Environ. Sci. Technol. 1996, 30, 3441-3447
Volatile Organic Compounds: Comparison of Two Sample Collection and Preservation Methods TERRY L. LIIKALA,* KHRIS B. OLSEN, STEVEN S. TEEL, AND DAVID C. LANIGAN Pacific Northwest National Laboratory, Richland, Washington 99352
Two soil sample collection and preservation methods for volatile organic compounds, used during site characterization activities, were evaluated using standard U.S. Environmental Protection Agency analytical methods. A conventional bulk method recommends completely filling a sample container with soil; a less commonly used methanol method recommends placing a soil aliquot into methanol. Analytical results showed large negative biases associated with the bulk samples as compared to the methanol samples for aromatic compounds. Order of magnitude differences in concentrations measured between the methods were observed for benzene and toluene. Lesser differences were noted for xylenes and ethylbenzene. Limited data for chlorinated compounds suggest behavior similar to the aromatic species. A limited spike recovery study was conducted using the methanol method on laboratory and field samples. Samples were analyzed 82 days after spike addition. Poorer spike recoveries were noted from spiked methanol vials transported to the field and used for collection of soil samples. Differences between mean recovery values for the laboratory and field samples appear to be the result of losses during sample collection and transport. Despite the 82-day holding time, spike recoveries were within 70% of initial spike concentrations. These results demonstrate the stability of using methanol as a preservative for soil samples.
Introduction Volatile organic compounds (VOCs), consisting of aromatic and chlorinated compounds with boiling points below 200 °C (1), are the most prevalent class of compounds found at hazardous waste sites throughout the United States (25). In addition, VOCs are commonly used in the home by the general population and are often improperly disposed of directly into the environment. Most of these compounds * Corresponding author telephone: (509) 376-4143; fax: (509) 3765368; e-mail:[email protected]
S0013-936X(95)00922-9 CCC: $12.00
1996 American Chemical Society
are classified as hazardous because of their carcinogenic and/or mutagenic effects, even at low concentrations. VOCs are persistent in the environment and are relatively mobile in the subsurface, being transported by gas phase, as a separate nonaqueous phase liquid, or dissolved phase in groundwater. The U.S. Environmental Protection Agency (EPA) considers VOCs to pose significant human health and ecological risks because of their physical and chemical properties and biological effects (6). VOCs have relatively high vapor pressures and thus greater volatilities at ambient conditions because of their relatively low boiling points. For example, vapor pressures at 20 °C for benzene, toluene, ethylbenzene, and xylene (BTEX) range from 76 mmHg for benzene to 5 mmHg for o-xylene (7). Vapor pressures at 20 °C for carbon tetrachloride (CCl4), trichloroethylene (TCE), and tetrachloroethylene (PCE) are 90, 60, and 14 mmHg, respectively (7). The volatilities of these compounds can, coupled with certain sampling and preservation methods, potentially result in large negative biases for soil analyses, where the measured values are significantly lower than the expected values. The negative biases are attributed to loss through volatilization during sample collection, handling, shipping, and preparation in the laboratory (6, 8-15). Among the factors facilitating VOC loss are sample collection and transfer into a container (disturbance and exposure of sample, compromised container seal due to over-tightening or presence of soil grains); sample degradation (loss through headspace, microbial activity); sample shipment (widely fluctuating pressure and temperature of aircraft cargo hold); and laboratory sample preparation (homogenization of sample and exposure of sample to air). Since the inception of the Resource Conservation and Recovery Act (RCRA) of 1976 (as amended by the Hazardous and Solid Waste Amendments of 1984) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980 (as amended by the Superfund Amendments Reauthorization Act of 1986), the quality assurance/quality control objectives of environmental characterization for VOCs have attempted to minimize analytical measurement errors to (20-30%. However, as presented in this paper, improper soil sample collection and preservation methods could have order(s) of magnitude negative bias on the analytical results. Such large negative biases could have serious repercussions if a VOC-contaminated site were to be designated clean based on erroneous data. During 1990, staff at the Pacific Northwest National Laboratory (PNNL) conducted a review of various soil sample collection and preservation methods that would reduce VOC losses during sampling and subsequent transport to the laboratory. Conventional practice has been to fill a glass container with bulk soil in the field, return the sample to the laboratory, and then place a sample aliquot in water before analysis. This method is referred to as the “bulk method”. The literature review highlighted an EPA procedure in SW-846 (1) where the sample aliquot is placed in methanol instead of water before analysis. Methanol acts as a preservative and extracts VOCs from the soil matrix, thus minimizing volatilization losses. PNNL modified the latter procedure by placing the sample in methanol at the
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sample site rather than in the laboratory. This modified method is referred to as the “methanol method” (15). PNNL conducted a followup study in 1990 to evaluate spike recoveries for a limited number of field and laboratory samples preserved with methanol. Results from the literature review and the spike recovery study were used as the basis for a direct comparison study of the bulk and methanol methods. The comparison study was conducted in 1993 and 1994, as PNNL characterized two sites contaminated with fuel-related and chlorinated compounds for the U.S. Air Force (USAF). The results of the spike recovery study and the comparison study are presented in this paper.
Experimental Section Spike Recovery Study. A limited spike recovery study was conducted in 1990 to determine the efficacy of placing a soil sample in methanol at the sample site rather than waiting to preserve the sample at the laboratory. For this study, 10 mL of Burdick and Jackson purge-and-trap grade methanol was added to nine 40-mL amber glass volatile organic analysis (VOA) vials with Teflon septa and screwcaps. Two of the nine VOA vials were stored as blank samples. Four of the remaining seven vials were spiked with 100 µL of EPA methanol standard solution WS 1082 containing bromochloromethane (BCM), chloroform (CHCl3), CCl4, m-dichlorobenzene (m-DCB), TCE, 1,2dibromomethane, 1,1,1,2-tetrachloroethane, and 1,2-dibromo-3-chloropropane. The spike level in the vials equaled 100 ( 1 ppb. The last three VOA vials were spiked with 200 µL of the same EPA standard solution, equaling 200 ( 2 ppb. Four of the spiked VOA vials (two at 100 ppb and two at 200 ppb) were stored with the blank samples. Three of the spiked VOA vials (two at 100 ppb and one at 200 ppb) were weighed and taken to the field on ice. Soil was added to these three vials, ranging in mass from 2.5 to 9.0 g. Duplicate soil samples were collected in VOA vials containing unspiked methanol to ensure none of the spike compounds were present in the soil samples. All spiked and unspiked methanol vials were stored at 4 °C until analyzed 82 days after spike preparation. The analytical system was calibrated for BCM, CHCl3, CCl4, m-DCB, and TCE. Thus, only these compounds had spike recoveries reported. Comparison Study. In 1993 and 1994, PNNL characterized two sites contaminated with fuel-related compounds and chlorinated compounds for the USAF. This characterization effort provided the opportunity to compare the bulk method with the methanol method. Over the course of this study, 50 soil sample pairs (100 total samples) including duplicates were collected at 50 sampling points during site characterization activities. For each pair of samples at a sampling point, one sample was collected by the bulk method and one sample was collected and preserved by the methanol method. Samples were analyzed for aromatic and chlorinated compounds using EPAapproved analytical methods (1, 16, 17). Sample Collection. Soil samples were collected during borehole drilling for both the bulk and methanol methods. Drilling was generally performed using a Central Mine Equipment Co. (CME) 75 hollow-stem auger rig, and sample cores were collected using a CME continuous core sampler. However, three sample pairs from one of the two sampling
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sites were collected with a continuous core sampler during air rotary drilling. Upon retrieval from the borehole, the continuous core samples were scanned with a photoionization detector (PhotoVac MicroTip) and a hydrocarbon analyzer (GasTech Tracetector) to check for organic vapors and to guide in selecting subsample locations. Instrument readings as well as visual and olfactory observations were recorded. Subsamples were collected within approximately 5 min after the continuous core sampler was retrieved from the borehole. Tools used for subsampling included stainless steel trowels, cork bores, and scoopulas. Subsamples for the bulk method were collected first using stainless steel trowels and were tightly packed into 4-oz. wide-mouth jars with Teflon-lined screw-top lids. Duplicates were collected on approximately 5% of the samples. Care was taken to ensure minimal headspace in the jars. The clear glass jars were furnished by the EPAcertified laboratory performing the bulk sample analyses. The jars were shipped directly from the laboratory to the field site, sealed in their original boxes. No preservatives were used. Bulk sampling took approximately 2 min. In preparation for subsampling using the methanol method, 10 mL of Burdick and Jackson purge-and-trap grade methanol was added to 40-mL amber glass VOA vials with Teflon septa and screwcaps. Each batch of methanol was verified clean by gas chromatography analysis before aliquoting into the VOA vials. The VOA vials were weighed, with the weight recorded onto the vial label, and then stored at 4 °C until required for sampling. During drilling operations, the VOA vials were removed from refrigerated storage and taken to the field in ice chests containing ice. Subsamples were collected at the same sampling point from the same core as those for the bulk method. Duplicates were collected on approximately 5% of the samples. After the continuous core sampler was opened, a VOA vial was removed from the ice chest, and approximately 4 g of soil was added to the vial for analysis by PNNL. Actual moist soil mass ranged from 3.67 to 11.40 g, with an average of 6.20 g. Several of the unused methanol vials were analyzed as field blanks. The type of subsampling tool used for the methanol method was dependent on the lithology. In finegrained silt and clay, a cork bore was used. Coarse and/or compacted soils were sampled using scoopulas. Sampling by the methanol method took approximately 1 min. All sample containers, whether collected for bulk or methanol methods, were immediately labeled and placed on the ice within the ice chest. The samples were transported to the respective laboratories for analysis via overnight express. Chain-of-custody forms were included with each shipment. All sampling equipment was decontaminated prior to the collection of each sample. Continuous core samplers were decontaminated by an initial wash in an Alconox/ process water mixture, followed by a process water rinse, and then a distilled water rinse. Subsample tools were decontaminated by an initial wash in an Alconox/distilled water mixture, followed by a distilled water rinse. Bulk Method Analysis. Samples collected by the bulk method were analyzed for aromatic and chlorinated compounds by gas chromatography in accordance with EPA SW-846 Proposed Method 8211 (16). The gas chromatograph was equipped with a purge-and-trap sample introduction system. A 5-g aliquot of soil sample was placed
into the purge vessel. Five milliliter of laboratory grade organic free water was spiked with the appropriate surrogate solution, and the content of the syringe was quickly transferred to the purge assembly. The purge assembly was heated to 40 °C and purged with ultra-high-purity helium. The trap was then desorbed at 225 °C for 4 min, and the sample was transferred to the gas chromatograph column through a heated transfer line. A Hewlett-Packard Model 5890 gas chromatograph, containing a 105 m long × 0.53 mm i.d. 502.2- or RTX-Volatile (Restek Corporation) megabore capillary column with a 2.0-µm film thickness, was used. Column effluent was detected with photoionization and electrolytic conductivity detectors in series. The system was calibrated using a five-point calibration. Internal standards were used to verify instrument stability. The resulting chromatogram from the sample analysis was compared to the standard chromatogram using relative retention times of the eluting peaks for compound identification and compound response factors to determine compound concentrations. Methanol Method Analysis. Samples collected by the methanol method were also analyzed for aromatic and chlorinated compounds by standard purge-and-trap, capillary column, gas chromatography using EPA Method 502.2 (17). For each analysis, sample VOA vials containing methanol and soil were removed from the refrigerator and allowed to warm to room temperature. The moist mass of soil in the vial was determined by the difference of the tare weight (including the bottle, cap, and methanol) recorded on the vial label and the final weight of the VOA vial. The VOA vial was placed in a sonication ice bath and sonicated for 5 min. The vial contents were agitated and then centrifuged to settle the suspended soil particles to the bottom. Initially, a 2.5-mL aliquot of the methanol was removed from the VOA vial with a glass pipet, placed into a 50-mL volumetric flask, and diluted to 50 mL with boiled Milli-Q water. A second VOA vial was then filled to zero headspace with the water-methanol solution from the flask and placed in an autosampler for direct injection to the purge-andtrap system. Dilutions, if necessary, were performed manually by placing a 0.1-5.0-mL aliquot of the watermethanol solution directly into the purge-and-trap with a syringe. The liquid volume in the purge assembly was adjusted to a final volume of 5 mL with boiled Milli-Q water. VOCs were purged from the purge assembly with ultrahigh-purity helium onto a Tenax trap containing a methyl silicone-coated packing and the following adsorbents: 2,6diphenylene oxide polymer, silica gel, and coconut charcoal. Samples were thermally desorbed from the trap and transferred to the gas chromatograph column through a heated transfer line. A Hewlett-Packard Model 5890 gas chromatograph, equipped with a 105 m long × 0.53 mm i.d. 502.2 (Restek Corporation) megabore capillary column with a 3.0-µm film thickness, was used. Column effluents were detected with photoionization and electrolytic conductivity detectors in series. The system was calibrated using a four-point calibration. External rather than internal standards were used to verify instrument stability. An IBM AT computer, employing Nelson Analytical Inc. Turbochrom chromatography software and Nelson Analytical Inc. dual-channel chromatography interfaces for chromatography data acquisition and analysis, was used to record the data.
Percent Recoveries for Spike Compounds Preserved in Methanol BCM
CHCl3 CCl4 m-DCB TCE 160a 90a 1a 60a
Methanol Vials Maintained in Laboratory spiked samples (100 ppb) 88 82 90 88 82 88 spiked samples (200 ppb) 86 78 89 90 81 81 mean recovery (%) 88.0 80.7 87.0 standard deviation 1.6 1.9 4.0
84 85 84 84 84.3 0.5
82 82 83 81 82.0 0.8
Methanol Vials Transported to Field soil/spiked samples (100 ppb) 77 72 77 80 73 79 soil/spiked sample (200 ppb) 84 78 81 mean recovery (%) 80.3 74.3 79.0 standard deviation 3.5 3.2 2.0 7.7 6.4 8.0 ∆b
77 79 84 80.0 3.6 4.3
71 73 79 74.0 3.6 8.0
a Vapor pressure at 20 °C in mmHg. b ∆ is the difference between mean recovery values of samples where the methanol vials were maintained in the laboratory and those transported to the field.
Results and Discussion Spike Recovery Study. Table 1 contains the results of the spike recovery study. Vapor pressures for the calibrated compounds are given in mmHg. Values are reported in percent recovery of the spike compounds determined 82 days after preparation. The top half of the table presents the recovery data for methanol that was spiked with the EPA standard solution and refrigerated at 4 °C until analyzed. The bottom half of the table presents the recovery data for the spiked methanol samples taken to the field and used to collect soil samples. No apparent trends dependent on the vapor pressures of the compounds were observed. Also, no significant differences were observed in the percent recovered of the spike compounds within and between the 100 and 200 ppb concentration levels from the methanol vials maintained in the laboratory. However, there were poorer spike recoveries from the spiked methanol vials transported to the field than from the spiked methanol vials maintained in the laboratory. Mean recoveries from the methanol vials maintained in the laboratory ranged from 88.0% for BCM to 80.7% for CHCl3. Mean recoveries from the methanol vials transported to the field ranged from 80.3% for BCM to 74.0% for TCE. Differences between mean recovery values from methanol vials transported to the field and methanol vials maintained in the laboratory ranged from 4.3 to 8.0 percentage points for m-DCB and CCl4/TCE, respectively. Because of the limited number of samples, the data may not be significant, but the negative biases suggest losses associated with sample collection and transport. Contaminant concentrations in the blank samples and the unspiked duplicate soil samples were all less than 1 ppb for the compounds of interest. The holding time of 82 days for these samples clearly exceeded the 14 days recommended by EPA for soil VOA in SW-846 (1). Despite the longer holding time, spike recoveries were within 70% of initial spike concentrations. The results of the spike recovery study demonstrate the stability of using methanol as a preservative for soil samples and the feasibility of using methanol in the field. However, these data also demonstrate the loss of volatile compounds from the methanol stored under ideal conditions (refriger-
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Scenarios Used To Determine If Sample Pair Analytical Data Could Be Used for Detected Compound Correlation Plots example concn in µg/kg scenario
data used/not used
1 2 3 4 5 6 7