Anal. Chem. 2000, 72, 5847-5851
Flow Analysis Method for Determining the Concentration of Methanol and Ethanol in the Gas Phase Using the Nitrite Formation Reaction Ha Thi-Hoang Nguyen, Norimichi Takenaka,* Hiroshi Bandow, and Yasuaki Maeda
College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
This paper presents a flow determination method for low molecular weight alcohols (methanol, ethanol) in the gas phase using the nitrite formation reaction, which was developed from an earlier method using a glass bottle. In this method, the ambient air and nitrogen dioxide (1000 ppmv) were allowed to continuously flow in a glass tube, which had been filled with 10 g of Pyrex glass beads. The flow rates of the ambient air and nitrogen dioxide were 30 and 20 cm3/min, respectively. The gas-phase alkyl nitrites produced by the dark reaction of atmospheric alcohols and nitrogen dioxide on the Pyrex glass beads were then analyzed by gas chromatography with an electron capture detector. The alcohol concentrations of the samples were calculated using a calibrated conversion factor for each alcohol to its nitrite. The detection limits for the methanol and ethanol are 0.7 and 0.5 ppbv, respectively. This flow method was used to determine the atmospheric alcohol concentrations and was found to have the advantages of a short sampling time and simple quantitative procedure compared with the previously reported method (glass bottle method). The feasibility of this method was also established. The emission sources of alcohols are significant from plants.1-5 Recently, the use of alcohol-fueled vehicles has increased. More than 3 000 000 light-duty cars run on ethanol-blended fuel in Brazil.6,7 Due to the emission from these alcohol-fueled vehicles, the atmospheric alcohol concentrations are expected to be higher than that in the past from plant emission sources. Determination of the concentration of atmospheric alcohol is important. The detection of low molecular weight ambient alcohol at trace levels is still a difficult problem. Methods such as the conductometric method,8 electrochemical fuel-cell method,9 and infrared absorp(1) Kirstine, W.; Galbally, I.; Ye, Y.; Hooper, M. J. Geophys. Res. 1998, 103, 10605-10619. (2) MacDonald, R. C.; Fall, R. Atmos. Environ. 1993, 27A, 1709-1713. (3) Lamanna, M. S.; Goldstein, A. H. J. Geophys. Res. 1999, 104, 21247-21262. (4) Singh, H.; et al. J. Geophys. Res. 2000, 105, 3795-3805. (5) Warneke, C.; et al. Global Biogeochem. Cycles 1999, 13, 9-17. (6) Grosjean, E.; Grosjean, D.; Gunawardena, R.; Rasmussen, R. A. Environ. Sci. Technol. 1998, 32, 736-742. (7) Chang, T. Y.; Hammerle, R. H.; Japar, S. M.; Salmeen, I. T. Environ. Sci. Technol. 1991, 25, 1190-1197. (8) Maekawa, T.; Tamaki, J.; Miura, N.; Yamazoe, N.; Matsushima, S. Sens. Actuators B 1992, 9, 63. (9) Criddle, W. J.; Jones, T. P.; Neame, M. J. H. Meas. Control. 1984, 17, 10. 10.1021/ac000538n CCC: $19.00 Published on Web 11/02/2000
© 2000 American Chemical Society
tion method10 have detection limits at several ppmv of alcohol concentration, and therefore, it is difficult to apply these methods to the determination of the typical concentrations of alcohols in ambient air at the ppbv level. Another method by Huang et al.,11 which can successfully determine trace level of alcohols, is preferred to the aqueous sample. Another two methods for the quantitative analysis of atmospheric alcohol are the method using a dehydrogenase-based biosensor and the method using gas chromatography analysis through cryogenic trap processes. The former has a detection limit of 50-250 ppm.12 The latter can detect atmospheric alcohol at the ppbv level,2,5,13 and the GC-FID method was used in several studies.14-17 However, polar compounds such as methanol and ethanol have an affinity for surfaces, leading to irreversible adsorption on the column. In their results, methanol and ethanol exhibited badly tailing peaks and it was found to suffer from integration errors.17 While the concentrations of atmospheric alcohols are at the ppbv levels, the directly analyzed detection limit for alcohols is the ppmv level. Therefore, a 1000 time preconcentration is required. Because alcohols easily dissolve in water, the problem due to a loss during preconcentration is not easily solved. For example, a cryotrap is plugged by ice when more than 22 mm3 of liquid water is frozen. This amount of water corresponds to ∼1.2 dm3 of air sampled on a rainy summer day.13 In most cases, a water trap was used prior to the alcohol preconcentration in order to avoid this problem. Goldan et al. reported 15% or less of light alcohols was lost in the water trap.14 In an earlier study, we used Pyrex glass bottles for ambient air sampling and the atmospheric alcohols were allowed to react with nitrogen dioxide on the glass surface of the bottle to convert (10) Jones, A. W.; Beylich, K. M.; Bjorneboe, A.; Ingum, J.; Morland, J. Clin. Chem. 1992, 38, 743-747. (11) Huang, G.; Deng, G.; Qiao, H.; Zhou, X. Anal. Chem. 1999, 71, 42454249. (12) Dennison, M. J.; Hall, J. M.; Turner, A. D. F. Analyst 1996, 121, 17691773. (13) Leibrock, E., Slemr, J. Atmos. Environ. 1997, 31, 3329-1339. (14) Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C. J. Geophys. Res. 1995, 100, 25, 945-25963. (15) Goldan, P. D.; Kuster, W. C.; Parrish, D. D.; Carpenter, J.; Roberts, J. M.; Yee, J. E.; Fehsenfeld, F. C. J. Geophys. Res. 1995, 100, 22771-22783. (16) Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C. J. Geophys. Res. 1997, 102, 6315-6324. (17) Apel, E. C.; Calvert, J. G.; Greenberg, J. P.; Riemer, D.; Zika, R.; Kleindienst, T. E.; Lonnerman, W. A.; Fung, K.; Fujita, E. J. Geophys. Res. 1998, 103, 22281-22294.
Analytical Chemistry, Vol. 72, No. 23, December 1, 2000 5847
the alcohols into the corresponding alkyl nitrites.18 The nitrite thus produced was selectively analyzed by gas chromatography with an electron capture detector (GC-ECD). The ppbv concentration level of alcohols in ambient air could be detected without preconcentration. In the alkyl nitrite formation method, because the directly analyzed detection limit for the alkyl nitrite is ppbv, we could avoid the problem of preconcentration, and the tailing peak problem was also eliminated. In this study, we propose a method, which is developed from the method using the glass bottle, for measuring the atmospheric alcohol using the nitrite formation reaction, whose mechanism was discussed in many references;19-24 alcohols adsorbed on the glass surface react with N2O4, which is also adsorbed on the glass surface from NO2, to form nitric acid and alkyl nitrite in the gas phase. The glass surface area for the alkyl nitrite formation is increased using Pyrex glass beads. Therefore, the reaction time could be reduced and the yield of the alkyl nitrites increased. The main advantage of this method is that the time spent for each sample was less than 3 min while the reaction time to yield alkyl nitrite in the previous study was 30 min. EXPERIMENTAL SECTION Methanol, ethanol, and 2-propanol were purchased from Wako Pure Chemicals, Co. Ltd. Nitrogen dioxide and the other gases were from Takachiho Shoji Co., Ltd. Materials and all gases were research grade and were used without further purification. A schematic diagram of the apparatus for the continuous determination of atmospheric alcohols using the alkyl nitrite formation reaction is shown in Figure 1. Air was drawn in by a sampling pump. A nitrogen dioxide cylinder (1000 ppmv in nitrogen) provides the nitrogen dioxide used for alkyl nitrite formation. Atmospheric alcohols were allowed to react with the nitrogen dioxide in a glass tube which had been packed with Pyrex glass beads (6-8 mesh). These clean glass beads had been previously treated with a chromic acid mixture, rinsed with water, and dried. Diluted alcohol standards for the calibrations were prepared in a 5-dm3 Tedler bag by diluting gaseous alcohol in nitrogen with dry air; dry air from the cylinder was pumped into a 5-dm3 bag at 1 dm3/min while a known concentration of gaseous alcohol in nitrogen, which had been previously prepared in a glass bottle, was introduced into the flow route by a gastight syringe to get 5.00 ( 0.03 dm3 air in the bag. The method of preparing diluted alcohol in the nitrogen sample in a glass bottle has been mentioned in the literature.18 The alcohol and nitrogen dioxide reaction occurred on the surface of the glass beads to yield alkyl nitrite. A 0.5-mL air mixture containing alkyl nitrites produced in the glass tubes were injected into the GC-ECD by a gastight syringe. The sensitivity of the ECD was sometimes checked with HCFC-123a. The GC (18) Nguyen, H. T.; Fujio, Y.; Takenaka, N.; Bandow, H.; Maeda, Y. Anal. Chim. Acta 1999, 402, 233-239. (19) Jonson, A. Environ. Sci. Technol. 1982, 16, 106-110. (20) Koda, S.; Yoshikawa, K.; Okada, J.; Akita, K. Environ. Sci. Technol. 1985, 19, 262-264. (21) Akimoto, H.; Takagi, H. Environ. Sci. Technol. 1986, 20, 393-397. (22) Schuck, E. A.; Stephens, E. K. Environ. Sci. Technol. 1967, 1, 138-143. (23) Glasson, W. A. Environ. Sci. Technol. 1975, 9, 1048-1053. (24) Silverwood, R.; Thomas, J. H. Trans. Faraday Soc. 1967, 63, 2476.
5848 Analytical Chemistry, Vol. 72, No. 23, December 1, 2000
Figure 1. Schematic diagram of the apparatus for continuous determination of the atmospheric alcohols.
analysis was done using a Shimadzu GC-4CM gas chromatograph which was fitted with a Teflon column (3 mm inner diameter and 4 m length). The carrier gas was nitrogen with a flow rate of 40 mL/min. The column was packed with tricresyl phosphate on Chromosorb W-AW (60-80 mesh). The temperature of the injection port, detector, and column was 25 °C. The response time of the analysis system to the change in the atmospheric alcohol concentrations was examined. A 5-dm3 bag of dry air (from pure air cylinder) and an 100-dm3 bag of a known alcohol concentration in ambient air were exchanged at the inlet of the air sample (Figure 1). From exchanging of the alcohol and nonalcohol samples, the yields of nitrites versus time were recorded. We prepared known alcohol concentrations of ambient air in a 100-dm3 bag by collecting ambient air into the bag using the sampling pump. The bag was tumbled to make the alcohol concentrations uniform. The alcohol concentrations in the bag were checked by a previously reported method.18 RESULTS AND DISCUSSION Optimum Concentration of Nitrogen Dioxide for the Formation of Alkyl Nitrites. The result for estimating the optimum concentration of nitrogen dioxide for the formation of alkyl nitrites is shown in Figure 2. The open triangles and closed circles show the results for methanol and ethanol, respectively. The results show that the optimum concentration of nitrogen dioxide for alkyl nitrite formation reaction is 400 ppmv. This result is double the optimum concentration of nitrogen dioxide in the method using the nitrite formation reaction in the glass bottle, which was reported as 200 ppmv.18 In the flow method, because of the short residence time of the gas, nitrogen dioxide has the strongest effect on the yield of the nitrite formation reaction at concentrations higher than that in the glass bottle method.
Figure 2. Optimum concentration of nitrogen dioxide for alkyl nitrite formation. The examination is made with optimum conditions of residence time: (4) CH3ONO; (b) C2H5ONO.
Yield of Alkyl Nitrites versus Residence Time of an Air Mixture in the Pyrex Glass Beads. To examine the optimum residence time of the air mixture in the glass beads, the yields of the alkyl nitrites were recorded at various residence times, which was arranged by changing the glass bead amount from 0 to 20 g to get residence times from 0 to 23 s, and sample air flow rate was kept constant at 30 cm3/min with the optimum nitrogen dioxide of 400 ppmv. Moreover, in the range of the optimum residence time, the yield of alkyl nitrites versus residence time was checked by changing the flow rates of air from 60 to 15 cm3/ min with the optimum concentration of nitrogen dioxide of 400 ppmv and with a constant glass bead amount of 10 g. The percentage yields of the alkyl nitrites from their alcohols versus residence time of the air mixture are shown in Figure 3 A for methanol and (B) for ethanol. The closed symbols show the yields of alkyl nitrites versus residence time for the case of changing the residence time by changing the glass bead amount, and the open symbols show the yields of the alkyl nitrites versus residence time for the case of changing the residence time by changing the flow rate of the air mixture while the glass bead amount was constant (10 g). Similar results were obtained for both cases. When the residence time was zero, the alcohol and nitrogen dioxide reaction took place on the glass tube surface, and the yields of the methyl and ethyl nitrites were 19 and 35%, respectively. These results show that the percent yield of the alkyl nitrites reached a maximum at a residence time of 6 s with 5 g of glass beads. No improvement in the percent yield of the alkyl nitrites from their alcohols has been found for a residence time longer than 6 s. The results show that the yields of the alkyl nitrites reaches a maximum with a residence time of more than 6 s. From the start the system, the yield of alkyl nitrite changed and then became stable. The stabilization time is defined as the time from starting the system until the time when the response of produced nitrites becomes stable. Within the range of the optimum residence time, we chose the sample air flow rate of 30 cm3/min with a residence time of 15 s (in the case of 10 g of glass beads) to get a stabilization time of 5 min. Yield of Alkyl Nitrite from Alcohol and Detection Limit under Optimum Conditions. The optimum conditions for alkyl nitrite formation were determined from the optimum concentration of nitrogen dioxide of 400 ppmv and the optimum residence time of 15 s, with the flow rates of nitrogen dioxide and ambient air of 20 and 30 cm3/min, respectively. The glass bead amount was 10 g. The optimum condition for alkyl nitrite formation was used for the atmospheric alcohol determination. The yield of alkyl nitrite
Figure 3. Yield of alkyl nitrite versus residence time. The examination is made with an optimum concentration of nitrogen dioxide (400 ppmv). (A) Methanol (b) changing glass beads amount from 0 to 20 g and (O) changing flow rate of sample air (10 g glass beads). (B) Ethanol (2) changing glass beads amount from 0 to 20 g, (4) changing flow rate of sample air (10 g of glass beads), and (9) stabilization time vs air flow rate. Stabilization time: the time from starting the system until when yielded nitrites became stable. Table 1. Yield of Alkyl Nitrites from Alcoholsa and Detection Limit (DL)a with S/N ) 3 under Optimum Conditionsb alcohol
nitrite yield (%)
DL (ppbv)
methanol ethanol
78 ( 8 90 ( 7
0.7 0.5
a For five replicates. b Optimum conditions: NO flow rate 20 dm3/ 2 min; air flow rate 30 dm3/min; glass beads 10 g).
from alcohol and the detection limits are listed in Table 1. The percent yields of methyl and ethyl nitrites from methanol and ethanol were 78 ( 8 and 90 ( 7%, respectively. The yield of methyl nitrite from methanol is lower but the yield of ethyl nitrite from ethanol is higher by this method than those by the glass bottle method. This can be explained by the nitrite-formation-reaction time of each alcohol and that the residence time of the reactant is significantly reduced in this flow method. In the glass bottle reaction method, it was found that the reaction time of methanol with nitrogen dioxide is much longer than that of ethanol, and the reaction time of 2-propanol is the longest.18 Right after putting reactants in the glass bottle, the reaction of methanol, ethanol, and 2-propanol occurred at 65, 80, and 30%, respectively.18 The reduction in the residence time by the flow analysis method does not decrease the yield of ethyl nitrite from ethanol. Moreover, the ethyl nitrite yield tends to increase due to the increase in the reaction surface area. The yield of 2-propyl nitrite from 2-propanol Analytical Chemistry, Vol. 72, No. 23, December 1, 2000
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Figure 4. Response of the alcohol determinated system while changing alcohol concentrations. The open symbols indicate the starting point of alcohol air sample or no alcohol dry air inlet: (A) CH3OH; (B) C2H5OH. Table 2. Recovery of Spiked Samples
new glass beads 24-h-used glass beads
replicate
ambient concn (ppbv)
added ethanol (ppbv)
concn found (ppbv)
recovery (%)
4 5 3
31.2 7.0 6.4
33.0 48.8 45.9
62.7 53.9 48.2
95.4 ( 1.8 96.2 ( 2.3 91.0 ( 4.7
is reduced so much in this method that the ambient level of 2-propanol is under the detection limit. The detection limits for alcohols in this method determined by the signal-to-noise ratio of 3:1 were 0.7 and 0.5 ppbv for methanol and ethanol, respectively. However, for measurement in the upper troposphere or rural areas, where the alcohol concentrations (especially ethanol) were reported lower than these detection limits,4,13 for example, below 50 pptv ethanol concentration in the upper troposphere4 or 250 pptv ethanol concentration in rural Germany,13 the preconcentration method is preferred. Calibration graphs for methanol and ethanol are straight lines with zero intercepts. Calibration equation for methanol and ethanol were y ) 0.0211x + 0.0099 (R2 ) 0.9962) and y ) 0.0367x + 0.0123 (R2 ) 0.9978), respectively. Repeatability of the Glass Beads for Alcohol Measurements. Figure 4 shows the concentration of alkyl nitrite versus time when the inlet alcohol concentrations were exchanged from a known alcohol concentration to zero alcohol dry air. For 10 exchange runs within 12 h, excellent reproducibility was shown for the yield of nitrite from alcohol. The recorded response time was 4-9 min for methanol and 3-5 min for ethanol while the alcohol concentration changed from 3 ppbv methanol and 6 ppbv ethanol in ambient air to zero alcohol dry air. Moreover, the signals of the produced alkyl nitrites were rather stable during a 1-h examination. The glass beads can be continuously used for at least 12 h with good reproducibility. Interference of Atmospheric Trace Constituents. The presence of water vapor and many trace gases in the atmosphere, which is a very complicated gas mixture, may have some effect 5850 Analytical Chemistry, Vol. 72, No. 23, December 1, 2000
on the nitrite formation reaction that produces differences from that studied for the standard alcohols. The influence of atmospheric trace constituents on the quantitative analysis of atmospheric alcohol by the alkyl nitrite formation reaction was examined in an earlier work. An insignificant effect was found for the 21, 43, and 64% relative humidity samples. The alkyl nitrite formation reaction rate was also found independent of temperature (17-21 °C) by Koda et al.20 Moreover, the variations in the produced alkyl nitrites were less than 2.5% for the samples with added hydrocarbons, aldehydes, and ether.18 In the GC-ECD analysis, no effect of other GC-ECD-sensitive compounds such as alkyl nitrates was found because of the short retention time of the alkyl nitrite compared with those of the alkyl nitrates. The yield of nitrite from only the ethanol standard and from the same ethanol standard with an ambient air mixture was compared. Under the optimum conditions for alcohol analysis, the produced outlet alkyl nitrites were recorded while standard alcohol, the ambient air sample, and the ambient air/standard alcohol mixture were introduced into the inlet of the analysis system. The concentration of standard alcohol in the ambient air/ standard alcohol mixture was deduced from balancing the alcohol concentration in only ambient air and ambient air in the ambient air/standard alcohol mixture. The results were examined for the case of new glass beads and for the case when the glass beads had been continuously used for 24 h. Table 2 lists the results, which are obtained from two examinations of new glass beads and one examination of the 24-h-old glass beads. For each examination, from three to five analyses of the same inlet were done to get an average result. The ambient levels of ethanol from two examinations of the new
glass beads were 31.2 and 7.0 ppbv, respectively, and that of the 24-h-old glass beads was 6.4 ppbv. In all cases, the results show that more than 90% of the standard alcohol, which was intentionally mixed with the ambient air, was detected. The results of the system in response time to the inlet alcohol concentration and the results by the standard addition method proved the applicability of this method to determine the concentration of alcohols in the gas phase. Measurement of Alcohol in the Ambient Atmosphere. The alcohol concentrations in the atmosphere were determined for 24 h at the Osaka Prefecture University campus from noon, November 19h, to noon, November 20, 1998. Atmospheric alcohols were also simultaneously measured by the glass bottle method and by this flow analysis method. The results are shown in Figure 5 for both methods. The results show that these two methods presented similar determination results. The advantage of the flow analysis method can be seen; the ambient level of alcohols can be recorded every 3 min, though it is very difficult by the glass bottle method because of the complicated procedure and long reaction time. The peaks of the alcohol variation by the flow method are sufficiently shown while those of the glass bottle method are just a few scattered points. Flow analysis by the nitrite formation reaction method can be a promising method for determining atmospheric alcohol. CONCLUSION A method for the continuous determination of atmospheric alcohol using the alkyl nitrite formation reaction was developed in this study. This method shows significant improvement compared with the other methods for measuring ambient alcohol due to its high sensitivity, no required concentration process, and
Figure 5. Diurnal variation of alcohols by this method and compared with glass bottle method. The graph shows all measurement results determined by each 3 min: (-) CH3OH by this method, (- -) C2H5OH by this method, (0) CH3OH by the glass bottle method, and (b) C2H5OH by the glass bottle method.
rather high yields of the alkyl nitrite from alcohol. Moreover, this method can be an automated analysis system for atmospheric alcohol. ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid for International Scientific Research (Field Research), FY 1997-1998, from the Ministry of Education, Sciences, Sports and Culture, Japan. Received for review May 11, 2000. Accepted September 19, 2000. AC000538N
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