.1 w t %; n = 10. Average RSD,0.8%. Average percent accuracy, 0.5 %.
gram of the ISTDs and analytes. Note that there is partial separation of some of the pairs such as toluene-& and toluene because of differences in boiling points. There were three coelution problems which had to be addressed. 1-Methylcyclopentene coeluted with benzene, 2,3,3-trimethylpentane coeluted with toluene, and m-xylene coeluted with p-xylene. 1-Methylcyclopentene and 2,3,3trimethylpentane can both be very abundant in gasoline,
Table 111. Detection Limits of Analytesa detectn limit compd ( a t injected, ng) compd benzene toluene ethylbenzene
50 125 100
m-xylene p-xylene o-xylene
detectn limit (amt injected, ng)
200 100 100
a l 0 0 n g = -O.lwt%.
usually in the 0.5-2.Owt % range. The narrow reconstruction frequencieslisted in Table I gave high selectivitiesfor benzene, toluene, and m-xylene (>lOOO), but the selectivity ofp-xylene over m-xylene was only 14. Selectivity was calculated by dividing the respective selectiveabsorbance chromatographic peak area of 1wt % of an analyte by the area of the peak produced by 1 wt 5% of the interfering compound after reconstruction a t the analyte's wavelength region. Calibration curves developed for p-xylene with and without an equal amount of m-xylene present had the same slopes. Tests with
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Table IV. Analyses of Fuel fuel A GC/FTIR benzene toluene ethylbenzene m-xylene p-xylene o-xylene total C8 aromatics
1.80 11.21 1.75 3.18 1.46 1.57 7.96
Piona 1.95 11.33 a a a a 8.15
fuel B GC/FTIR Piona 1.84 11.39 1.74 3.20 1.47 1.58 7.99
1.96 11.57 a a a a
8.04
fuel C GC/FTIR Piona 0.51 17.13 1.82 2.89 1.28 1.72 7.71
fuel D GC/FTIR Piona
0.56 17.36 a a a
a 7.97
2.21 8.11 2.41 4.62 2.03 2.49 11.55
2.54 8.25 a a
a a
11.79
Piona analyzer reports all C2 benzenes aa one value. Results are in weight percent.
solutions in which the amount of m-xylene was varied from 1/3to 3 times the amount of p-xylene did not show increased errors in the measurement of either isomer. The concentration of m-xylene was usually found in gasolines at twice the concentration of p-xylene. Unlike the GC/FTIR oxygen-18 isotope dilution work done by Olson et al.,5 all calibration curves were linear. Precision and accuracy data are presented in Table 11. The analyses had an average relative standard deviation of 0.8% and an average percentage accuracy of 0.5% ’ . All analytes including m-xyleneand p-xylene were prepared together in one solution for the tests. The calibration curves were found to be stable for several months. Detection limits for these compounds ranged from 50 ng for benzene to 200 ng for m-xylene (Table 111); 100 ng was equivalent to approximately 0.1 wt 5% , assuming a gasoline density of 0.8 g/mL. Detection limits were based on a chromatographic signal-to-noise ratio of 1O:l. It should be noted that column and light pipe flows were optimized for chromatographic separation and speed, and different conditions might lower these limits. The FTIR had an appropriate dynamic range for gasoline analyses. Samples often contained 0.5 wt % or less benzene but up to 25 wt % toluene. No sample dilution or concentration was required to handle this wide concentration range of analytes. It is preferred that sample handling be minimized, because of the many volatile compounds in gasolines. (12)DiSanzo, F. P.; Giarrocco, V. J. J. Chromatogr.Sci. 1988,26,258.
Table IV contains the analyses of a number of gasolines along with the results of a Piona analyzer.12 The Piona analyzer uses flame ionization detection with theoretical response factors and eliminates interferences for aromatic hydrocarbons by multidimensional GC and reversible chemical traps. The results in Table IV agree very well, and any differences are within the precisions and accuracies of the two techniques. This indicated that the GC/FTIR was definitely overcoming any coelution problem. The Piona analyzer cannot deal with the presence of compounds such as alcohols because they are irreversibly absorbed by some of the chemical traps and/or valves and interconnecting tubing. Ethers and alcohols do not cause a problem for GC/ FTIR and can be measured simultaneously with t h e BTEX.7 Oxygen-containing compounds are now being blended with gasolinesin some states as required by the Clean Air Act.
CONCLUSION Deuterium isotope dilution GC/FTIR is a precise and accurate technique for measuring BTEX in gasoline. The proper selection of reconstruction frequencies overcomes chromatographic coelution problems and produces calibrations which are linear and stable. Further work is in progress to extend the technique to the determination of total aromatics in reformulated gasolines. RECEIVEDfor review March 8, 1993. Accepted June 9, 1993.
