Environ. Sci. Technol. 1997, 31, 1224-1228
Bottle-in-Bag Technique for Gas Sampling with Analysis by Autoheadspace GC J U L I A M . B R I X I E , * ,† L E S L I E M . N E I L S O N , ‡ BYUNG J. KIM,§ AND BLAINE F. SEVERIN‡ 2294 Hamilton Road, Okemos, Michigan 48864-1641, MBI International, 3900 Collins Road, Lansing, Michigan 49010-0609, and USA-CERL-EPD, Champaign, Illinois 61826-9005
A new (bottle-in-bag) method for sampling gases is presented, examined, and discussed. The bottle-in-bag (BiB) method has broad application in air pollution control, industrial hygiene, soil remediation, groundwater remediation, and biofiltration of industrial waste streams. The BiB method is more cost-effective than the Tedlar bag capture method and allows analysis by autoheadspace GC. The method entails pumping the gas source and injecting it into a standard 22-mL headspace vial, held within a Ziploc bag, followed by crimp sealing of the bottle through the sealed bag. Gases from three separate sources (isobutylene calibration gas and pilot- and field-scale biofilter gas) were sampled to test the accuracy and precision of the BiB method under different sampling conditions. The BiB method was compared to the Tedlar bag and manual gas syringe methods. Sampling with the BiB technique produced results that were more accurate (recoveries of 98% and greater) and precise (coefficients of variability as low as 0.006) than the other methods. Highly reproducible and accurate results, even when the samples were subjected to temperature extremes and the rigors of airplane travel, are a strength of the BiB method, which combined with its simplicity and cost-effectiveness give it the potential for wide use.
Introduction Gas sampling and analysis is a major component of environmental monitoring and cleanup of volatile organic compounds (VOCs) and is crucial to the fields of air pollution and industrial hygiene. Many remediation treatment systems also produce gas streams that must be monitored. Some of these treatment systems include vapor extraction of gas from soil remediation (1), air scrubber gas from groundwater cleanup, and biofiltration of VOCs in industrial waste streams (2). Traditionally, these gas sources have been collected in Tedlar bags (3). Some studies using this technology have shown satisfactory results and stability (4). Other researchers, however, have experienced problems (3, 5, 6). In general, problems with the bags center around the need for manual gas chromatography (GC) analysis. The bags are also expensive, cumbersome, fragile, and difficult, if not impossible, to clean (3, 5). Tedlar bags also are prone to leaks during transport and should be used for short-term storage (32 °C) in the trunk of a car for several hours and were returned to the lab after 4 days. One set of samples was checked in the baggage compartment, while the other was transported in the passenger cabin as carry-on luggage. The third set of samples was aged in the lab for the same amount of time (4 days), and all three sets were then analyzed for isobutylene (Table 1) and compared to the fourth set of 20 freshly prepared samples. Pilot-Scale Biofilter Experiments: Comparison of BiB Method to Manual Sampling with a Gas-Tight Syringe. A pilot-scale biofilter experiment was designed to test the accuracy and precision of the BiB method and to compare it to manual sampling with a gas syringe in a realistic sampling scenario. The pilot-scale biofilter was a 92.5 cm × 10.2 cm glass column with a total volume of 18.5 L and a flow rate of
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15 L min-1. The support medium in the column was made of polyurethane material incorporated with nutrients. Approximately 2 L of aqueous liquid containing nutrients was present below the support medium in the biofilter column. The pilot-scale biofilter was fed liquid ethyl acetate continuously to obtain an influent concentration of 267 µg L-1 ethyl acetate in the air stream in the biofilter column. The biofilter was sampled at the influent port 20 times by both the manual syringe method and the BiB method. The effluent port was sampled 10 times by each method. Effluent samples were taken before influent samples, and BIB samples were taken before manual samples. The results were compared to test the accuracy and precision of the two sampling methods. The sample ports on the biofilter column were equipped with septa that allowed direct access to the gas stream by the needle. The manual syringe method consisted of pulling a sample of the biofilter gas with an SGE gas-tight syringe (10 mL) directly from the ports on the biofilter column. The sample was then taken immediately to the GC and injected for analysis. The BiB method was used, as described above, with the use of a pump. The pump rate was 4.7 L min-1. All BiB samples were taken first (30 min total sampling time); all of the manual samples were taken after waiting 1 h for the system to reach equilibrium. We experimented with a number of different times for the system to reach equilibrium from 15 min to 24 h and did not find any significant differences, so we arbitrarily chose 1 h as the equilibrium time. Ambient air blanks were taken prior to sampling for each method and after every 10 samples to check for tubing contamination. Effluent samples were taken before influent samples for both methods. All samples were analyzed for ethyl acetate. Because both the reproducibility and the accuracy of the manual syringe sampling method were very poor, as will be described below, additional experiments were conducted to determine the source of this error. The manual syringe method was modified and the experiment repeated. However, in the modified gas syringe method experiment, the biofilter gas was sampled by both methods simultaneously. The modified syringe method consisted of pulling the manual sample from the Ziploc bag during the BiB method sampling. A septum fashioned from silicone sealant (732, Dow Corning Corp., Midland, MI) was placed on the outside of the Ziploc bag to minimize leakage from the additional needle hole. The modified manual sample was taken after the bag was full of gas but before the headspace vials were crimped. The same numbers of influent, effluent, and blank samples were taken in the order described above. To further determine error sources in the manual syringe method, the syringes used for these methods were tested for leaks by repeatedly injecting standards with the different syringes. Duplicate or triplicate standards of ethyl acetate were injected with each syringe used in the manual syringe method. Blanks were also run between standards to determine if carryover was a problem. Field-Scale Biofilter Experiment: Comparison of BiB Method to Tedlar Bag Sampling and Effect of Shipment Method on Analytical Results. This applied, field-scale experiment was designed to (1) compare the BiB method to Tedlar bag sampling, (2) determine the effects of air transport on BiB sample degradation, and (3) test the BiB method for reproducibility in a field situation. The full-scale biofilter was 28.3 m3 in volume with an airflow capacity of 0.47 m3 s-1. The air stream fed to the biofilter had a variable concentration of roughly 300-400 µg L-1 ethyl acetate. Three sets of 12 samples were collected from the influent port of the fieldscale biofilter. Two of the sets of samples were collected using the BiB method; the third set was collected using the Tedlar bag method. The 12 Tedlar bags and one set of the BiB samples were shipped from the site to the lab via overnight air delivery. The other set of BiB samples was returned to the lab in the carry-on luggage of the sampling personnel. Three
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TABLE 2. Sampling of Isobutylene Using BiB and Tedlar Bag Methods sampling method
no. of samples
BiB Tedlar bag
20 21
isobutylene rel peak area meana SD 9.14 14.10
0.05 2.10
coef of variation 0.006 0.149
a BiB and Tedlar bag method used different GC (see Table 1) with different ranges, thus the wide discrepancy in peak area results.
other sampling excursions were made to the site, wherein both influent and effluent ports were sampled (10-20 replicates) by the BiB method. A pressurized sample line (15 L min-1) located on the field biofilter, capable of sampling both influent and effluent samples, was sufficient for collection of both Tedlar bag and BiB samples without the use of an external pump. The Tedlar bags were filled by using 61 cm of Norprene (Cole-Parmer, Niles, IL) tubing with a disposable needle on one end and connecting the other end to a barbed sampling port on the discharge of the biofilter. The influent samples were collected by taking a Tedlar bag sample followed by two BiB method samples until all 12 Tedlar bag samples and 24 BiB samples had been taken. On the subsequent sampling trips, effluent samples were taken before influent samples. All samples were analyzed for ethyl acetate.
Analytical Section The gas samples discussed in this paper were analyzed on one of two instruments, under the conditions found in Table 1. Samples collected by the BiB method were analyzed on the autoheadspace analyzing GC (Varian Genesis/3500, Walnut Creek, CA), and samples collected by all the other methods were analyzed by manual injection on a Varian 3600 GC. Standards for the two different instruments were prepared differently. Gas standards run on the autoheadspace analyzer GC were made by injecting different aliquots (0.51.5 µL) of a freshly prepared stock standard solution (451 µg L-1 ethyl acetate in water) into empty headspace vials. Care was taken to inject the solution with the needle touching the side of the vial and to immediately seal the vial in order to minimize volatilization of the standard solution. The small amounts of stock solution in the headspace vials evaporated in the closed vial prior to analysis. Standards were analyzed immediately to prevent breakdown of the ethyl acetate in water. Standards run on the manual injection GC (Varian 3600) were made by injecting different aliquots (0.5-10.0 µL) of pure ethyl acetate into a modified 2-L volumetric flask. The flask was equipped with a gas-tight valve and a septum port. After injecting the ethyl acetate into the flask, the flask was shaken until the ethyl acetate had evaporated (approximately 1 min). Triplicate samples of the gas standard were withdrawn from the flask (5 mL each) and injected onto the GC.
Results and Discussion Isobutylene Experiment. The results of the isobutylene experiment illustrate the reproducibility and precision of the BiB method (Table 2). Isobutylene concentrations in samples collected by the BiB method were highly reproducible as demonstrated by the very low coefficient of variability (CV ) 0.006). In this isobutylene experiment, the BiB method of sampling was found to be more reproducible than the Tedlar bag method (CV ) 0.149). The minimal effects of time and travel on BiB samples illustrate the suitability of this method for gas samples that cannot be analyzed immediately (Table 3). All of the samples showed excellent stability after 4 days, with 98% or better recovery of isobutylene (as compared to the fresh samples).
TABLE 3. Effects of Time and Air Travel on Analytical Results from BiB Samples sample age travel method
fresh none
mean isobutylene concn (rel peak area) standard deviation (rel peak area) coef of variation no. of samples % recovery (from fresh samples)
9.14 0.05 0.006 20
TABLE 4. Comparison of BiB Method to Manual Syringe Methods of Sampling Pilot-Scale Biofilter Gas sampling method manuala BiB Mod-Manualb Mod-BiBc known all
sample port
mean ethyl acetate concn (µg L-1)
influent influent influent influent influent effluent
53 272 75 287 267 NDd
none
4 days old carry on
baggage
9.05 0.16 0.018 20 99
9.08 0.17 0.019 19 99
8.97 0.24 0.026 20 98
TABLE 5. Syringe Test of Ethyl Acetate Recovery using the Manual Syringe Method
n
CV
% recovery
20 20 20 20
0.524 0.142 0.730 0.382
20 102 28 107
syringe no.
mean ethyl acetate (µg L-1)
1 2 3 4 5 6 7
661 725 622 553 873 815 666
40
a
Manual gas syringe method. b Modified manual gas syringe method. c Modified BiB method. d ND, not detected.
The variability in the data obtained from the BiB samples that were carried on the airplane was similar to those that were aged in the lab (CV ) 0.019 and 0.018, respectively). The data from the samples that were checked in the baggage compartment were more variable than the others (CV ) 0.026), indicating perhaps that the larger pressure changes in the baggage compartment had a small effect on BiB samples. However, an analysis of variance (ANOVA) showed that the mean isobutylene concentration did not differ significantly among the three time delayed BiB samples. There was a small, though statistically significant, difference between the data from the fresh and the aged samples, indicating that the differences observed are merely from aging of the samples and not the transportation method (Table 3). These results strongly suggest that the BiB method is a suitable method for gas sampling, especially when the samples cannot be immediately analyzed and/or when they need to be transported long distances for analysis. Pilot-Scale Experiments. The results from the pilot-scale experiments are shown in Table 4. The mean concentration of ethyl acetate in the influent gas sampled by the BiB method was 272 µg L-1, whereas the known feed value for the influent was 267 µg L-1 ethyl acetate, resulting in a 102% recovery of ethyl acetate. Statistically, the mean of the BiB data is not significantly different from the known value. The manual syringe method of sampling the influent gas, however, resulted in a mean concentration of 53 µg L-1 ethyl acetate, which is only 20% recovery of the known feed value; thus, data from the manual syringe influent method were significantly different than the known influent feed value at P ) 0.05. The concentration of ethyl acetate in the effluent was below detection limits for the instruments for both methods (Table 4). The results from the pilot-scale experiments illustrate the increased accuracy of the BiB method over the manual syringe method for sampling of this biofilter. The extremely low recovery of ethyl acetate using the manual method of sampling was disconcerting, and thus we sought an explanation for this discrepancy. The solid support medium in the pilot-scale biofilter had compressed to a point well below the influent port. We hypothesized that the low recovery of ethyl acetate from the manual method could, in part, be due to stagnant air at the sample port and thus devised the modified experiment described above to test this hypothesis. Sampling from the pumped air in the Ziploc bags did, in fact, increase recovery for the modified manual syringe
a
N
% recoverya
blank ethyl acetate (µg L-1)
3 2 2 2 2 2 2
71 78 67 60 94 88 72
none 47 87 94 94, 34 100 106
Ethyl acetate standard concentration was 929 µg L-1.
method. However, the recovery was still low (28% vs 20% for the manual syringe method) (Table 4). The results of the experiment to test the syringes illustrate some of the difficulties surrounding the manual syringe method (Table 5). Upon further investigation, we discovered that only two of the seven syringes used for the pilot-scale biofilter experiments were acceptable. The ethyl acetate standard injected by these syringes was 929 µg L-1. The recoveries for the seven syringes ranged from 60 to 94%. The blanks injected by these syringes had ethyl acetate contamination ranging from 34 to 106 µg L-1. The poor quality of the syringes most likely contributed significantly to the poor reproducibility and accuracy of the manual syringe method of sampling for this pilot-scale biofilter experiment, since all seven syringes were used during the experiment. Fortunately, only good syringes were used in the isobutylene and fieldscale biofilter experiments. These findings demonstrate the difficulty in using sampling methods requiring manual injection GC analysis. The statistical analysis of the results from the pilot-scale experiment shows that the BiB method is more accurate and precise than either the manual or modified manual syringe method (Table 4). The CV of the BiB samples in the first experiment was 0.142 whereas for the manual method the CV was 0.524. Thus, the variability of the manual syringe method was almost 4 times greater than that of the BiB method. Although modifying the manual syringe method improved the recovery of ethyl acetate, it also acted to increase the variability (CV ) 0.730). In addition, modifying the manual method also increased the CV for the BiB method (0.382), probably because there was an additional hole in the bag for the gas to escape and more time had elapsed before the vials were crimped. These results indicate that the BiB method is more reproducible than either the manual gas syringe method or the modified manual gas syringe method. The manual methods are more variable in their reproducibility because of several potential factors: syringe failure, operator error on sample draw and/or injection, and/or difficulty in manually drawing the sample. Hopefully, other laboratories do not have as difficult a time as we did in utilizing a direct sample method. We were suprised at how quickly the quality of gas syringes deteriorated during the experiments. This underscores the need for close examination of syringes used in gas sampling and analysis.
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TABLE 6. Comparison of BiB Method to Tedlar Bag Method for Sampling Gas from a Field-Scale Bioreactor sampling method
shipment method
sampling port
n
CV
BiB Tedlar BiB
CO overnight overnight
influent influent influent
12 10a 9b
0.231 0.286 0.096
ethyl acetate (µg L-1) SD mean 82 46 36
354 162 371
a Some samples were removed from the statistical data set because bags were flat after shipping. b Samples less than the mean values minus three times the standard deviation were removed from data set.
TABLE 7. Reproducibility of BiB Method for Sampling Gas from a Field-Scale Bioreactor sampling event
sampling method
sampling port
n
CV
1a
BiB BiB BiB BiB BiB BiB BiB
influent influent effluent influent effluent influent effluent
10 20 19 10 9 10 10
0.157 0.306 0.212 0.368 0.164 0.328 0
2 2 3 3 4 4 a
ethyl acetate (µg L-1) SD mean 51 94 56 166 29 100 0
324 308 265 452 176 305 0
Effluent was not sampled.
Field-Scale Biofilter. The results for the field-scale biofilter sampling experiment are shown in Tables 6 and 7. There were several difficulties encountered during the first sampling experiment. The air temperature during sampling was -7 °C. The gas expanded after the samples were brought inside and warmed to room temperature (23 °C). Although the Tedlar bags were not completely filled to allow for the expansion, two of the bags were, nonetheless, practically flat when they arrived at the lab. The loss of sample was probably from leaks caused by overexpansion of the bags during repeated heating and cooling and pressure changes during transport. Both bags had low levels of ethyl acetate and were discarded from the data set for statistical purposes. Three of the overnight BiB samples also had low levels of ethyl acetate. Close examination of the vials themselves revealed that these vials were not sealed tightly because the Ziploc bag had been crimped under the seal of the vial. Therefore, we also discarded these three samples from the data set. During the course of these experiments, we discovered that crimping the Ziploc bag under the seal of the headspace vial usually resulted in leaky vials. In addition, if the seal on each vial was not checked by twisting and recrimping when necessary,
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vials could also leak (presumably from poor crimping). The results of the field experiment clearly illustrate the utility of the BiB method in the collection of gas samples in a field situation (Table 6). The mean influent concentration of ethyl acetate was 354 and 371 µg L-1 for the BiB carry on and BiB overnight samples, respectively, as compared to 145 µg L-1 for the Tedlar bag samples. Statistically, there was not a difference in the shipment method on BiB samples (P ) 0.745). The reproducibility of the BiB method, for both carry on and overnight samples, was also exceptional (CV ) 0.231 and 0.096, respectively). Continued sampling of the field-scale biofilter influent and effluent ports using the BiB method further reinforces the utility, reliability, and versatility of this sampling method (Table 7). Mean influent concentrations of ethyl acetate ranged from 305 to 452 µg L-1. Effluent concentrations ranged from 0 to 265 µg L-1 ethyl acetate. The CV values for these sampling events ranged from 0.157 to 0.368. The biofilter was sampled monthly from the start to monitor removal efficiency, explaining the change in effluent from high values to values below the detection limits of the instrument. These data repeatedly show the reliability of the BiB method for gas sampling, even under difficult field conditions.
Acknowledgments The authors would like to thank Randall Schaetzl for his support of the research and thoughtful comments on the manuscript. We also thank Bruce Pigozzi for statistical assistance and Stephen Boyd for comments on the manuscript. Financial support for this project was provided through Contract DACA 88-94-C-002 from USA-CERL-EPD, Champaign, IL.
Literature Cited (1) Buck, F. A.; Seider, E. L. Remediation 1991, 39 (winter), 39-50. (2) Severin, B. F.; Shi, J.; Cybul, B. G.; Neilson, L. M.; Furstenberg, J. L. Presented at the Water Environment Federation; Industrial Wastes Technical Conference; Multimedia Pollution Control and Preventions, Pittsburgh, PA, March 1995. (3) Posner, J. C.; Woodin, W. J. Appl. Ind. Hyg. 1986, 1 (4), 163-168. (4) Maskarinec, M. P.; Johnson, L. H.; Bayne, C. K. J. Assoc. Off. Anal. Chem. 1989, 72 (5), 823-827. (5) Dietz, E. A., Jr.; Hoffman, V. J. Am. Ind. Hyg. Assoc. J. 1984, 46 (6), 382-385. (6) Butler, F. E.; Coppedge, E. A.; Suggs, J. C.; Knoll, J. E.; Midgett, M. R.; Sykes, A. L.; Hartman, M. W.; Steger, J. L. J. Air Pollut. Control Assoc. 1988, 38 (3), 272-277.
Received for review September 9, 1996. Revised manuscript received December 11, 1996. Accepted December 16, 1996.X ES960758B X
Abstract published in Advance ACS Abstracts, February 15, 1997.
