Environ. Sci. Technol. 2003, 37, 4994-5000
Avoiding Hydrolysis of Fuel Ether Oxygenates during Static Headspace Analysis ZHIXUN LIN,† JOHN T. WILSON,‡ AND D E N N I S D . F I N E * ,† Shaw Environmental and Infrastructure, Inc., P.O. Box 1198, Ada, Oklahoma 74821-1198, and U.S. EPA National Risk Management Research Laboratory, Ground Water and Ecosystems Restoration Division, P.O. Box 1198, Ada, Oklahoma 74821-1198
The determination of fuel ether oxygenates in groundwater was found to be problematic when samples are preserved at pH < 2 and then analyzed using heated headspace sampling. Acid catalyzed the hydrolysis of tert-amyl methyl ether, ethyl tert-butyl ether, and methyl tert-butyl ether during headspace sampling when aqueous samples were heated at 80 °C, a typical temperature used for heated headspace sampling. Hydrochloric acid at pH 2 did not cause hydrolysis of oxygenate ethers in samples stored for 28 d at 4 °C. When trisodium phosphate was used to preserve the sample or to adjust the pH of samples preserved with acid before headspace sampling, the recovery of spiked ethers was excellent. The heated headspace method was also applicable for the determination of other fuel oxygenates including ethanol, tert-butyl alcohol (TBA), tertamyl alcohol (TAA), isopropyl alcohol (IPA), acetone, and monoaromatic compounds found in gasoline including benzene, toluene, ethylbenzene, xylenes, and trimethylbenzenes. The method detection limits range from 0.1 to 0.2 µg/L for the ethers and aromatics. For alcohols and acetone, the method detection limits were 0.8 µg/L for TBA, 18 µg/L for ethanol, 1.2 µg/L for TAA, 5.5 µg/L for IPA, and 3.3 µg/L for acetone. The heated headspace method yielded accurate results for ether oxygenates in samples containing a wide range of gasoline concentrations (2500-100 000 µg/L).
Introduction The use of oxygenated fuels has created growing concern about the fate of fuel oxygenates and related compounds in groundwater. Methyl tert-butyl ether (MTBE) and ethanol are the additives used most often to increase oxygen content in gasoline and thereby reduce harmful emissions from vehicles. Other compounds that can be used as fuel oxygenates include ethyl tert-butyl ether (ETBE), diisopropyl ether (DIPE), tert-amyl methyl ether (TAME), tert-butyl alcohol (TBA), tert-amyl alcohol (TAA), isopropyl alcohol (IPA), acetone, and methanol. In addition to being present in fuel, TBA, TAA, IPA, acetone, and methanol can be produced from biodegradation of the ether oxygenates in the environment. In this paper, these compounds are collectively referred to * Corresponding author phone: (580)436-8669; fax: (580)436-8635; e-mail:
[email protected]. † Shaw Environmental and Infrastructure, Inc. ‡ U.S. EPA National Risk Management Research Laboratory. 4994
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as fuel oxygenates. Groundwater contamination with these fuel oxygenates could occur from any leaking underground storage tank (LUST). As of January 2000, public water systems are required by the U.S. EPA to monitor for MTBE in drinking water (1). Each year new state regulatory cleanup levels of oxygenates in groundwater are being implemented. The State of New York has set enforceable drinking water standards for MTBE, TBA, DIPE, TAME, ETBE, ethanol, and methanol at 50 µg/L (2). Michigan has set residential drinking water criteria for DIPE at 30 µg/L, TAME at 190 µg/L and ETBE at 49 µg/L (3). The increasing number of analytes and the need for analytical methods that are applicable to all these fuel oxygenate compounds make method development challenging. In 1999, the California State Department of Health Services established a drinking water action level for TBA at 12 µg/L (4). Determination of TBA at this low concentration is particularly challenging. Preparative methods that show promise for determining TBA at low concentrations include direct aqueous injection (5, 6), and solid-phase microextraction (7, 8). However, reviews of analytical methods for fuel oxygenates (9-11) indicate that EPA Method 8260 (GC/MS) (12) with preparative methods EPA Method 5030 (purge-and-trap) (13) or EPA Method 5021 (static headspace) (14) are also generally appropriate. Purge-and-trap GC/MS can provide method detection limits for TBA as low as 3 µg/L when a sample volume of 10 mL is purged at 40 °C (15) or 0.5 µg/L when a sample volume of 25 mL is purged at 65 °C (16). In contrast to purge-and-trap, the static headspace method has not been widely applied for analysis of fuel components in water samples. In our hands, the static headspace method with GC/MS is a sensitive and robust method for determination of all the fuel oxygenate compounds, when the water sample is heated to 80 °C before the headspace is taken for analysis. Table 1 compares the method detection limits attained in this study when the sample was prepared from water heated to 80 °C in a static headspace sampler to method detection limits reported in the literature for purge-andtrap at ambient temperature, for purge-and-trap from a sample heated to 20, 40, and 65 °C, for direct aqueous injection, and for solid-phase microextraction (SPME). Methods using direct aqueous injection or SPME provided method detection limits for TBA that were adequate to determine TBA at California’s action level of 12 µg/L. However, purge-and-trap at ambient temperature was not adequate to determine 12 µg/L TBA. The performance of purge-and-trap was adequate to determine TBA at 12 µg/L when the water sample was heated to 40 or 65 °C, and static headspace sampling was adequate when the water sample was heated to 80 °C. EPA purge-and-trap methods specify that samples be preserved by addition of hydrochloric acid to obtain a pH of 2 or less than 2 (17-19). O’Reilly et al. (20, 21) showed that MTBE, ETBE, and TAME readily hydrolyze at pH near 2 when the sample is heated. Calculations using rate constants (20) as well as laboratory evaluations indicate that acid hydrolysis has little practical importance during purge-and-trap from water at ambient temperature or from water that is heated to 40 °C (22, 23), but hydrolysis can be substantial if the water sample is heated to 80 °C during static headspace analysis (11). Acid hydrolysis of MTBE, ETBE, and TAME can be avoided if a base is used as the preservative instead of hydrochloric acid (11, 24). Kovacs and Kampbell (25) used trisodium phosphate dodecahydrate (TSP) present at 1 wt %/vol in the sample as an alternative to hydrochloric acid. 10.1021/es030375v CCC: $25.00
2003 American Chemical Society Published on Web 10/08/2003
TABLE 1. Comparison of Method Detection Limits of Ethers, Alcohols, and Acetone (µg/L)
compd
EPA Method 8260B
ASTM D4815 SPME (15) (7)
ethanol -g TBA 35a (3.0b); 0.5c IPA acetone 0.6f
27
0.2f 1a (0.2b); 0.04c 0.6a (0.2b); 0.05c 0.5a (0.1b); 0.04c 0.6a (0.1b); 0.04c
1.1
TAA MTBE ETBE DIPE TAME
-
0.9 1.1 0.7
DAI-GC/MS
HS/GC/MS (this paper)
0.96d (1.2e); 0.1f 40 1.76d (0.99e); MTBE > ETBE > TAME. This trend in stability agrees with results reported by O’Reilly et al. (21), who showed formation of TBA and TAA resulting from the hydrolysis of TAME, ETBE, and MTBE at pH 2 and 25 °C. Comparison of Preservatives. Groundwater samples from wells typically contain microorganisms that are capable of degrading aromatic hydrocarbons relatively quickly when oxygen is available. As a general practice, groundwater samples are preserved when they are collected in the field before they are delivered to the laboratory. Acidification is the most common method for preserving groundwater samples intended for volatile analysis; concentrated hydrochloric acid is added drop by drop to a 40-mL water sample to produce a pH that is near or below 2. As an alternative to acid, water samples can be preserved with TSP instead of acid (25). Water samples preserved with TSP have pH above 11, which effectively prevents the biodegradation of organic compounds in the samples. Even alkalinophilic bacteria, which multiply at or tolerate a high pH range of 8-10, are not viable at pH above 11 (27). When samples that were preserved with hydrochloric acid were analyzed immediately after preparation, 2.6 ( 3.5% of DIPE, 14 ( 8.5% of MTBE, 23 ( 9.0% of ETBE, and 29 ( 8.9% of TAME were hydrolyzed in the heated headspace sampler [treatment (HCl) in Figure 2]. Values are means and 95% confidence intervals of triplicate analyses. When samples were stored at 4 °C for 28 d and then analyzed without neutralization, the ether losses increased to 15 ( 8.5% of DIPE, 31 ( 3.5% of MTBE, 33 ( 6.0% of ETBE, and 48 ( 2.0% of TAME. When the samples were preserved with TSP, there was no evidence of hydrolysis of the four ethers at 95% confidence [treatment (TSP) in Figure 2]. When the samples were analyzed immediately after preparation, the losses of DIPE, MTBE, ETBE, and TAME were 0.4 ( 6.5%, 2.0 ( 3.0%, 9.8 ( 5.5%, and 4.4 ( 8.0%, respectively. The losses during analysis with TSP were less than the losses with hydrochloric acid. When the samples that had been stored for 28 d were analyzed, the losses of DIPE, MTBE, ETBE, and TAME were 0.2 ( 5.0%, 3.0 ( 9.9%, 7.6 ( 18.4%, and 3.0 ( 6.0%, respectively. There was no evidence at 95% confidence that the oxygenate ethers degraded during storage when preserved with TSP. When the samples that were preserved with hydrochloric acid were adjusted with 1% TSP [treatment (HCl + TSP) in Figure 2], the sample pH increased to greater than 7.0. When the samples were analyzed immediately after preparation, the losses of DIPE, MTBE, ETBE, and TAME were 3.2 ( 7.5%, 3.2 ( 3.5%, 15 ( 12%, and 7.8 ( 7.0%, respectively. The losses after 28 d were -0.4 ( 2.0%, 1.8 ( 4.5%, 9.8 ( 9.9%, and 4.2 ( 6.5%, respectively. The losses during analysis were no larger than the losses in samples preserved with TSP from the beginning. The losses in the samples that were preserved with acid and then stored for 28 d were no greater than the losses in the samples that were analyzed immediately. This
FIGURE 1. (a) Hydrolysis of MTBE and ETBE in water at pH 2 at 80 °C. Hydrolysis of MTBE and ETBE produced TBA, and hydrolysis of ETBE produced ethanol. Concentrations are mean values. The error bars are the standard deviation of the means. (b) Hydrolysis of TAME and DIPE in water at pH 2 at 80 °C. Hydrolysis of TAME produced TAA. DIPE is stable and produced little IPA. Concentrations are mean values. The error bars are the standard deviation of the means. indicates that water samples can either be preserved with TSP at collection or preserved with acid at collection and then adjusted in the lab before analysis using a heated headspace sampler. Both techniques were effective in preventing ether hydrolysis. Care should be taken that the pH be above 7 after neutralization. When NaOH was used to neutralize the acidified sample and the pH was increased to only 5.9, the variability of recoveries increased, and evidence of hydrolysis of ETBE was found during headspace analysis (28). Method Detection Limits and Spike Recovery. The performance of the static headspace sampler with GC/MS analysis is presented in Table 2. Operational parameters used for the static headspace analysis including sample heating temperature, heating time, phase ratio, amount of salt added, and salt selected were typical of parameters cited in the literature (29-31). Judgments involved in selecting the operational parameters are discussed in the Supporting Information. The method detection limits (MDLs) for the ethers and monoaromatic compounds were between 0.1 and 0.2 µg/L.
Ethanol, IPA, TAA, and TBA had MDLs of 18, 6, 1, and 0.8 µg/L, respectively. Comparison of the MDLs with values determined in five other published methods reveal that the heated static headspace, direct aqueous injection, and purgeand-trap methods had similar sensitivities (see Table 1). A SPME method (7) had lower MDLs. The MDLs for TBA using EPA Method 8260B at ambient temperature was 35 µg/L (15). Lower method detection limits were obtained when larger purge volumes and higher purge temperatures were used (15, 16). For ASTM D4815, the method detection limit for TBA was 27 µg/L (15). These limits are higher than the drinking water action level for TBA in the State of California of 12 µg/L (2). The calibration curves were strongly linear. The correlation coefficient (R 2) was 0.996 or greater for each of the analytes. The lowest calibration standard for the ethers and aromatic compounds was 0.5 µg/L. For the other alcohols and acetone, the lowest calibration standards ranged from 2.5 to 50 µg/L. The relative standard deviation at the lowest calibration standards varied from 8.2% to 14% for all the analytes. VOL. 37, NO. 21, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Stability of four ethers in samples preserved with HCl and TSP. (TSP) indicates that the samples were preserved with TSP. (HCl + TSP) means that the samples were preserved with three drops concentrated HCl and neutralized with 0.4 g of TSP. The neutralization was performed just prior to GC/MS analysis. (HCl) means that the samples were preserved with HCl and analyzed without neutralization. 0-day means that the samples were analyzed after preparation. 28-days means that the samples were stored in the dark at 4 °C for 28 d before GC/MS analysis. Each sample experienced headspace heating at 80 °C for 30 min. The initial concentrations were 500 µg/L. Concentrations are mean values (n ) 3). The error bars are the standard deviation of the means.
FIGURE 3. Increasing gasoline content in water showed no significant matrix effect on determination of 20 µg/L ethers. PUG, premium unleaded gasoline. Concentrations are mean values (n ) 3). The error bars are the standard deviation of the means. Recovery of the Fuel Oxygenate Compounds from Water Containing Gasoline. Previous studies showed that high concentrations of gasoline in the sample matrix could affect quantitative determination of ethers and TBA (15, 32). The static headspace GC/MS method was evaluated to determine the effect of the gasoline matrix on ether recovery. Water was spiked with MTBE, ETBE, DIPE, and TAME at a concentration of 20 µg/L and with concentrations of gasoline ranging from 0 to 100 000 µg/L. There was little effect of PUG (Figure 3) and RUG (33) on the recovery of the ether oxygenates. The recovery of MTBE, ETBE, DIPE, and TAME was determined in triplicate at each concentration of gasoline. The average recovery of the ether oxygenates from premium gasoline was 98.2% of the spiked concentration. The range 4998
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of recoveries extended from 73% to 118% of the spiked concentrations. The average sample standard deviation was 3.4% of the spiked concentration. The highest sample standard deviation was 20.3%. The recovery from regular gasoline was very similar. Here, the average recovery was 101.3% of the spiked concentration. The range of recoveries extended from 72% to 115% of the spiked concentrations. The average sample standard deviation was 2.8% of the spiked concentration. The highest sample standard deviation was 20.3% of the spiked concentration. The matrix effects due to gasoline content were minimal. These results are comparable with those presented by Halden et al. (15), who evaluated EPA Method 8260B, a purge-andtrap GC/MS method for the analysis of MTBE, other ethers, and TBA in the presence of gasoline matrixes. Their evaluation
TABLE 2. Performance of a Heated Static Headspace Sampler with Analysis by GC/MS for Determination of Aromatic Hydrocarbons and Oxygenate Compounds analytes
calib. std. range (µg/L)
ethanol acetone IPA TBA MTBE DIPE ETBE benzene TAA TAME toluene ethylbenzene m+p-xylene o-xylene 1,3,5-TMB 1,2,4-TMB 1,2,3-TMB
50-10000 10-1000 15-1000 2.5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000 5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000 0.5-1000
retention corr time quantitation coeff MDLa RSDb (min) mass (R 2) (µg/L) (%) 3:03 3:27 3:42 4:12 4:22 5:04 5:34 6:59 7:02 7:11 9:59 12:35 12:48 13:30 15:18 16:00 16:48
45 43 45 59 73 45+46 59 78 59 73 91 91 91 91 105 105 105
0.999 18 0.999 3.3 0.999 5.5 0.999 0.79 0.999 0.21 0.996c 0.14 0.999 0.21 0.999 0.12 0.999 1.2 0.999 0.17 0.999 0.17 0.999 0.22 0.999 0.18 0.999 0.22 0.998 0.16 0.999 0.20 0.999 0.17
10.4 11.4 12.0 10.2 13.3 8.6 13.0 8.2 8.3 10.8 11.6 13.8 12.2 14.0 10.6 12.8 11.1
a For acetone and tert-amyl alcohol, the method detection limit ) 2.896 × SD of the lowest concentration standard. For all other compounds, the method detection limit ) 3.14 × SD of the lowest concentration standard. b Relative standard deviation of the lowest calibration standard (values in the second column). Calculated as the sample standard deviation divided by the sample mean of nine replicate analyses for acetone and tert-amyl methyl ether and seven replicates for the other analytes. c When only m/z 45 used as quantitation ion mass, the R 2 is 0.981.
showed that Method 8260B produced excellent results in high TPH contents ranging from 0 to 50 000 µg/L. However, their evaluation did not include ethanol, IPA, and TAA. The recoveries of ethanol, IPA, TAA, and TBA in the presence of 1000 µg/L of gasoline were determined. The spiked concentration of ethanol, IPA, and TAA was 100, 20, and 10 µg/L respectively; TBA was spiked at 2 and 10 µg/L. The recovery of 100 µg/L of ethanol was 109% (SD 2.0%) and 108% (SD 5.2%) for RUG and PUG, respectively (34). The recovery of all the alcohols in both gasolines varied from 85% to 124% with relative standard deviations varying from 2 to 17%. Caveat about Trisodium Phosphate. Although TSP can be used to preserve samples containing ethers and aromatic compounds, base hydrolysis of some halogenated volatile compounds including 1,1,2,2-tetrachloroethane and 1,1,2trichloroethane should be expected (35). Significant losses of bromomethane have occurred when TSP was used as a preservative during sample storage at 20 °C for 27 d (24). Surrogate standards other than dibromofluoromethane should be used if TSP is used as a preservative.
Acknowledgments The authors are grateful to Dr. Greg Jungclaus for his careful review of our manuscript. All of the experimental work and initial manuscript preparation was done through EPA Contract 68-C-98-138 with ManTech Environmental Research Services, Inc. This work was funded wholly by the U.S. Environmental Protection Agency through EPA Contracts 68C-98-138 and 68-C-03-097 (Project Officers Georgia Sampson and Stephen Kovash, U.S. EPA National Risk Management Research Laboratory, Ground Water and Ecosystems Restoration Division, Ada, OK); however, it has not been subjected to Agency review and does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
Supporting Information Available Operational parameters for static headspace analysis and additional tables and figures referenced in the text. This
material is available free of charge via the Internet at http:// pubs.acs.org/journals/esthag/index.html.
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(33) Supporting Information, Figure 2. Determination of 20 µg/L ethers with increasing regular unleaded gasoline content. (34) Supporting Information, Table 1. Recoveries of alcohols in the presence of 1000 µg/L gasoline. (35) Jeffers, P.; Ward, L.; Woytowitch, L.; Wolfe, N. Environ. Sci. Technol. 1989, 23, 965-969.
Received for review February 24, 2003. Revised manuscript received August 22, 2003. Accepted August 22, 2003. ES030375V
