Environ. Sci. Technol. 1996, 30, 188-195
Viability of Using SUMMA Polished Canisters for the Collection and Storage of Parts per Billion by Volume Level Volatile Organics D A V I D A . B R Y M E R , * ,† L A R R Y D . O G L E , CHRIS J. JONES, AND DAVID L. LEWIS Radian Corporation, 8501 North MoPac Boulevard, P.O. Box 201088, Austin, Texas 78720-1088
The collection of whole air samples using SUMMA polished canisters has been a commonly used technique for over a decade. This technique has been examined for sample storage, analyte stability, and recovery for up to 45 compounds routinely detected in ambient air. However, this technique is often used for a more extended set of target analytes for which little or no published validation data exist. This paper reports the accuracy, precision, and storage stability results for 194 volatile organic compounds generated in humidified ambient air and collected in SUMMA polished canisters. In addition, instrument and method detection limits for the gas chromatography/multidetector (GC/MD) analytical system were determined from the resulting data. A small percentage of the study compounds displayed high variability, low recovery, or poor storage stability for which qualitative only data can be generated with this technique. However, 168 of 194 (86%) compounds studied appear to be amenable to the canister technique. This study shows that canisters are a viable collection and storage media for a variety of compound classes including alkanes, alkenes, alkynes, aldehydes, ketones, alcohols, aromatics, and sulfur-containing compounds.
Introduction SUMMA polished canisters have been used to collect whole air samples to be analyzed for volatile organics for over 15 years (1, 2). These stainless steel vessels, which have been treated to expose a smooth nickel and chromium oxide surface, have remained a popular collection tool since they allow for the collection of a whole air sample, can hold sufficient sample for multiple analyses, can be reused many times, and are easy to use. The use of canisters has expanded to include both point source (stacks, vents, engine exhausts, flux chambers, and soil gases) sample collection † Present address: Texas Natural Resource Conservation Commission, P.O. Box 13087, Austin, TX 78711-3087.
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and ambient air analysis with detection limits from parts per million by volume (ppmv) to parts per trillion by volume (pptv) levels. The application of the canister technique to levels at or below parts per billion by volume (ppbv) has resulted in the need to evaluate storage stability, recovery, accuracy, and precision at these low levels. Storage stability may be affected by physical adsorption or absorption on the canister surface, reactivity with other chemicals in the sample matrix, or instability of the compound. Sample integrity in whole air samples collected in canisters is a key issue in determining applicability of the technique to a wide range of compounds. Past studies of sample integrity in canisters have primarily addressed a select group of hydrocarbons and halocarbons. Rasmussen (1, 3) reported on the stability of several halocarbons in canisters. Harsch (2), Cox (4), Westberg (5), and Gholson (6) have documented the stability of various hydrocarbons and volatile aromatic hydrocarbons in passivated containers. Oliver et al. (7) have determined the storage characteristics of 15 halogenated and aromatic VOCs at levels less than two ppbv. These studies document the applicability of canisters to a small subset of volatile organics but do not include a large number of compounds across many chemical classes including oxygen-, nitrogen-, and sulfur-containing polar compounds. More recent studies (8-11) have addressed selected polar compounds but are rather limited in scope or describe theory more than actual measurement data. Brymer et al. (12) presented data on a preliminary stability study for over 150 volatile hydrocarbons, including alkanes, alkenes, alkynes, aldehydes, alcohols, ketones, aromatics, halogenates, nitrogenated compounds, and sulfur-containing hydrocarbons. This study included a large number of compounds in the low ppbv level concentration range but only addressed stability and not recovery or method detection limits. This initial study consisted of a limited number of replicates and was primarily designed to gather data to determine which compounds may and may not be applicable to this particular methodology. The work reported by Brymer et al. (12) led to the more extensive study reported in this paper. The current study was designed to provide data on the relative recovery, storage stability, and instrument and method sensitivity for a large number of volatile organics from multiple chemical classes. This study was also designed to gather sufficient data to determine the applicability of the canister GC/MD technique for the majority of volatile analytes of interest. The study design does not address sampling systems or compare various analytical systems or approaches. Target lists includes components from the EPA Compendium Method TO-14, Urban Air Toxics Monitoring Program, Photochemical Assessment Monitoring Sites Program, and additional compounds that may be of interest from a health effects or odor standpoint.
Experimental Methods Study compounds were initially divided into five groups comprised of 43-51 compounds per group to minimize the potential for analytical interference. Compounds in each group were organized by elution time and standard
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1995 American Chemical Society
TABLE 1
Analytical Approach to VOC Validation Study elapsed time from sample collection (days)
canisters pressurizeda
canisters analyzeda
0 0 6 7 13 14 21 29 30
none none 1, 4, 7 none 2, 5 none none 3, 6 none
test stream (precollection) test stream (postcollection) none 1, 4, 7 None 7, 1, 4, 4, 2, 2, 5b 4, 7, 7, 1b none 3, 3, 1, 1, 7, 4, 6b
a Numbers indicate the canister location on the manifold during sample collection. Refer to Figure 1 for locations. b Duplicate numbers indicate duplicate analyses for that canister.
FIGURE 1. Dynamic system used to prepare test mixtures.
availability without considering the potential for reactivity between compounds. Test mixtures were comprised of selected target compounds initially prepared at ppmV levels and then dynamically diluted using humidified zero-grade air as shown in Figure 1. Final test mixtures had nominal concentrations of 1-5 ppbv per compound for nonpolar compounds and 2-20 ppbv per compound for polar analytes in a 70% relative humidity (RH) air matrix. Test mixtures (0.5-L samples) were loaded directly onto an automated cryofocusing interface designed and custom built by Radian and subsequently desorbed into a gas chromatographic system equipped with a flame ionization detector (FID)/photoionization detector (PID)/electrolytic conductivity detector (ELCD) for precollection analysis. Dual cryo traps of 80/100 mesh silanized glass beads were held at -185 °C during sample collection and then desorbed at 210 °C onto separate J&W Scientific DB-1 (1-µm film) 60 m × 0.32 mm i.d. capillary columns. A Radian designed moisture management system as described by Ogle et al. (13, 14) was employed to minimize the effect of moisture on the chromatography. The oven program used for separation was -50 °C for 2 min, ramped at 6 °C/min to 100 °C, with a second ramp of 25 °C/min to 250 °C. After the test mixture analysis verified that the diluted compound concentrations were near the theoretical concentrations, eight evacuated (-14.1 ( 0.2 psig) canisters were filled approximately to ambient pressure (-0.3 ( 0.3 psig) with the test mixture. The eight test canisters were attached to the standard preparation device via a common manifold and filled at a flow rate of 0.5 L/min (Figure 1). This test mixture was then loaded directly onto the GC/ MD system from the manifold vent and reanalyzed (postcollection analysis).
The eight conditioned (not new) canisters in each study group were subsequently pressurized with UHP nitrogen to 30 psig ((0.5 psig) and analyzed as shown in Table 1. This pressurization resulted in a test stream containing approximately 25% RH at analysis. This dilution process ensures adequate sample volume for replicate analyses over time and minimizes the potential for water vapor condensation. A method surrogate, 1,4-difluorobenzene, was added to each canister via a fixed loop injection system at the time of pressurization to provide an estimate of method precision. After sample pressurization, the samples were allowed to equilibrate at least 12 h and analyzed on the same analytical system described above. Analytical precision was monitored by adding an analytical surrogate, 4-fluorotoluene, to the cryotrap during sample concentration. This technique and that of surrogate addition are described in more detail by Brymer et al. (15). Subsets of three to five canisters each were analyzed at days 7, 14, 21, and 30 according to the analytical scheme shown in Table 1. This sample handling and analytical scheme was utilized to evaluate any obvious differences between results for canister samples stored at ambient pressure versus those stored under positive pressure. Pressurization and/or analyses were not performed during the first 6 days after sample collection to simulate normal sample handling and shipping procedures. One of the eight canisters was not included in the study design and was designated as a backup sample in the event of sample loss during the study. Recovery of the test mixture compounds from canisters was determined by comparing the mean of the measured values at day 7 to the mean of the pre- and postcollection analyses from the sample generation manifold. Compound stability was evaluated using the measured concentrations from days 7, 14, 21, and 30. A multivariate analysis of variance (ANOVA) provided an estimate of the amount of variability in measured concentration attributable to time of storage and tested this value for statistical significance at the 0.05 significance level. The instrument detection limits (IDL) and method detection limits (MDL) were estimated from the ANOVA data. MDLs were estimated by multiplying the square root of the variance (i.e., variability) from the ANOVA measured between canister samples by 3.3 (the appropriate t-test value for seven replicates at 99% level of confidence), and IDLs were expressed as the instrument variability from duplicate analyses multiplied by a factor of 3.3.
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To ensure high-quality data, numerous quality control measures were incorporated into the study design. Because of the low test concentrations utilized, contamination was of the foremost concern and addressed in several different ways. First, the canisters used for test mixture collection were cleaned, filled with humidified UHP grade N2, and certified to a total non-methane hydrocarbon (TNMHC) value of less than 3 ppbv of C of using a dedicated GC/FID instrument performing U.S. EPA Compendium Method TO12 as described by Shaulis et al. (16). Next, the humidified diluent gas was loaded directly onto the GC/MD system and analyzed to ensure that the analytical system was void of significant contaminants that would interfere with target compound quantitation prior to preparing or analyzing the test mixture. A humid nitrogen blank was analyzed on the analytical system daily prior to sample analysis to demonstrate a TNMHC value of less than 10 ppbv of C with no more than 0.2 ppbv of any given target analyte. In addition, a set of eight humid blanks was collected and analyzed in a manner consistent with the other test groups to assess contamination levels during dilution, storage, and analysis. These method blanks did not contain any target analytes in excess of the system blanks, and the background contribution was thus considered negligible. Quantitation was achieved using compound-specific response factors, which are derived daily where available, or a generic carbon factor based on hexane for the FID when certified calibration standards were not available. Analytical linearity was assessed by the analysis and evaluation (acceptance criteria of r > 0.995) of multipoint calibration curves. Accuracy was verified through the use of second source standards for the carbon response factor ((15%) and other selected halogenated, polar, and straight chain hydrocarbons ((30%). In addition, the daily individual response factors used for quantitation were checked for statistical control and compared to the multipoint response ((30% acceptance criteria). Sample pressurization accuracy was verified by using a NIST certified gauge test kit ((0.5% accuracy) to assess gauge calibration. This gauge check in conjunction with data on compounds shown to be stable in previous work (6, 7, 9) were used to screen for bias introduced during sample dilution. Precision was assessed during the study through replicate analysis of at least one sample on each analysis date. In addition, method (1,4-difluorobenzene) and analytical (4fluorotoluene) surrogates were utilized to determine overall method and analytical precision.
Results and Discussion During the course of the study, several compounds were found to have analytical interferences, degraded so quickly they were at concentrations too low to provide reliable response, or were not detected at all after 7 days. A set of 13 compounds for which meaningful data were not initially collected due to interferences or that appeared to degrade quickly to low concentrations were included in a sixth study set. Methyl mercaptan, ethyl mercaptan, butyl mercaptan, dimethyl acetal, and bis(chloromethyl) ether were verified in this sixth set as unstable to the point of being undetectable at day 7. For those compounds stable enough to be detected after 7 days, the average concentration obtained from the preand postcollection analysis, the average percent recovery of the cans analyzed at 7 days relative to the average initial concentration, the instrument and method detection limits,
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and an indication of compound stability were calculated as shown in Table 2. Compounds included in more than one test set are reported with the average initial concentration for each test set. Percent recoveries reflect the average across all study groups for an individual compound. In order to determine the significance of the measured recovery of each compound, system variability was determined during the study period through the evaluation of surrogate data. The analytical and method surrogates for the 133 analyses (not including analytical duplicates) during the 17-week study period yielded a relative standard deviation of 14.22% and 14.28%, respectively. Thus, any observed average percent recovery that is outside the surrogate variability of (28.56% of the average initial concentration can be considered to be a function of the compound and not the measurement system with a 95% degree of certainty. Of the 189 compounds stable enough to be detected after 7 days in the canisters, 13 had recoveries outside the expected recovery range of 71.5-128.5%. These compounds can be divided into three groups based upon the suspected reason for low/high measured recovery of the compound from the canisters. Seven of these 13 compounds yielded higher than expected recoveries, which may have been caused by analytical interferences. These compounds are cis-2-pentene, carbon tetrachloride, cyclohexanone, iodomethane, m-diethylbenzene, methyl isobutyl ketone, and tert-2-pentene. The six remaining compounds exhibited a significant loss in concentration after 7 days. Four of these compounds are polar and are generally characterized by higher analytical variability than the surrogates due to the difficulty in quantitating the nonsymmetrical peak shape on the nonpolar (DB-1) column. These compounds may also have been lost to water condensate or on active sites in the analytical system or canisters. The affected compounds are butyl acrylate, diethyl ether, hexanal, and isophorone. The two remaining compounds with low recovery are propyne and isopentyl mercaptan, which may not be amenable to this technique due to their suspected reactivity in air matrices. A multicriteria approach to determining compound stability was developed to ensure that the compound was truly stable in the canister and not rejected as unstable as a function of instrument variability. Compounds were considered stable if (1) the ANOVA analysis temporal variability had a RSD
