Anal. Chem. 1994,66,924-927
GWMS Analysis of MTBE, ETBE, and TAME In Gasolines Hlromltw Kanai,' Veronica Inwya,t Reglnald Goo,* Rendy Chow,# Lester Yatawa, and Jlm Maka Measurement Standards LWvision, Hawaii State Department of Agriculture, 725 Ilalo Street, Honolulu, Ha wail 968 13 To improve octane performance and reduce emissions, MTBE and to lesser degree ETBE and TAME are blended into gasolines. These oxygenates coelute with hydrocarbon components of gasoline in GC analysis. It is known that oxonium ions are formed in the electron impact mass spectrometry of aliphaticalcohols and ethers. The base ions of t-BuOH, NfTBE, ETBE, TAME, and BEE are either m/z 59 or 73 ions, and fragment ions of alkanes, alkenes, and naphthenes are at m/z 41,43,55,57,69,71,83, and 88. The maximum background mlz59 and 73 ion abundancesof three diluted (1:20) gasolines which are used to determine percent oxygenates in gasoline were less than 0.04%that of the lowest analyte standard used to determine the linear regression coefficients of these ethers. The background m/z 59 and 73 traces of undiluted gasoline were also studied. An ACN/gasoline partitioning cleanup technique was used to remove hydrocarbon interferences of less than 2%(v/v) oxygenated gasoline prior to identification by GC/MS. Two types of oxygenated compounds, either aliphatic alcohols or ethers, can be blended into gasoline to improve octane performance and reduce emissions. While gasolinecontaining alcohols require careful handling to avoid or minimize water content, ethers tend to be relatively troublefree as gasoline blend components. For this reason methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) are blended into gaso1ines.l Gasoline is a complex mixture of hundreds of compounds. Further, depending on the refining process, the proportion of paraffins, olefins, naphthenes, and aromatics (PONA) of different gasolines vary. The three oxygenates coelute with PON portions of most types of gasolines when analyzed by gas chromatography (GC). Several GC methods have been studied to solve this problem. These include solvent extraction followed by GC,2 GC oxygen-specific dete~tion,~ and multidimensional capillary column GC4adopted by the ASTM.S Methods using GC chemical ionization mass spectrometry6 and membrane introduction chemical ionization mass spectrometry7 also have been studied. f EnvironmentalChemistry Laboratory, Hawaiian Electric Industries, Honolulu, HI 96782. Toxics Laboratory, Dept. of Public Works,City and County of Honolulu, Honolulu, HI 96813. I Chemistry Branch, Hawaii State Dept. of Health, Honolulu, HI 96813. (1) Owen, K.; Coley, T. Automotive Fuel Handbook; Society of Automotive Engineers: Warrendale, PA, 1991; pp 221-232. (2) Paula, R. E.; McCoy, R. W . J . Chromatogr. Sci. 19fJ1, 19, 558-561. (3) Verga, 0. R.;Sironi, A.; Schneider, W.;Frohne, J. C. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 248-252. (4) Di Sanzo, F. P. J. Chromatogr. Sci. 1990, 28, 73-75. ( 5 ) ASTM D4815, Determination of C1 to C, alcohols and MTBE in gasoline by gas chromatography. ASTM Standards, 1990; Vol. 05.03. (6) Orlando, R.; Munson, B. Anal. Chem. 1986,58, 2788-2791. (7) Lauritsen, F. R.; Kotiaho, T.; Choudhury, T. K.; Cooks, R. G. Anal. Chem. 1992, 64, 1205-1211.
*
924
Arnelytlcel Chemistry, Vol. 66, No. 6, March 15, 1994
Quantitation of complex mixtures by monitoring the integrated area of a selected ion in electron impact (EI) MS has been used for number of year^.^^^ Aliphatic alcohols and ethers contain common fragment ions in EL MS: oxonium ions at m / z 45, 59, 73, or 87.loJ1 The base ions in the E1 spectra of MTBE, TAME, and ETBE are m / z 73, 73, and 59, respectively, and common fragment ions of alkanes, alkenes, and cycloalkanes (naphthenes) are CnHzn+~and CnH~n-1,respectively, at m / z 41,43, 5 5 , 57, 69, 71, 83, and 85.12 In this study we wanted to demonstrate that table-top model GC/MS in E1 mode could be used to quantitate oxygenates in gasolines by measuring the ion abundance of either m / z 59 or 73 ions. Three oxygenate-free gasolines with different PONA compositions were selected for this study. The background abundances of m / z 59 and 73 ions in the retention time interval used in this analysis were determined for the three gasolines. MTBE, ETBE, and TAME were spiked to another set of these gasolines and their concentrations were determined. A linear concentration range was determined for these oxygenates using tertiary butyl alcohol (t-BuOH) and butyl ethyl ether (BEE) as internal standards. When the concentration of an oxygenate was greater than 2% (v/v), identification by searching the NBS spectral library was possible. However, for concentrations lower than 2%, the identification is not definite using this method. The acetonitrile (ACN)/petroleum ether partitioning method is used to remove interferences in the GC analysis of organochlorine pesticides in fatty matrix.13 This method was applied to remove hydrocarbons prior to analyzing the oxygenate to allow MS identification.
EXPERIMENTAL SECTION Gas Chromatograph/Mass Spectrometer. A HewlettPackard (HP), Model 5890 G C was i n t e r f a d to an H P Model 5970 mass spectrometer equipped with an E1 source and a quadrupole mass analyzer. The ionizing voltage was 70 eV. The source temperature was set at the factory to 250 OC. H P Models 300 and 7946 formed the data station, which also Sweeley, C. C.; Elliot, W.M.; Fries, I.; Ryhage, R. Anal. Chem. 1966, 38,
1540-1553.
De Leenheer, A. P.; Cruyl, A. A. Biochemical Applicotions of Moss Spectrometry (First Supplementary Volume); Wiley: New York, 1980; pp
1170-1207.
McLafferty, F. W .Interpretationof MassSpectra, 3rded.; UniversityScience Books: Mill Valley, CA, 1980; pp 178-189. Budzikiewicz, H.; Djermi, C.; William, D. H. Interpretation of MassSpectra of Organic Compounds; Holden-Day: San Francism, 1964; pp 50-52. McLafferty, F. W .Interpretationof MassSpectra, Jrded.; UniversityScience Books: Mill Valley, CA, 1980; p 286. Horowitz, W. OJJlcial Methods of Analysis of the Officio1 Assodotion of Analytical Chemists, 12th ed.;George Banda: Menaaha, WI, 1975; pp 524525. QQQ3-27QQf 941Q366-Q924$Q4.5Q/Q
c6 1994 Amerkan Chetnkal Society
contained the mass spectra of 42 261 chemical compounds in the NBS REVF library. A 50 m X 0.2 mm HP-PONA fused column coated with 0.5-pm cross-linked methyl silicone was used. Our previous studies showed thecapillarycolumn coated with poly(ethy1ene glycol) did not improve resolution of oxygenates from hydrocarbons. Injector temperature and the transfer line were set at 250 and 280 OC, respectively. The GC oven was programmed from the initial temperature of 35 OC and ramped at the rate of 10 OC/min to the final temperature of 240 OC, which was maintained for 15 min. The helium gas flow rates were 1.4-1.5 mL/min. Reagents. BEE, ETBE, MTBE, TAME, t-BuOH, and toluene (all 99% purity) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Three gasoline samples, each with different PONA values, were collected from local gasoline stations. Procedure. Calibration standards of -0.2-1.2 pg/pL MTBE, ETBE, and TAME were prepared in toluene with internal standards of 0.38 pg of BEE/pL and 0.40 pg of t-BuOHlpL. Three gasolines of different PONA values were diluted (1:20) with toluene, and the background ion abundances at m / z 59 and 73 were analyzed from 3 to 8.5 min for both undiluted and diluted gasoline samples. Three oxygenated gasoline samples [4% (v/v), 1% (v/v), 0.1% (v/v)] each containing MTBE, ETBE, and TAME in gasoline C were prepared. After dilution two internal standards, BEE and t-BuOH, were added to the 4% and 1% samples. The two internal standards were added directly to the 0.1% sample. The exact mass of each ether was determined on the analytical balance. Aliquots (0.1 pL) of spiked samples, sample blanks, ACN/ oxygenated gasoline partitioned samples, and calibration standards were analyzed on the GC/MS. These were scanned from m / z 35 to 200 Da at 1.65 scans/s. The total areas of the m / z 59 and 73 ions were integrated for quantitation. ACN/Casoline Partitioning. Aliquots (50-mL) of Gasoline C containing 1% (v/v) each of MTBE, ETBE, and TAME were transferred into 250 mL separatory funnel. Aliquots (50 mL) of ACN were added and the resultant mixture was shaken for 5 min. Three layers were formed when 10 mL of saturated sodium chloride solution was added into the separatory funnel containing the ACN/gasoline mixture. The top hydrocarbon and the lower aqueous phases were discarded. The middle ACN layer containing the ethers was passed through a disposable pipet fitted with glass wool to remove residual water trapped in this solution. The small amount of hydrocarbons coextracted into ACN was removed by evaporation of the sample for -25 min. After the final volume was measured, the ACN sample was analyzed by GC/MS.
RESULTS AND DISCUSSION Quantitation. When 1% (v/v) or greater oxygenated gasoline was analyzed it was diluted (1 :20),and the data show no significant background interference from hydrocarbon components of the three similarly diluted oxygenate-free gasolines in the retention time interval where t-BuOH, MTBE, ETBE, TAME, and BEE elute in the ion chromatogram. The m / z 73 background for diluted (1:20) gasoline C from 3 to 8.5 min is shown in Figure 1A. Figure 1B shows similar background ion abundance for m / z 59 ion of this samesample.
1 76001
X
lIi!l
L
8
.L
10300-
I
7725
1
I
5150-
Figwo 1. Ion abundanCbs of mlz of 73 (A) and 59 (8) bns of d l M (1:20) gasoline C scanned from 3.0 to 8.5 min. (a-e) are retmtbn time intervals where *OH, MTBE, ETBE,TAME, and BEErerpectkty elute. (X) represents the mlz 73 lon due to benzene. CBuOH, ETBE, and BEE were quantitated by integrating the mlz 59 ion peak; the quantities of MTBE and TAME were determined by integrating the m/z 73 lon. More hydrocarbon components which elute in these retention time intervale are listed in Figure 4. The amount of benzene was 0.8%.
Points a-e of Figure 1 indicate where the three oxygenates and two internal standards elute in the chromatogram. The maximum background is 5000 ion counts, which is -0.04% of the (12-15) X lo6 ion counts for the lowest concentration of the five oxygenate standards analyzed to determine the linear regression coefficient (LRC) for each oxygenate, MTBE, ETBE, and TAME. The most intense m / z 73 background occurs at a retention time between 5.8 and 6.1 min from benzene. However, interference from benzene is not a problem because benzene does not elute in the retention time interval of the analytes. The largest m / z 59 background is due to 2,3-dimethylpentane at a retention time of -7.3 min. When gasoline C is diluted (1 :20), the 2,3-dimethylpentane background interference to BEE is negligible. The ion count difference between gasoline background and the oxygenate is important in finding out the determination limit. Gasoline C was analyzed for this study because it has maximum m / z 59 and 73 background interferences among the three gasolines. Figures 2B and 3B show the m / z 73 and 59 background traces, respectively, of undiluted gasoline C. The chromatographic peaks b, d, a, c, and e shown in Figures
-
Analyfcal Chemistry, Vol. 66,No. 6, Wrch 15, 1994
025
1 6.0ES-
c
d
b
I
S.OE5-
I!
4.0138.OE5-
P.OEb 1.085-
i
1 **lJ:2.0E4
1. O E 4
7
;@-
Flgura 2. (A) The m/z 73 lon abundance8 min of 0.1% (v/v) MTBE and Flgurr (A) The m/z 59 lon abundance of 0.1% (v/v) t-BuOH, MTBE, and BEE each splked into gamline C and marked as point6 a, c, and e, respectivelyy.(B) The mlz 59 ion backgroundtrace of the undlMed gasoline C scanned from 3 to 8.5 mln; (y) lei due to 2,3dimethyipentane.
T i b k 1. Quantkatlon of 8pik.d MTBE, ETBE, and TAME in Qa8dlnO'
OXYmaas MTBE ETBE TAME MTBE ETBE TAME
(me) 738.0 749.9 788.8 374.5 370.0 387.3
OXY (%,v/v)
AVM (%)
SD
(%)
4.0 4.0 4.0 1.0 1.0 1.0
3.9 3.9 3.9 1 1 1
0.04 0.05 0.05 0.01 0.02 0.02
1 1.2 1.2 1 2.0 2.0
RSD
a Data are baaed on seven determinations. SD,standarddeviation; RSD, relative standard devation;AVM,mean value;OXY,oxygenate.
2A and 3A represent 0.1% (v/v) of MTBE, TAME, t-BuOH, ETBE, and BEE, respectively, spiked into gasoline C. The background where t-BuOH and ETBE elute each has -5000 ion counts, which is -0.04% of the ion counts for the lowest concentration of the oxygenate standards used to determine the LRC. This background is -0.01% of the ion counts of 0.1% (v/v) r-BuOH and ETBE spiked into gasoline C. The background where MTBE and TAME each elute is 300 000 ion counts, which is -2.5% of the ion counts of the lowest concentration of the oxygenate standard used to determine the LRC. However, 300 000 ion counts is -0.5% of the ion counts of 0.1% (v/v) MTBE and TAME sample prepared from gasoline C. The m / z 73 ion background interferences in the chromatographic retention intervals, 3.8-4.3 and 6.77.1 min, where MTBE and TAME elute are due to 4-methyl-
-
028 A m W I CYmWy, Vd. 66, No. 6, Mrch 15, 1004
2-pentene and 2,2,3-trimethylpentane, respectively, of the gasoline sample. The ion counts of the m / z 73 background which are due to benzene are much greater than the diluted (1:20) sample. However, this did not cause an analytical problem. Both BEE and 2,3-dimethylpentane have m / z 59 ions, and these peaks partially overlap in undiluted sample. Table 1 shows the recovery data of 4% (v/v) and 1% (v/v) each of MTBE, ETBE, and TAME spiked into gasoline C. The analysesof three oxygenatesshow slight determinate error. Except for MTBE, the relative standard deviations of ETBE and TAME of 4% (v/v) are lower than the 1% (v/v) samples. The LRC obtained from the ratio of an oxygenate to either t-BuOH or BEE vs the corresponding oxygenate quantity for the three ethers were mostly 0.999. Although there is tailing in the ion chromatographic peaks of these oxygenates, satisfactory results were obtained because total integrated ion abundance were used in all of these calculations. ACN/Gasoline Partitioning prior to Identification. When the individual oxygenate concentration in gasoline is less than 2%,identification of its mass spectrum by searching through a reference mass spectral library, such as the NBS REVF library, was incomplete. Even background subtraction was not helpful in most cases. Probably, this arises from the tailing characteristic of these oxygenate peaks. An ACN/gasoline partitioning cleanup technique was used to remove hydro-
2.OE7
1 OE?
0'
4
6
f
b
TYmet3nin.Y Figure 4. (A) A portion of an RIC of undlluted gasoline C spiked with 1% (v/v) each of MTBE, ETBE, and TAME. (B) Same sample purified through the ACN/gasoiine partitioning technique. Points m, e, t, and g refer to MTBE, ETBE, TAME, and benzene, respectively. Peaks: (a) 4-methyl-2-pentene, (b) 1-hexene, (c) 2-methyl-1-pentene, (d) 3methylenepentane, (k) 2-methylpentene, (f) 2,4dimethylpentane, (I) 2,3dimethylpentane, (h) 3-methylhexane, and (i) heptane. Sample analyzed, 0.1 pL each for (A) and (B). Recoveries of MTBE, ETBE, and TAME were 12%.
-
carbon interferences prior to GC/MS identification. The results are shown in Figure 4. Although recovery of these 1 % (v/v) MTBE, ETBE, and TAME in the gasoline sample was only 12%,a baseline separation of these ion chromatographic peaks was obtained and complete matching was observed with the reference mass spectral library.
-
(14) Ghosh, A.; Andcregg, J. R.Anal. Chem. 1989, 61, 13-11.
CONCLUSION This study was conducted to analyze percent quantity of MTBE, ETBE, and TAME in gasoline because pending legislation in the US. Congress is targeting organic-bound oxygen content of gasoline to be minimum of 2.7% (v/v). Although correlation studies between the multidimensional GC method adopted by the ASTM4g5and this E1 MS method were not conducted in this laboratory, the relative standard deviations of the data obtained through this study and the published data of multidimensional GC4 and oxygen-specific detection GC3 methods are similar. The alkanes, alkenes, and naphthenes (cycloalkanes) in three gasolines which were analyzed for m / z 59 and 73 background ion abundance included hydrocarbon components typically found in most gasolines. Therefore, when one is quantitating percent of oxygenates in most gasolines a background subtraction is not needed. Both t-BuOH and BEE behaved as good internal standards and any of the two can be used. The sample must be analyzed for the presence of t-BuOH before it can be used as the internal standard. Because of slight interference in the chromatographic retention intervals of the undiluted gasoline sample C where MTBE and TAME elute, only one determination was done on the sample containing 0.1%(v/v) each of MTBE, ETBE, and TAME. The background interfering m / z 59 and 73 ions are not the common fragment ions of the alkanes and alkenes of the gasoline sample. The operating conditions of the GC/ MS instrument, such as the amount of vacuum, the condition of the source, and the quantity of the analyte, will affect the abundance of different ions. Therefore, additional background interference studies on a large number of undiluted gasoline samplesover period of time are needed before a determination limit of -0.1% (v/v) can be set. The GC/MS identification of all components in a gasoline mixture, even with a high-resolution capillary column, is difficult. Most laboratories do not have access to specialized software, such as the differential mass spectra.I4 Although recovery was very poor, the ACN/gasoline cleanup technique removed most of the interfering hydrocarbons, which allowed definite identification of the oxygenatesthrough the reference spectral library. Once identified, these oxygenates can be quantitated through either m / z 59 or 73 ions. Received for review August 11, 1993. Accepted December 15, 1993.' *Abstract published in Aduance ACS Absrracrs. February 1, 1994.
Analytical Che"ty,
Vd. 66,No. 6,March 15, 1994
021
