Gas

Environ. Sci. Technol. 1997, 31, 853-859

Performance Evaluation of a Thermal Desorption/Gas Chromatographic/ Mass Spectrometric Method for the Characterization of Waste Tank Headspace Samples C H E N G - Y U M A , * ,† J . T O D D S K E E N , † AMY B. DINDAL,† CHARLES K. BAYNE,‡ AND ROGER A. JENKINS† Chemical and Analytical Sciences Division and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

The operating principles for the thermal desorption and gas chromatographic analysis were based on methods reported in the literature (5-13) with modifications made for our specific application (14). Method performance was evaluated by analyzing vapor-phase standards generated from 25 target analytes that had been identified in one of the most problematic tanks (15, 16). The method was validated in terms of desorption efficiency, reproducibility, stability, and linearity of the calibration, method detection limits, preanalytical holding times, and quality control limits for surrogate standard recoveries. This paper documents the performance of the TD/GC/MS method for the confident determination of volatile organic compounds (VOCs) in the headspace, reports results on a typical tank, and establishes quality control limits required to ensure the validity of the sampling and analytical processes.

Instrumentation and Analysis A thermal desorption/gas chromatography/mass spectrometry (TD/GC/MS) method was validated for the determination of volatile organic compounds collected on carbonaceous triple sorbent traps and applied to characterize samples of headspace gases collected from underground nuclear waste storage tanks at the U.S. Department of Energy’s Hanford site, in Richland, WA. Method validation used vapor-phase standards generated from 25 target analytes, including alkanes, alkyl alcohols, alkyl ketones, alkylated aromatics, and alkyl nitriles. The target analytes represent a group of compounds identified in one of the most problematic tanks. TD/GC/MS was carried out with Hewlett-Packard 5972A or 5995 GC/MS systems with modified injectors. Performance was characterized based on desorption efficiency, reproducibility, stability, and linearity of the calibration, method detection limits, preanalytical holding time, and quality control limits for surrogate standard recoveries. Desorption efficiencies were all greater than 82%, and the majority of the analytes (23 out of 25) had reproducibility values less than 24% near the method detection levels. The method was applied to the analysis of a total of 305 samples collected from the headspaces of 48 underground waste storage tanks. Quality control procedures were implemented to monitor sampling and TD/GC/MS method.

Introduction The U.S. Deparment of Energy Facility in Hanford, WA, houses 177 underground storage tanks containing a total of approximately 60 million gallons of radioactive mixed wastes generated from 40 years of nuclear weapons production (1, 2). The fugitive emissions from these tanks may pose a safety hazard due to flammability and an occupational exposure hazard due to the presence of toxic chemicals (3). Headspace samples were collected from the risers installed in the tanks (4) and were analyzed by the thermal desorption/gas chromatographic/mass spectrometric (TD/GC/MS) method described in this paper to addresss these concerns. * Corresponding author telephone: 423-576-7909; fax: 423-5767956; e-mail address: [email protected]. † Chemical and Analytical Sciences Division. ‡ Computer Science and Mathematics Division.

S0013-936X(96)00517-2 CCC: $14.00

 1997 American Chemical Society

Triple Sorbent Trap Preparation. Triple sorbent traps (TSTs; 6 mm o.d., 76 mm × 4 mm i.d. stainless steel tubing) were prepared in batches of 10. The traps were plugged at the upstream end of the sampling flow with approximately 15 mm of silanized glass wool and filled with 14-mm lengths of each of three carbonaceous adsorbents (Supelco, Bellefonte, PA) in order of increasing absorbency: Carbotrap C (20-40 mesh), Carbotrap (20-40 mesh), and Carbosieve S-III (6080 mesh). Another 15-mm plug of silanized glass wool was inserted in the downstream end. Swagelok stainless steel caps and nuts and Vespel/Graphite ferrules (1/4 in. i.d.) were used to seal the traps. Each batch of TSTs was conditioned by thermal desorption on a manifold inside a gas chromatograph oven. The traps were heated at 385 °C overnight (ca. 16 h) with helium (high purity, 99.9999%) flowing opposite to the sampling direction at a rate of 10-60 mL/min. One trap was randomly selected from each batch of 10 and analyzed by TD/GC/MS prior to spiking and/or sample collection to ensure the cleanliness of the batch. Gas-Phase VOC Standard Preparation and Spiking of TSTs. The methodology for VOC standard generation used in this study is similar to those described in the static dilution method (17) and the EPA Method TO-1 (12), except for differences in analytes and the standard bottle. Table 1 lists the gas standard mixture used for this study that was generated from a mixture of neat liquid standards in a concentration range of 0.2-1.0 µg/mL. Deuterated standards were obtained from Isotec, Inc. (Miamisbury, OH), and target standards were obtained from Aldrich Chemical Co. Inc. (Milwaukee, WI) except for heptanenitrile, which was purchased from Pfaltz & Bauer, Inc. (Waterbury, CT). Gas standards were generated by injecting 1-3 µL of neat liquid standard mixture through a Mininert valve into a 250-mL preheated (70 °C) gas-tight glass bottle. The mixture was stirred with a magnetic stirring bar and glass beads for 30 min to ensure complete vaporization of the liquid. Aliquots of this gas standard mixture were spiked onto TSTs via a preheated (70 °C) injector to obtain concentrations of 20-1000 ng/trap. The injector consisted of a 1/4-in. stainless steel Swagelok tee; the TST was attached to one end of the tee, while helium flowed through it from the opposite end at 200 mL/min. Using a preheated (70 °C) syringe, gas standard was slowly injected into the stream of helium through a septum attached to the third end of the tee (at a 90° angle to the trap). A total of 400 mL of helium was passed through the trap after the injection to ensure even distribution of the VOCs on the trap. Each of the TSTs shipped to Hanford for headspace sampling was spiked in the same manner with a vapor phase mixture of three surrogate

VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Desorption Efficiency, Reproducibility, Linearity, and Detection Limits for Target Analytes and Internal and Surrogate Standards Analyzed by TD/GC/MS compda

analyte

CAS No.

desorption efficiency (%)

reproducibilityb (%RSD)

linearityc ( R 2)

MDLd (ng/trap)

CRLe (ng/trap)

MDLf (ppbv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

benzene-d6 (IS)g acetonitrile acetone vinylidene chloride methylene chloride propanenitrile butanal hexane hexafluorobenzene (SS)i benzene 1-butanol butanenitrile 2-pentanone heptane ethylbenzenene-d10 (IS) toluene-d8 (SS) toluene pentanenitrile 2-hexanone octane hexanenitrile 2-heptanone nonane bromobenzene-d5 (SS) heptanenitrile 2-octanone dodecane-d26 (IS) decane undecane dodecane tridecane

1076-43-3 75-05-8 64-67-1 75-35-4 75-09-2 107-12-0 123-72-8 110-54-3 392-56-3 71-43-2 71-36-3 109-74-0 107-87-9 142-82-5 25837-05-2 2037-26-5 108-88-3 110-59-8 591-78-6 111-65-9 628-73-9 110-43-0 111-84-2 4165-57-5 629-08-3 111-13-7 112-40-3 124-18-5 1120-21-4 112-40-3 629-50-5

97 105 106 114 109 97 85 104 96 93 113 88 90 92 102 98 99 97 97 99 99 100 95 99 118 93 91 104 101 96 82

nah 0.6-10.4 4.2-10.1 3.3-10.6 4.1-19.4 0.9-7.9 9.4-25.9 1.3-24.6 na 1.0-13.2 1.6-22.1 0.9-12.0 1.4-16.0 1.2-8.8 na na 0.9-16.5 2.5-5.4 2.3-23.9 2.2-6.1 0.7-7.9 0.5-16.7 0.8-5.7 na 5.0-10.0 2.0-13.1 na 2.8-13.3 2.6-13.2 6.3-27.2 13-33.8

na 0.990-0.998 0.987-0.999 0.969-0.995 0.991-0.997 0.994-0.999 0.994-0.996 0.983-0.991 na 0.978-0.999 0.993-1.000 0.996-0.999 0.997-0.999 0.979-0.997 na na 0.976-0.985 0.997-0.999 0.993-0.998 0.979-0.997 0.994-0.997 0.989-0.998 0.968-0.999 na 0.996-1.000 0.980-0.990 na 0.888-0.977 0.914-0.978 0.957-0.997 0.981-0.997

na 3.6 5.9 10.3 19.8 3.7 15.3 10.3 na 6.0 7.0 6.4 4.3 2.1 na na 6.5 2.2 3.4 2.0 2.5 3.2 1.4 na 4.1 4.5 na 2.6 3.7 7.5 12.1

na 4.4 7.1 12.3 23.3 4.5 18.1 12.1 na 7.3 8.3 7.7 5.2 2.6 na na 7.8 2.7 4.1 2.5 3.1 3.9 1.8 na 5.0 5.4 na 3.2 4.5 8.9 14.5

na 2.2 2.3 2.3 5.3 1.6 4.7 2.6 na 1.7 2.1 1.9 1.0 0.5 na na 1.7 0.5 0.7 0.4 0.7 0.6 0.2 na 0.8 0.7 na 0.5 0.6 0.9 1.5

a Compound number refers to labeled peaks in Figure 2. b % RSD range of RFs for triplicate analyses on TSTs that were spiked with target analytes at concentration of 5, 10, 20, 40, 100, and 200 ng/trap. c Range of correlation coefficients (R 2) for 9 five-point calibration curves over a nominal concentration range of 20-500 ng/trap. d Method detection limit (ng/trap). e Certified reporting limit (ng/trap). f Estimation is based on highest sampling volume analyzed (1000 mL). g IS, internal standard. h na, not applicable. i SS, surrogate standard.

standards at a nominal concentration of 300 ng/trap. Quality control checks of surrogate spiking consistency were performed periodically during spiking by analyzing spiked TSTs using TD/GC/MS. Surrogate spiking consistency was typically within 10% RSD. Sample Collection at the Hanford Site. The vapor sampling system (VSS) was used to collect VOCs on TSTs from the waste tank headspaces by the collaborators at the Hanford site (1, 4). The VSS consists of a mobile laboratory, a hot-water jacketed stainless steel probe that was inserted into the tank headspace through a riser, and heated stainless steel transfer tubing that connects the mobile laboratory with the probe (4). A riser is a stainless steel pipe (4 in. i.d., 20 ft long) penetrating through the soil layer and tank shell. Approximately 3 ft of the riser extends above ground level and serves as a headspace sampling port. Headspace vapor was drawn from the tank and through the transfer tubing and sampling manifold by an air pump. Air flow within the VSS is directed by electrically or pneumatically actuated valves. Instrumentation housed in the mobile laboratory provides system temperature monitoring and control, absolute and differential system pressure monitoring, mass flow metering and controlling, and hydrocarbon vapor monitoring. A key feature of the VSS is its use of heated sample transfer tubing and a heated sampling manifold. Maintaining the system at an electronically controlled, elevated temperature prevents vapor condensation and reduces vapor adsorption on surfaces exposed to sample air. Headspace vapor samples were collected on TSTs at the sorbent trap station of the sampling manifold. Most samples were taken in duplicate. Highly accurate mass flow controllers were used to measure and control the flow rate of sample air through the TSTs. These

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were located downstream of the sorbent trap station and of in-line driers, which removed water vapor from the air before it was metered. Depending on the VOC concentrations in the tank headspace, multiple air samples of 0.2-1.0 L were collected onto TSTs at flow rates of 50-200 mL/min. Trip blank and field blank TSTs provided evidence that the samples were not contaminated during shipping and handling. Three TST blanks and three system blanks were also analyzed concurrently with samples. Because of concerns about radioactive contamination, all headspace samples were taken through small HEPA filters to screen out particulate-borne activity. In addition, sorbent traps was sacrificed at the end of each sampling run. Its contents were expelled and scanned for radioactive vapor-phase components. Typically, these readings indicated radioactivity at background levels. TD/GC/MS Analysis of VOCs Collected on TSTs. TD/ GC/MS analysis was performed on either a Hewlett-Packard 5972A or 5995 GC/MS system. A modified injector port was used to adapt an in-house manufactured short-path desorber to transfer desorbed VOCs directly into a cryofocusing loop in a splitless mode (Figure 1). A male Swagelok 1/4-in. nut was welded on the septum retainer nut for direct connection of TST traps. A section of aluminum-clad fused silica capillary tubing (0.53 mm i.d., 5 cm long) was inserted through the septum to serve as a transfer line to the glass liner in the injector port. A capillary inlet adaptor (Restek, Port Matilda, PA) with a 1/16-in. Swagelok male fitting was installed at the base of the injector port. A cryofocusing loop, made with 15 cm of stainless steel tubing (0.04 in. i.d., 1/16 in. o.d.), was connected to the inlet adaptor with a 1/16-in. Vespel/graphite ferrule. A capillary fused silica column (Rtx-5, 0.32 mm i.d., 60 m, 1 µm film thickness) was connected to the stainless

FIGURE 1. Schematic diagram for thermal desorption/gas chromatography/mass spectrometry system. steel cryofocusing loop via a Valco three-way ZDV switching valve. A flash heating tube furnace, capable of heating TSTs to 400 °C in 1.5 min, was constructed from a coiled resistor cable heater (Watlow Cable Heater Co., St. Louis, MO, 50 mm × 7 mm i.d., 120 V, 240 W, 2 A) encased in a ceramic tube using high-temperature cement (14). In a typical analysis, the cryofocusing loop was first immersed in a liquid nitrogen bath. A TST trap was then placed in the tube furnace and connected to the modified septum retainer with the upstream end of the trap nearest the injector. During the flash desorbing process, the carrier gas (helium) flow was increased by switching the three-way valve to the “vent” position, and the trap was purged at a flow rate of 50 mL/min for 7 min in the direction opposite to the sampling flow. The liquid nitrogen bath was removed from the cryofocusing loop, and the oven temperature program (10 °C for 10 min, 3 °C/min to 230 °C, hold for 17 min at final temperature) was initiated immediately. The three-way valve was switched back to the injection position to transfer the desorbed VOCs from the cryofocusing loop to the head of the column. The flow rate of carrier gas was held at 1.00 mL/min throughout the GC run by an electronic pressure controller with vacuum compensation. The injector temperature was held at 250 °C, and the GC/MS transfer line temperature was held at 280 °C. Electron impact spectra were obtained with an electron energy of 70 eV, and mass spectral data were acquired with a scan rate of 1.6 scan/s over a mass range of 29-300 amu. The integrated area of the total ions or of a selected ion was obtained for each component for data manipulation. Prior to analysis of the TST samples, the traps were dried by passing approximately 2 L of dry helium through the trap to prevent water vapor from freezing in the cryofocusing loop. The sample traps were then spiked with a vapor phase mixture of three internal standards (ca. 300 ng each) for quantification of target analytes by the internal standard method. All target analytes were quantified using a validated software package (Hewlett-Packard EnviroQuant, Version C.00.02).

Results and Discussion Selection of Target Analytes and Reference Standards. During the initial phase of this study, one of the underground waste storage tanks was selected for a detailed chemical

FIGURE 2. Total ion chromatogram (a) for target analyte standards, internal standards, and surrogate standards and total ion chromatogram (b) for a headspace sample collected from a representative underground storage tank at DOE’s Hanford site. The legend for labeled peaks appears in Table 1. characterization. This tank was chosen because it contained a large volume of very complex organic nuclear waste (1, 2) and was the source of some health and safety problems for tank farm workers (3). A team of toxicologists established a list of target analytes based on the VOCs identified in the TST samples collected from the headspace of this tank during several sampling events (15, 16). The VOCs on this list (Table 1) are relevant to the tank headspace flammability and/or to the safety and health issues for fugitive emissions. Prior to sampling at Hanford, three surrogate standards (identified by SS in Table 1) were spiked on the traps in order to monitor the shipping, handling, sampling, and analysis process. Internal standards (identified by IS in Table 1) were spiked on the TSTs just prior to TD/GC/MS analysis to quantify the target analytes. The selection of SS and IS was based on the following characteristics: (1) thermal stability, (2) absence in the samples (deuterated analogs of the target analytes are ideal), and (3) chemical similarity to the target analytes (volatility, GC retention time, etc). In addition, the internal standards were chosen to bracket the target analytes at appropriate retention time intervals for quantification. Figure 2 illustrates the total ion chromatogram for 25 target analytes, 3 surrogate standards, and 3 internal standards. Thermal Desorption Efficiency. Air-based VOC standards, generated by the static dilution method (17), have several advantages over the standard method of spiking a liquid standard onto a trap. They more accurately represent the air sample matrix than do liquid standards. The active sites on the sorbent are available to trap small molecules because the large deposition of solvent on the sorbent bed has been eliminated. The capillary column performance is also improved because injection of a large volume of solvent


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can cause flooding of the column (18). For each gas standard, the desorption efficiency was calculated as the ratio of integrated area (AD) for a selected ion generated from TD/ GC/MS to that generated from direct injection (AI) of the same gas-phase standard followed by GC/MS analysis. As indicated in Table 1, the desorption efficiency (AD/AI × 100%) is greater than 82% for all analytes. Recently, a number of investigations (19, 20) have examined the issue of artifact formation from VOCs that have been collected on the carbonaceous media. The large surface area of carbonaceous media may act as a catalytic surface under the thermal desorption conditions to facilitate a thermal decomposition reaction for VOCs. In order to address this issue, we compared the total ion chromatogram obtained from direct injection of a gas-phase standard mixture with chromatograms obtained from TSTs spiked with the same standard mixture. The two sets of total ion chromatograms were virtually identical, suggesting that there is no chromatographable artifact formation from thermal desorption. The only indication of an artifact formation is a small frontal peak observed when 2-propanol was in the standard mixture. Systematic examination of the desorption of 2-propanol from TSTs revealed that approximately 5% of 2-propanol is converted to acetone during thermal desorption. Linearity and Reproducibility. TST samples were spiked in triplicate with gas-phase standards that included three internal standards (IS) and 25 target analytes at six concentrations ranging from 5 to 200 ng/trap. TST samples were spiked and analyzed by TD/GC/MS in random order over a period of 2 weeks. The integrated area of each target analyte and corresponding IS was used to calculate the response factor [RF ) (amount of IS) × (area of analyte)/(amount of analyte) × (area of IS)]. Table 1 summarizes the percent relative standard deviations (% RSDs) of the RFs that were observed at the six concentrations. The majority of % RSDs for the entire procedure (including gas standard generation, TST spiking, and desorption) are less than 20% for the TSTs spiked at concentrations greater than 5 ng/trap. As expected, reproducibility decreases as the concentrations of target analytes approach the method detection limits. For example, hexane exhibits fairly poor reproducibility (RSD ) 25%) at a concentration of 5 ng/trap, which is below its method detection limit (Table 1). In addition, large variations seem to be associated with the stability and volatility of the compounds. Butanal and 1-butanol, once spiked on the TST, degrade within 1 day (described in detail in the Pre-analytical Holding Time section). These two compounds are expected to have fairly high RSDs at low concentrations. Tridecane also exhibits an abnormally high RSD (33.8%) as well as low desorption efficiency (82%), presumably due to its high boiling point (234 °C), which may lead to incomplete evaporation in the gas standard preparation process. A five-point calibration curve was constructed for each target analyte with concentrations ranging from 20 to 1000 ng/trap. The area ratios (target analyte to internal standard) of a selected ion were measured at each concentration. A linear regression curve was fitted to the data to construct a target analyte calibration curve. Nine calibration curves were constructed for each target analyte over a period of 8 months. The correlation coefficient (R 2) for all of the target analytes fell in a range of 0.888-1.000 (Table 1). In addition, longterm reproducibility of response was evaluated for selected target analytes. This evaluation was made by carrying out a mid-level calibration (180 ng/trap) when field samples were analyzed. The response factors (RFs) for 39 mid-level calibrations generated over a period of 8 months deviate less than 15% from the average response factors for the five-point calibrations (Table 2). These deviations served as an indicator of the calibration RF stability. If the deviations were greater than 25% for more than three target analytes, the five-point

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TABLE 2. Long-Term Reproducibility of Mid-Level Calibration Response Factor for Target Analytes Analyzed by TD/GC/MSa target analyte

av % deva

min % dev

max % dev

acetonitrile acetone vinylidene chloride methylene chloride propanenitrile hexane benzene butanenitrile 2-pentanone heptane toluene pentanenitrile 2-hexanone octane hexanenitrile 2-heptanone nonane heptanenitrile decane undecane

9.9 15 14 7.4 6.6 10 4.7 8.2 10 11 3.3 5.3 7.9 9.0 5.3 9.4 6.2 13 12 15

3.4 1.8 1.2 1.9 0.1 0.3 0.7 0.3 0.2 0.7 0.2 0.4 0.8 0.1 0.0 0.2 0.1 0.7 1.0 0.5

26 27 26 18 16 21 11 22 30 27 13 15 19 17 19 29 16 24 29 22

a Deviation (%) of the continuing calibration response factor (CCRF) from the average five-point calibration response factor (AVGRF); 39 CCRF and 9 AVGRF obtained over a period of 8 months.

calibration curve was reconstructed (21). In general, the calibration RF remained constant for at least 1 month. Method Detection Limit and Certified Reporting Limit. Two common estimates of low concentration measurement capabilities are the method detection limit (MDL) specified by the U.S. Environmental Protection Agency (22) and the certified reporting limit (CRL) specified by the U.S. Army Environmental Center (23). The principles for calculating the MDL and CRL are described in detail by Grant et al. (24) and Hubaux and Vos (25). The MDL will have a 1% falsepositive error and a 50% false-negative error. By contrast, the CRL will have both a false-positive and a false-negative error of 5%. Determination of MDLs and CRLs for this study was carried out by analyzing triplicate calibration TSTs that had been spiked with internal standards and target analytes at concentrations of 5, 10, and 20 ng/trap. On three consecutive days, the calibration TSTs were analyzed by TD/ GC/MS in a random order. The ratios of the integrated area of a selected ion for each target analyte to the integrated area of a selected ion for an appropriate internal standard were calculated. A linear regression between the area ratios and the spiked concentration was performed by the method of least squares using a SAS software package (26). Statistical analysis of these data also included testing for zero intercept, lack-of-fit, and variance homogeneity. Table 1 presents MDLs and CRLs for the 25 target analytes. In general, for analytes with area ratios that have small % RSD in the low concentration range (i.e., 5-20 ng/trap), the MDL and CRL estimates show remarkably good agreement, as in the cases of propane nitrile, heptane, pentane nitrile, octane, hexane nitrile, and nonane. However, for analytes with a large % RSD in the low concentration range, the CRLs are much greater than MDLs, as in the cases of methylene chloride, vinylidine chloride, butanal, hexane, 1-butanol, dodecane, and tridecane. Based on the highest sampling volume (1000 mL) analyzed, the MDLs range from 0.4 to 5 ppbv, a range normally achieved in most of the EPA air monitoring methods (12). Determination of VOCs in Headspace Samples Collected from a Hanford Underground Storage Tank. During the period from January 1994 to September 1995, a total of 305 sample TSTs were analyzed. These samples were collected from the headspace in 48 underground nuclear waste storage tanks. A majority of the tanks that contain significant concentrations of VOCs in the headspace exhibited charac-

TABLE 3. Summary of Target Analyte Concentrations of a Representative Hanford Underground Storage Tank target analytes

CAS No.

mean concn (mg/m3)a

mean concn (ppbv)a

% RSD

acetonitrile acetone propanenitrile butanal hexane benzene 1-butanol butanenitrile 2-pentanone heptane toluene pentanenitrile 2-hexanone octane hexanenitrile 2-heptanone nonane heptanenitrile 2-octanone decane undecane dodecane tridecane

75-05-8 67-64-1 107-12-0 123-72-8 110-54-3 71-43-2 71-36-3 109-74-0 107-87-9 142-82-5 108-88-3 110-59-8 591-78-6 111-65-9 628-73-9 110-43-0 111-84-2 629-08-3 111-13-7 124-18-5 1120-21-4 112-70-3 629-50-5

0.63 3.20 0.17 1.10 0.63 0.04 15.00 0.18 0.48 0.30 0.05 0.09 0.27 0.22 0.09 0.47 0.22 0.12 0.24 0.23 1.80 8.50 17.00

340 1200 70 36 170 11 4600 59 130 67 13 23 59 43 20 93 39 23 43 37 260 1100 2000

5.3 0.6 5.1 36.1 11.2 5.5 8.9 8.3 10.8 16.4 6.9 6.5 7.8 5.3 11.0 9.9 8.7 11.7 12.8 7.3 5.4 12.7 14.0

a

TABLE 4. Practical Holding Times (PRTS) for Selected Target Analytes Spiked onto TSTs PRT (days)

Mean concentration of three determinations.

teristic total ion chromatograms as shown in Figure 2. Rather than reporting the findings resulting from the analysis of each tank, we have reported the results from the determination of the target analytes and tentatively identified compounds (TICs) in the headspace samples collected from one representative underground storage tank (Table 3). Based on triplicate analysis, 1-butanol was the most prevalent target analyte, present at a measured mean concentration of 4600 ppbv. Acetone, dodecane, and tridecane levels also were significant, measured at mean concentrations between 1000 and 2000 ppbv. All other target analytes were measured below 400 ppbv. An additional 66 compounds were tentatively identified. Tetradecane was the most predominant TIC, estimated at a mean concentration of 410 ppbv. Approximately 46% of the TICs were long, branched-chain alkanes (C8-C14). Approximately 15% of the TICs were alkanones and 20% were cyclic hydrocarbons. Total organic concentrations were estimated at 51 mg/m3 for the target analytes and 34 mg/m3 for non-target analytes, yielding a mean total organic concentration of 85 mg/m3 (27). Similar target analytes were also found in samples collected from the remaining storage tanks. The mean concentrations of the analytes ranged from less than 1 ppbv to 4.0 ppmv, with acetonitrile, acetone, butanal, 1-butanol, dodecane, and tridecane being the most abundant components found in the tank headspaces.

a

compd

refrigerated

ambient

acetone methylene chloride propanenitrile butanal hexane bezene 1-butanol 2-pentanone toluene pentanenitrile nonane heptanenitrile 2-octanone undecane dodecane tridecane

51 84 84 1a 84 84 17 34 84 56 84 84 84 84 84 84

32 84 30 5b 84 84 2b 2b 84 30 84 84 1b 84 84 84

Log term.

b

Inverse term; all others, zero-order.

VOC concentration measurements were carried out over a period of 84 days (30). A total of 56 TSTs were spiked with 16 selected target analytes at concentrations ranging from 330 to 663 ng/trap. The spiked TSTs were divided into two groups with one group stored under refrigeration (about 2 °C) and the other at ambient temperature (about 22 °C). Four TSTs were then selected randomly at day 0, 7, 14, 21, 28, 63, and 84 to perform replicate analyses by thermal desorption/ gas chromatography/flame ionization detection for each of the storage temperatures. Just prior to the analysis, each TST was spiked with three internal standards (hexafluorobenzene, ethylbenzene-d10, and dodecane-d26) to quantify the target analyte concentration against the three-point calibration curve constructed on the same day.

Quality Assurance and Quality Control

PRTs depend on approximating models to represent the degradation of analyte concentrations with time (28, 29). The zero-order kinetic model (concentration ) intercept + slope × day) represents the majority of the analytes’ degradation. This linear model was fitted to all the data using the methods of least squares (31). The PRT method also includes statistical tests to examine if the slopes were significantly less than zero at the 5% significance level. Analytes with slopes that were not significantly less than zero showed no decrease in analyte concentrations and were assigned PRT values of 84 days. If the slopes were significantly less than zero, the statistical analysis also examined the models for lack of fit to determine if alternative models should be investigated that might better approximate the data. Due to the lack of fit at the 5% significance level, four analytes (butanal, 1-butanol, 2-octanone, and 2-pentanone) had to be fitted with different approximating models (28), either the log-term model [concentration ) intercept + slope × loge (day)] or the inverseterm model [concentration ) intercept + slope/day].

Pre-analytical Holding Time Determination. A holding time study was conducted on a representative subset of the target analytes (Table 4) using the practical reporting time (PRT) method developed by Bayne et al. (28, 29). The purpose of a PRT is to specify how long a sample can be stored with reasonable assurance that the initial analyte concentration has not changed significantly. The PRT is defined as the time (in days) when there is a 15% risk that the analyte level will fall below a critical concentration. The critical concentration is determined on day zero of the holding time study; it is the concentration below which there is only a 5% chance, due to measurement error, that a measured concentration would be observed. A significant change has occurred when the concentration falls below this critical concentration (28). The

Table 4 summarizes PRTs at refrigerated and ambient temperatures and the approximating models used for their determinations. Eleven of the 16 analytes (acetone, butanal, 1-butanol, 2-pentanone, and pentane nitrile were the exceptions) stored at refrigerated temperatures showed no significant degradation during 84 days. Nine of the 16 analytes (acetone, propanenitrile, butanal, 1-butanol, 2-pentanone, pentanenitrile, and 2-octanone were the exceptions) stored at ambient temperatures showed no significant degradation during 84 days. The PRTs for these nine analytes under ambient conditions were at least 84 days. 1-Butanol, 2-pentanone, and 2-octanone were stable for 17, 34, and 84 days, respectively, under refrigerated conditions and became unstable (e2 days) under ambient conditions.


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Surrogate Standard Recoveries. Surrogate standard recoveries were determined by comparing the surrogate quantity on the sample traps with those on the archived traps (reserved from the original spiking process prior to TST shipment). The surrogate recoveries determined from 140 samples (24 tanks) were statistically examined in order to establish quality control (QC) limits. These QC limits were used to evaluate the recovery precision as well as the accuracy of future sampling events. After eliminating 13 samples that were determined to be statistical outliers (32), the mean surrogate recoveries with one standard deviation in parentheses were calculated to be the following: 88.4% (14.9%) for hexafluorobenzene, 93.6% (12.6%) for toluene-d8, and 97.8% (8.1%) for bromobenzened5. Based on these calculated mean recoveries, the QC limits for each sample mean recovery for surrogates can be established. The QC limits use a 95% confidence interval calculated by

mean recovery ( 2 ×

σˆ xn

where σˆ is the estimated standard deviation from the recovery study (e.g., 14.9% for hexafluorobenzene) and n is the number of samples analyzed in the sampling event (usually n ) 3 or 4). The multiplying factor 2 is used because a large number of samples (n ) 140) were used to estimate the standard deviation, and a normal distribution can be used to approximate the probabilities for the mean recovery values (ref 33, pp 201-206). The observed mean recoveries from n samples should be in the QC limits (0.95 probability). If the observed mean recoveries are within the 95% confidence interval, one can assume that no bias has been introduced in the sampling and the analysis processes. These QC limits can also be used to evaluate the recovery accuracy for the entire process, including sampling and analysis. The observed variance (S2 ) can be compared with the estimated variance (σˆ 2) to determine whether S2 is below the upper 95% confidence limit:

[

]

σˆ 2χ2 (0.95, n - 1) ) 0.95 n-1

Pr S2 e

The upper 95% confidence limit on S2 is based on the χ2 distribution with n - 1 degree of freedom (ref 33, pp 239241) where χ2 (0.95, n - 1) is the upper 0.95 percentile value of the χ2 distribution. If the observed variance (S2) is below the upper 95% confidence limit, one can assume that the recovery precision is adequate, otherwise the measurement variation should be investigated. Individual results of the surrogate recoveries for additional 165 samples collected from 24 underground storage tanks were statistically analyzed. Groups of samples from individual tanks were evaluated against the QC limits and the upper 95% confidence interval for S2 values to determine whether the recovery efficiency and precision were adequate. For the data from 12 tanks, all three mean surrogate recovery values were within the established QC limits. Of the remining tanks, nine tanks had one mean recovery outside the QC limits, two tanks had two mean recoveries outside the QC limits, and one tank had three mean recoveries outside the QC limits. Approximately 77% of the tanks had observed surrogate recovery S2 values below the upper 95% confidence limit. High surrogate recoveries indicate that the source matrix had little or no effect on the efficient retention of analytes on the solid sorbent bed and that sample handling and analysis activities were executed properly.

Acknowledgments This research was sponsored by Westinghouse Hanford, Richland Operations Office, U.S. Department of Energy, under

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Contract DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. We thank Dr. M. R. Guerin of the Chemical and Analytical Sciences Division for his critical comments.

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Received for review June 17, 1996. Revised manuscript received September 13, 1996. Accepted October 22, 1996.X ES9605174 X

Abstract published in Advance ACS Abstracts, January 1, 1997.


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