Environ. Sci. Technol. 1902, 26, 1573-1580
Applequist, J.; Carl, J. R.; Fung, K. K . J . Am. Chem. Soc.
(39) Yalkowsky, S. H.; Valvani, S. C. J.Chem. Eng. Data 1979, 24, 127. (40) Yalkowsky, S. H.; Mishra, D. S. Environ. Sci. Technol. 1990, 24, 927.
1972, 94, 2952.
Weil, L.; Dwe, G.;Quentin, K. E. Wasser Abwasser Forsch. Praz. 1974, 7, 169.
Miller, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Enuiron. Sci. Technol. 1980, 14, 1524. Dickhut, R.M.; Andren, A. W.; Armstrong, D. E. Environ. Sei. Technol. 1986, 2, 807.
Received for review October 23, 1991. Revised manuscript received March 20, 1992. Accepted March 23, 1992.
On-Line Measurement of Benzene and Toluene in Dilute Vehicle Exhaust by Mass Spectrometry Mark A. Dearth,' Christine A. Glerctak, and Walter 0. Slegl
Analytical Sciences Department, Research Staff, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48 121-2053 Tandem mass spectrometry (MS/MS), using either an atmospheric pressure ionization (API) or a low-pressure chemical ionization (LPCI) source, has the potential for measuring concentrations of individual hydrocarbon species in real time. In this study, a triple quadrupole mass spectrometer is used to monitor two environmentally important exhaust hydrocarbons species, benzene and toluene. Real-time data for concentrations ranging from 5 ppb to 30 ppm can be measured, with a time resolution of as little as 100 ms/data point. The target compounds are quantitated in dilute vehicle exhaust monitored over the Urban Dynamometer Driving Schedule (UDDS) of the Federal Test Procedure (FTP). Two ionization methods (API and LPCI) are compared. The analytical results obtained with the two ionization methods are also compared to time-integrated off-line analyses by capillary gas chromatography. Of the real-time API- and LPCI/MS measurements, 94% (36 out of 38) fell within 2 standard deviations of the off-line GC/FID analyses. Real-time plots of tailpipe concentration levels for benzene and toluene over the UDDS show much higher concentrations for toluene during the first 90 s of the cold-start test (bag 1) and the hot-start test (bag 3), before the catalyst has reached full-performance temperature (light-off). However, higher concentrations of benzene relative to toluene are consistently observed after catalyst light-off. Considering the high ratio of toluene to benzene in the fuel (7:1), this observation is consistent with recent suggestions that benzene is produced in the catalytic converter by hydrodealkylation of alkylbenzenes. After catalyst lighboff, most of the benzene and toluene occurs in spikes corresponding with transitions (accelerations and deceleration) in the vehicle operation.
Introduction Pressure continues on designers and manufacturers of vehicle power train systems to reduce overall hydrocarbon emission levels and to minimize selected species considered to be air bxics. To accomplish this reduction, an improved understanding of the sources and detailed chemical nature of exhaust emissions is essential. New analytical methods are needed that can provide data on transient levels of chemical species at the low concentrations expected in future vehicle exhaust. Applying these new methods to measuring individual chemical species in auto exhaust will provide a powerful tool for studying the effect of power train and catalyst components and operating conditions on emissions. Instrumentation currently employed in vehicle emission testing can provide limited data. For the regulated 0013-936X/92/0926-1573$03.00/0
emissions carbon monoxide, oxides of nitrogen (NO,), and total hydrocarbons (THC), on-line measurements are available. However, these techniques do not provide information on individual hydrocarbon species ( I ) . Fourier transform infrared (FTIR) spectroscopy can provide online data for several hydrocarbons of C4 or less, but this method is limited by spectral overlap of similar species and sensitivity (about 1 ppm). Off-line methods, such as gas chromatography with a flame ionization detector (GC/ FID) or with a mass spectrometer detector (GC/MS), are currently the only way to obtain speciated data for vehicle exhaust hydrocarbons (2). GC methods, because of the length of time required per analysis, are limited to the determination of time-integrated emission levels. Analysis times for GC methods are usually 1-2 h and often require multiple analyses on different stationary phases to achieve complete speciation. Tandem mass spectrometry (MS/MS) with a triple quadrupole mass spectrometer using an atmospheric pressure ionization (API) source has the potential for measuring concentrations of individual hydrocarbon species in real time (3,4). However, vehicle exhaust, with high concentrations of water and other contaminants (NO,, SOz, and others), presents a significant challenge to the analysis of trace-level hydrocarbons. In this report, improvements to a Sciex API-I11 mass spectrometer are assessed by quantitating benzene and toluene in Tedlar bag samples with GC/FID, and then comparing these results to API/MS analyses of the same Tedlar bag samples (off-line). After validation of the API/MS method and improvements by off-line comparisons to GC/FID, benzene and toluene are measured continuously, by on-line sampling of dilute vehicle exhaust during vehicle testing. On-line data for concentrations ranging from 5 ppb to 8 ppm are reported for the target compounds, for emissions from vehicles driven over the Urban Dynamometer Driving Schedule (UDDS) of the Federal Test Procedure (FTP) (5). In addition, two ionization methods are compared; atmospheric pressure ionization (API) using a corona discharge and low-pressure chemical ionization (LPCI) using a glow discharge. Benzene and toluene were chosen as the target compounds because of regulatory interests and their importance as fuel and exhaust gas components (6). Furthermore, a comparison of the benzene to toluene ratio in the fuel with that in the exhaust at various points in the test cycle provides insight into the sources of these two exhaust species. Reports of small amounts of benzene formed during incomplete combustion in a spark-ignited engine, even with nonaromatic fuels (7,8), suggest the enrichment
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of benzene (relative to other fuel-derived hydrocarbons) in the exhaust stream prior to the catalyst. It is also known that significant amounts of exhaust benzene result from the incomplete combustion of toluene and other alkylbenzenes, as observed in the recent study by Kaiser et al. (B), using a single-cylinder engine fueled with pure toluene and also with a five-component fuel containing toluene. They reported significant levels of benzene in the engine-out exhaust, especially under high-speed conditions, but the benzene to toluene ratio remained below 1:4. A similar result was reported by Pelz et al. (9),using a multicylinder engine fueled with a full-blend gasoline (benzene to toluene content, about 1:3). These authors also found that the benzene to toluene ratio in the engine-out emissions, under various operating conditions, was higher than that in the fuel. However, even under high-speed, high-load conditions the benzene to toluene ratio of the engine-out exhaust remained less than unity. The published informationjust cited suggests to us that the tailpipe ratios of benzene to toluene of greater than unity, if observed after the catalyst, would suggest the formation of benzene (relative to toluene) over the catalytic converter. Real-time measurements of the benzene, toluene, and their ratio could answer the question of whether these emissions occur in discrete events that correspond to fuel-rich transitions in vehicle operation or are they a constant proportion of the exhaust. Emissions occurring in discrete events would suggest that undesirable chemistry is occurring over the catalyst. Two recent reports supporting the latter conclusion describe measurement of benzene emission levels, before and after the catalyst, under steady-state operating conditions. Under rich conditions (air to fuel ratio 13.5:14.0) with a Pt/Rh catalyst at temperatures above 260 "C, Summers and Silver (10) observed higher levels of benzene exiting the catalytic converter than entering it. In an engine dynamometer study, Pelz et al. (9) also found that benzene levels increased over the catalyst under highspeed, high-load conditions. Under these conditions, toluene and other alkylbenzenes in the exhaust appear to undergo hydrodealkylation over the oxidation catalyst to form benzene (9). In contrast, catalyst efficiencies are reported to be benzene > toluene > branched alkanes > normal alkanes, from most to least efficient, when the catalyst feed gas is at stoichiometry or lean of stoichiornetry (9). Thus, the work detailed within was done to develop the capability of making real-time measurements of individual chemical species for a dynamically changing sample stream and to answer the question of when benzene and toluene are emitted during the UDDS (FTP) test cycles. Experimental Section
Vehicle Testing. The vehicles used were 1991 model Ford Escorts or Ford Explorers equipped with factoryinstalled, standard vehicle emissions control technology. The vehicles were operated on premium unleaded fuel (Ford M4C-322L). The data were collected while the vehicle was operated according to the Urban Dynamometer Driving Schedule, which is part of the Federal Test Procedure, consisting of a 505-s cold start (bag l ) , an 867-s hot transient (bag 2), and a 505-9 hot start (bag 3). For further details on the driving cycles, see ref 5. The exhaust from the vehicles was transported through a heated stainless steel hose to a dilution tube ( I ) . The volumetric flow through the dilution tube was maintained at about 400 standard ft3/min, by supplementing the vehicle exhaust with dehumidified and heated room air, which re-
sulted in a dilution of the exhaust by a factor of about 101. For off-line analysis, diluted exhaust gas was collected in 40-L Tedlar bags over the duration of each of the three portions of the test. A background bag sample of dilution air was also collected from the dilution tube, just before vehicle testing at the same sampling site. Off-Line GC/FID Analyses. Quantitative analyses were performed on the emission bag samples using a Hewlett-Packard Model 5890 gas chromatograph equipped with a split/splitless injector and a flame ionization detector. The samples were introduced by using an automated gas sampling valve with either a 2- or a 5mL sample loop. The column was a 60-m X 0.32-mm-i.d. capillary column with a 1-pm film thickness (DB-1, J&W Scientific Co., Folsom, CA). The oven was temperature programmed as follows: after 2 min at -30 "C, the oven was heated at 1.4 "C/min to 30 "C, and then heated at 6 "C/min to 108 "C, followed by a 20 "C/min ramp to 210 "C. The temperature program used for the analysis provided for the separation of benzene from 1-methylcyclopentene,a common coeluter during GC analysis. Peak areas obtained from an HP 3393A integrator were used for quantitation; no correction was made for known minor differences in FID response factors. The GC response was calibrated daily with a propane standard. Fuel Analysis. A sample of the fuel was speciated by GC/FID. The major aromatic components of the fuel and their relative contribution are as follows (in percentage of total carbon): benzene (l.l%), toluene (7.8%),ethylbenzene (2.2%), m- and p-xylene (6.9%), and o-xylene (3.0%). The aromatic content by fluorescent indicator adsorption analysis is 30.9%. On-Line Mass Spectral Analyses. All mass spectral analyses used an API-I11 triple quadrupole mass spectrometer manufactured by Sciex, Thornhill, ON, Canada. Two different ion sources were obtained from Sciex. One is a low-pressure chemical ionization source that utilizes a 5-1500-pA glow discharge to generate primary reagent ions. These primary ions are formed from the matrix gas of the sample and are typically N2+,OZ+,and NO+. These primary ions ionize the analyte species mainly by charge exchange or attachment of the primary gas (M + NO', for instance). The other ion source is an atmospheric pressure ionization source, which was modified to reduce sample volume, to increase instrument response to analyte concentration changes, and to improve overall sensitivity to benzene and toluene (these modifications are described below). The API source generates a corona discharge in the matrix gas, which then is ionized and is capable of secondary ionization reactions of analyte molecules. Under API conditions in normal room air, the primary reagent ions are protonated water clusters. The molecular ions of benzene ( m / z 78) and toluene ( m / z 92) were monitored in the selected-ion-monitoring mode, with a dwell of 100 ms each. Since single-stage mass spectrometry was performed for quantitative purposes, two other ions, mlz 108 and 122, the NO adducts of the benzene and toluene molecular ions, were monitored to check for potential interferences. Including software overhead, total cycle time was about 500 ms. For quantitation, the instrument response was calibrated with certified 1ppm gas standards (Airco) of benzene and toluene. During both calibration and analysis, samples and standards were diluted approximately 20:1, with Nz (boil-off from a liquid nitrogen dewar) to a standard volume flow of 5 L/min. Dilution of the exhaust sample reduced its concentration to within the calibration range, on average. A calibration curve was constructed based on
22 mm 0.0. Glass Tube Room Air Dilution Gas
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Figure 1. Ion plenum region of the atmospheric pressure ionization (API) source, wlth plexiglass sleeve shown in proper location. Note: This figure is not drawn to scale.
10,20,50, and 100 ppb benzene and toluene; the average slope (ion counts per s/ppb, icps/ppb) of the linear regression line was used to calculate concentration for the subsequent on-line analyses. The response factors were typically 2000-3000 icps/ppb and were determined three times a test: before a vehicle test, between bags 2 and 3 of the UDDS test procedure, and after a vehicle test. The response factors were, then averaged for each test, and the average was used for calculating concentration. The standard deviation (SD) of the response factors was about f10% on a daily basis. Limits of detection can be estimated from the average response of the instrument to benzene and toluene (2000-3000 icps/ppb) and by measuring the background level and standard deviation of the noise. An average background was 1100 icps with a standard deviation of 300 icps. Based on a limit of detection of 3 times the standard deviation of the noise, the minimum detectable concentration would be about 0.5 ppb (above background levels), for a single data point measurement. API Source Modifications. The ion plenum region of the API source has a volume of about 5 L and is pumped by a rotary vane pump to about 50 mmHg below ambient pressure (see Figure 1). To reduce the volume of the ionization region, a plexiglass sleeve 22-mm i.d. by 10 cm in length was fabricated to fit snugly over the 22-mm (0.d.) glass inlet tube supplied by Sciex. The sleeve is slotted to allow entry of the corona discharge needle, and the end is tapered to mate snugly with the interface plate. In operation, the plexiglass sleeve is pressed snugly up to the interface plate, leaving no gap for gases to exit around the end of the sleeve. The needle slot, which is approximately 6 mm wide and 67 mm long, is the major exit point for inlet gases which are then pumped efficiently away by the plenum pumping system. The ion volume within this sleeve, starting at the tip of the discharge needle and extending to the orifice, is about 3.8 mL with the discharge needle 1 cm from the interface. With a sample flow rate of 5 L/min, the sample residence time in the ionization region is about 2 ms (three volumes swept).
The ion plenum region was wrapped with heating tape and insulated with glass fiber cloth. A thermocouple and a simple feedback circuit were used to maintain the plenum region at 100-110 "C. The interface plate temperature was maintained at 60 "C, and the glass inlet line was not heated. Sampling Manifolds. A sampling manifold for the LPCI ion source is detailed in Figure 2A (the left section of Figure 2 is common to both ion source sampling manifolds). A stainless steel, gas-tight manifold was constructed of 12-mm tubing and Swagelock fittings. A needle valve is used to control the flow from this manifold into the ion source. The pressures in the manifold and ion source are routinely 30 and 1.1mmHg, respectively. The low pressure in the sample manifold creates a large pressure drop (about 500 mmHg) from the dilution tube sample line. This large pressure drop eliminates the need to pump the sample from the dilution tube sample line into the mass spectrometer manifold, so a mass flow controller (0-1 L/min) is all that is used to control the flow of sample. Samples in Tedlar bags are coupled directly to this mass flow controller. For dilution tube sampling, the sample line from the dilution tube is connected to a tee at the inlet of the sample mass flow controller, and a high-volume pump is connected downstream of the tee to pull the sample continuously past this tee. Excess sample is vented to atmosphere. In this way, the sampling line from the dilution t u y is flushed with the fresh sample more than once a second. A sampling manifold for the API source (see Figure 2B) was also devised to transfer diluted auto exhaust from either Tedlar bags or the constant volume dilution tube into the mass spectrometer. A high-volume sampling pump (not shown in Figure 2B) is used to draw the sample from the dilution tube to a point downstream of the mass spectrometer inlet. Because there is only a small pressure drop from the dilution tube sample line to the ion plenum chamber, a small bellows pump (see detail 2 in Figure 2B) is needed to supply flow from the sample line or the Tedlar bag, into the manifold. The remainder of the sample Environ. Sci. Technol., Vol. 26, No. 8, 1992
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Figure 2. LFCI and API sampling manifolds schematic. Atternative A depicts the sampling manifold details that are unique for the LPCI source, while alternative B depicts the sampling manifold sections unique to the API source.
drawn from the dilution tube is vented to atmosphere, as in Figure 2A. The delay time of the sample, from point of collection at the tailpipe to detection by the mass spectrometer, is about 2 s.
Results and Discussion The results presented below represent a first effort to adapt API- and LPCI-MS to fast on-line measurements of individual chemical species in auto exhaust. This work was motivated by a need for a real-time diagnostic capability, and therefore, accuracy and precision were important but not critical factors. In this regard, we do not advocate these methods as a replacement for more established methods of quantitation of vehicle emissions, but rather as a complementary tool to aid the designer, engineer, and scientist. There were two significant problems to overcome before benzene and toluene could be measured in auto exhaust. The first was interference of water with the ionization of benzene and toluene. In the presence of ambient air levels of water, neither species could be detected, even at ppm levels. The solution to this problem, described below, required the use of pure dry gases for dilution of the sample. This created the second problem: the unmodified API-I11 requires a sample flow of 15-30 L/s through the ion plenum chamber to keep sample residence time below 1s. This sample flow is too large for sampling out of 40-L Tedlar bags and would have made dilution with pure gases prohibitively expensive. It was decided that a total sample flow of 5 L/min was more reasonable, and the ion source was modified to accommodate this flow. A millisecondrange response time is just one benefit of the modifications described below. The solution to the second problem, sample volume, is detailed first. API Source Modifications. The volume of the ion plenum chamber was reduced by fitting a plexiglass sleeve to the end of the glass inlet tube (see Figure 1). To test 1576
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the instrument response time, a sample stream was dynamically varied by the opening and closing of a mass flow controller (see Figure 1for experimental setup). Pyridine was spiked at about 10 ppb into a 40-L Tedlar bag of zero air (grade 1.0, Airco, Murray Hill, NJ). The spiked sample was pumped from the bag through the mass flow controller (100 mL/min full scale), which was cycled between 5 and 95 % open with an externally generated squarewave signal. The sample stream was diluted with 4 L/min of room air. Pyridine wm chosen as the analyte because of the extreme sensitivity of the API/MS for the protonated molecular ion. The protonated molecular ion of pyridine ( m / z 80) was monitored in the selected-ion-monitoring mode, with a scan time of 1ms. With software overhead, the instrument cycle time was less than 10 ms. Figure 3A represents the typical response obtained without the plexiglass sleeve. In Figure 3A, at 5 L/min flow rate, a change in analyte concentration takes several seconds to reach equilibrium, due to mixing in the ion source plenum. In Figure 3B, the same experiment is conducted with the plexiglass sleeve installed. Instrument response is now fast enough to determine the rise and settling time of the mass flow controller (270 ms). An additional modification was made to the sample inlet (other than the plexiglaw sleeve): a 0.32-cm 0.d. by 0.05cm i.d. Teflon sample tube which served to transport the sample from the mass flow controller to within 2 cm of the end of the glass inlet tube. Events as brief as 100 ms could be measured (with X10 oversampling) with this setup. Enhancement of Charge Exchange Ionization. In an effort to determine the API/MS sensitivity to benzene and toluene, the ion plenum region was baked out to reduce residual water adsorbed onto the inner surfaces. It was our intent to measure sensitivity under ultradry conditions and then compare the results to the same measurements in normally humidified air. We have found, by diluting the sample stream by at least 15:l (and usually 100:1),that instrument sensitivity to benzene and toluene
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is similar, whether in ultradry nitrogen or in a sample saturated with water, as long as the ion plenum region is previously well-baked and held at 100 "C. Thus, it became obvious that these simple procedures must allow charge exchange ionization of benzene and toluene to occur as long as the presence of water is minimized in the ion source. Off-Line Comparison of API- a n d LPCI/MS to GC/FID. Prior to attempting on-line analysis of benzene and toluene, mass spectral quantitation was tested by measuring dilute exhaust collected in Tedlar bags. Tedlar bag samples of diluted vehicle exhaust are routinely speciated by GC/FID in our laboratory; these samples were examined. Both API- and LPCI/MS compared favorably to GC/FID analysis of benzene and toluene. The bar plot in Figure 4 summarizes our findings. The sample concentration determined by MS was divided by the corresponding concentration determined by GC/FID. If the two measurements were in agreement, the ratio would equal 1 (MS/GC = 1). The standard deviation of the average ratio is also shown, just above the average ratio bar. For the off-line bars, seven test results were averaged. The relative standard deviation of the off-line MS to GC ratios was between 17 and 30%. The error of the GC/FID method for values above 1.0 ppm C was estimated to be 7% of the reported value and below 1.0 ppm C was assumed to be 0.1 ppm C; a reasonable assumption based on other published work from this laboratory (11) (note: the GC/FID limit of quantitation, in this work, is about 0.1 ppm C). The error ranges for MI- and LPCI/MS were estimated by performing nine replicate measurements of a Tedlar bag sample and were found to be about 8% for both techniques. Off-line analysis by API/MS produced 68% of the data values within 1SD and 94% of the data values within 2 SD of the GC/FID. This is more accurate than LPCI/MS, with
SDAPIMS
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4. MS 10 GC comparison for both APIIMS and LPCIIMS measuemmt of benzene and toluene in auto exhaust are represented by Um bars, with perfect agreement equal to 1. The groups of bars labeled '"offlinebz" and "offline tol" represent the ratio obtained by dvidng the mass s w a l concankakm by the OClFlD c o n m a t k n . and ea& bar repesents the average of seven indMdual comparisons. Since Um on-line mass spectral measurements contain Musands of hdlvkbal OaXxMtradMl values. the bars labeled "online bz" and " o n h tol" represent Um integrated average concentration during a vehicle test divided by its corresponding bag sample analyzed by GCIFID. Each on-line bar represents ths average of 11 IndMdual comparisons. The standard deviation of each average is labeled "SD A P I I M S and "SD LPCIIMS and is represented by the cross-hatched bars above the cwrespondlng average bar. FbWO
53% of the data within 1SD and 73% of the data within
2 SD of the correspondingGC/FID data A matched pairs t-test of the difference between the GC/FID value and the corresponding API- or LPCI-MS value was not significantly different a t the 99.9% confidence level. Because it was not our intent to develop a new off-line speciation method, these results were sufficient to show that quantitation was feasible by both API- and LPCI-MS. Therefore, on-lime measurements were conduded without further optimization of the off-line methods. On-Line Benzene a n d Toluene Emission Profiles for UDDS Driving Cycles. Figures 5-7 contain plots of the timevariant concentrations of benzene and toluene in diluted vehicle exhaust and the speed of the vehicle during the three phases of a UDDS test. Bag 1,the first 505 s ('the cold-start bag") of the UDDS test, is shown in Figure 5; bag 2, the next 870 s ("the hot-transient bag") of the UDDS test, is shown in Figure 6; and bag 3, the fmal 505 s (%e hotstart bag"), is shown in Figure 7. The upper and middle traces represent benzene and toluene concentration and are scaled in parts per million carbon (ppm C) (which is obtained by multiplying the actual concentration of a species by the total number of carbons in a molecule of that compound; thus 1 ppm of benzene is equivalent to 6 ppm C benzene). These data points were summed, and the average was taken to compare to off-line GC/FID analysis of samples collected simultaneously in Envlron. Sci. Technol.. Voi. 26. No. 8. 1992
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Flgure 5. Real-time UDDS (FTP) bag 1 traces of the dilution tube concentration of benzene (A) and toluene (B) in ppm C (y-axis) vs tlme (x-axis). Trace C is the speed of the vehicle in mph (y-axis) vs time.
Flgure 6. Real-time UDDS (FTP) bag 2 traces of the dilution tube concentration of benzene (A) and toluene (B) in ppm C (y-axis) vs tlme (x-axis). Trace C is the speed of the vehicle in mph (y-axis) vs time.
Tedlar bags (see above). All on-line data was background subtracted. The lower trace (C) is the speed of the vehicle in miles per hour. From the bag 1on-line traces shown in Figure 5A,B, we see that most of the benzene and toluene are emitted during the first 90 s of the test cycle. I t is also apparent that the emission of these two species, after the first 90 s, occurs during transitions in the vehicle operation (i.e., during accelerations and decelerations). The transitory nature of the benzene and toluene emissions is even more evident from the traces in Figure 6, obtained for bag 2. For the first time, it is also apparent that the benzene emissions exceed the toluene emissions after the vehicle reaches hot operating conditions, at least with the fuel used in these tests. Finally, the bag 3 traces (Figure 7) exhibit a large pulse of toluene and benzene at the start of the vehicle test, but because the car is still quite warm from the previous bag 2 test, it takes much less time for the catalyst to reach normal operating temperatures. Once at full efficiency, increases in the benzene and toluene emissions again correlate with transitions in the vehicle operation. On-line Comparison of API/MS and LPCI/MS to GC/FID. On-line quantitation agreed quite closely with the GC/FID off-line analyses. The data are summarized
in the bar plot in Figure 4. The average difference in concentrations measured by on-line API/MS and off-line GC/FID was 13% for benzene and 21% for toluene; 75% of all the on-line API/MS values fell within 1 SD of the GC/FID values, and 94% fell within 2 SD (the MS error was estimated as described in the off-line measurements above). All but one API/MS to GC/FID comparison agreed within 2 SD of the GC/FID value. The average differences between the LPCI/MS on-line and GC/FID off-line values were 15% and 21% for benzene and toluene, respectively, with 82% of the data falling within 1SD and 95% within 2 SD of the GC/FID values. This is, again, excellent agreement. In fact, both the API/MS and LPCI/MS methods produced only 2 measurements out of 38 that were more than 2 SD from the respective GC/FID measurement. As with the off-line measurements, a matched pairs t-test was performed on the difference between the GC/FID value and the corresponding API- or LPCI/MS value, and the MS values were not significantly different at the 99.9% confidence level. Based on the comparisons made to off-line GC/FID analyses, we conclude that the LPCI/MS and the API/MS on-line methods are of comparable accuracy. More significant, perhaps, is that the LPCI method is more robust and is less sensitive to changes in the sample matrix (data
1578 Envlron. Sci. Technol., Vol. 26, No. 8, 1992
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Flgure 7. Real-time UDDS (FTP) bag 3 traces of the dilution tube concenlration of benzene (A) and toluene (B) in ppm C (yaxls) vs time (x-axis). Trace C is the speed of the vehicle in mph (y-axis) vs time.
not shown). Both we and other authors (12, 13) have observed significant matrix effects when trying to measure trace-level species by API/MS. We have found a 75% loss of sensitivity in API/MS mode, when the dilution ratio is decreased from 1OO:l to 151of dry diluent gas to sample. We have observed that the LPCI/MS sensitivity decreased by only 15% for this same change in dry diluent gas to sample ratio. Other differences include ease of operation. The LPCI source requires some daily adjustments to the source pressure and discharge current. The API source, however, requires careful adjustment of the corona discharge needle as well as optimization of the curtain gas flow, the position of the glass inlet, and various source and lens voltages. In all, the LPCI source is easier to operate and less susceptible to sampling vagaries. Benzene to Toluene Ratio. Examination of the benzene to toluene ratio can provide some insight into the chemistry occurring during the combustion process and over the catalytic converter. The ratio of benzene to toluene is plotted for each bag analysis in Figure 8. The ratio of benzene to toluene is 1:7 in the fuel. During the initial 90 s of bag 1, the benzene to toluene ratio is similar to that in the fuel, or roughly 1:4 (ignoring the anomalously high value right at the start of the test, which is due to dividing two very small but disparate numbers), but the
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Time [seconds]
Flgure 8. Ratio of benzene to toluene concentrations (ppblppb) vs time. Vehicle road speed is the upper trace In each plot and is scaled on the right y-axis. The plots A and B are the ratios for bags 1 and 3 and share the same time (x) axis, scaled 0-505 s. Plot C contains the bag 2 traces and is scaled 0-870 s on the x-axis. The y-axes are scaled in bz/toi ratio values.
ratio increases after the catalyst reaches operating temperature or light-off (temperature at which the catalyst achieves 50% efficiency). This observation is consistent with our understanding that unburned fuel is the major contributor to the cold-start emissions. After catalyst light-off, the benzene to toluene ratio actually rises above unity. This transition point is easily observed in Figure 8A, where the trace rises above 1 (marked with an arrow). The greater abundance of benzene relative to toluene continues into bag 2 (Figure 8C). In bag 3 (Figure 8B) the benzene to toluene ratio is again similar to that in the fuel for about 40 s into the vehicle test. Because the catalyst is still warm, lighboff is achieved in a shorter period of time than for bag 1. The benzene to toluene ratio is seen to continue to rise throughout the UDDS (FTP) test cycle and reaches its highest values (50) in bag 3, after the engine and exhaust system reach their maximum temperatures. The rise in the benzene to toluene ratio suggests that toluene is consumed relative to benzene and/or that benzene is generated relative to toluene in the engine or over the catalytic converter. Evidence has been reported Environ. Sci. Technol., Vol. 26, No. 8, 1992
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by others to support each conclusion (see Introduction).
Acknowledgments
Conclusions After modification, the API source and mass spectrometer can be used to accurately measure benzene and toluene in dilute vehicle exhaust, at acquisition rates up to 100 Hz. These measurements require as little as 250 mL/min of diluted exhaust and are unaffected by significant amounts of water in the undiluted sample. The potential exists to extend these methods to other trace-level organic species that are ionized most efficiently by charge exchange. We have also investigated the LPCI source and found it to be less sensitive to matrix effects when compared to the API source. On-line emissions data were collected for benzene and toluene over the UDDS (FTP) test cycle. To our knowledge, this is the first published, on-line measurement of individual hydrocarbons above C4 in vehicle exhaust at these time scales. From the on-line data collected it is obvious that, after the catalyst becomes functional, benzene and toluene emissions correlate with transitions in the steady-state operation of the engine. A closer look at the relationship between these excursions in benzene and toluene emissions and engine and catalyst operating conditions is in progress. The potential to identify the specific modes of operation that produce unwanted benzene and toluene emissions has been demonstrated by the data presented here. Such information may be useful in reducing emissions to meet future State and Federal emissions regulations. The benzene/toluene ratios observed in this study are consistent with the formation of benzene over the catalyst during fuel-rich excursions of the feed gas which occur during accelerations and decelerations in the driving cycle. These data are consistent with other recent reports that benzene is produced over the hot catalyst under fuel-rich conditions. Experiments are underway to measure these species both pre- and postcatalyst. The major contributor of benzene emissions remains the cold-start portion of the test and is associated with unburned fuel.
T. J. Korniski, J. E. Weir, and A. Forsgren, at Ford Motor
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We gratefully acknowledge the assistance of E. Chladek, Co., and Sciex for the use of equipment and timely advice. Registry No. CsHs, 71-43-2;PhMe, 108-88-3.
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(10) Summers, J.; Silver, R. SAE Tech. Pup. Ser. 1990, No. 902072. (11) Schuetzle, D.; Jensen, T.; Nagy, D.; Prostack, A.; Hochhauser, A. Anal. Chem. 1991,63, 1149A-1159A. (12) Penn, S.; Ketkar, S. Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Topics, Tucson,
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Received for review November 26, 1991. Revised manuscript received April 16, 1992. Accepted April 20, 1992.
