Anal. Chem. 2008, 80, 1328-1335
Continuous Determination of High-Vapor-Phase Concentrations of Tetrachloroethylene Using On-Line Mass Spectrometry Dennis Fine,*,† Michael C. Brooks,‡ Mustafa Bob,‡ Susan Mravik,‡ and Lynn Wood‡
Shaw Environmental and Infrastructure, 919 Kerr Research Drive, Ada, Oklahoma 74821-1198, and Ground Water and Ecosystems Restoration Division, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, Oklahoma 74821-1198
The instrumentation and methods described here were developed to support laboratory-scale experiments to investigate the benefits of partial removal of dense nonaqueous phase liquid (DNAPL) contaminants from impacted aquifers. Recently, the benefits of conducting such remediation activity have been questioned due to the impracticality of complete DNAPL removal, which consequently hinders the restoration of impacted groundwater to drinking water standards.1,2 However, one benefit that may be achieved through partial DNAPL mass removal is a reduction in the contaminant mass flux in the dissolved-phase
plume from impacted areas.3,4 Consequently, experiments were conducted to investigate the response of contaminant flux to DNAPL mass reduction using a two-dimensional (2D) flow chamber containing water and silica sand as a model of an impacted aquifer. Experiments were designed to measure changes in contaminant flux as a function of aquifer heterogeneity and as a function of DNAPL mass removal using a typical remediation technique (i.e., air sparging).5,6 The experimental chamber consisted of a small glass flow cell that contained silica sand that was saturated with deionized, deaerated water. Before adding water to the chamber, all the air in the system was replaced with carbon dioxide to ensure that any trapped gas would readily dissolve in the water. A volume of tetrachloroethylene (PCE) was added to the chamber, and once the PCE had achieved an apparent steady-state distribution, water was pumped horizontally through the cell. The water was sampled at an effluent port on the side of the chamber, and the concentration of dissolved PCE was measured to determine the contaminant flux (mg cm-2 min-1). After the initial flux measurement, the cell was sparged with carbon dioxide to remove a fraction of the PCE. The chamber was then flushed with water to reestablish steady-state conditions, and the flux measurements were repeated. The experiments were repeated with flow chambers containing different silica sand sizes and distributions. These experiments required accurate and real-time determination of the PCE mass removed during a sparging event. Therefore, an on-line sampling system and an ion trap mass spectrometer (MS) were assembled to determine the concentration of PCE vapor sparged from the 2D flow cell and, consequently, the total mass of PCE removed during each sparge cycle. Several requirements were placed on the development of the instrument and the method. First, during the initial sparge of the chamber, the concentration of the PCE vapor exiting the flow cell would be close to its saturation vapor concentration. The vapor pressure of
* Corresponding author. Phone: 580-436-8669. Fax: 580-436-8635. E-mail:
[email protected]. † Shaw Environmental and Infrastructure. ‡ U.S. Environmental Protection Agency. (1) EPA. The DNAPL Remediation Challenge: Is There a Case for Source Depletion?; EPA/600/R-03/143; NRMRL, ORD, USEPA, Cincinnati, OH, 2003. (2) Stroo, H. F.; Unger, M.; Ward, C.; Kavanaugh, M.; Vogel, C.; Leeson, A.; Marqusee, J.; Smith, B. Environ. Sci. Technol. 2003, 37, 224A.
(3) Rao, P.; Jawitz, J.; Enfield, C.; Falta, R.; Annable, M.; Wood, L. Technology integration for contaminated site remediation: clean-up goals and performance criteria. In Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution; Thornton, S. F., Oswald, S. E., Eds.; IAHS Press: Oxfordshire, U.K., 2002; pp 571-578. (4) Soga, K.; Page, J.; Illangasekare, T. J. Hazard. Mater. 2004, 110 (1-3), 13. (5) Totten, C.; Anabele, M.; Jawitz, J.; Delfino, J. Environ. Sci. Technol. 2007, 41, 1622. (6) Suchomel, E.; Pennell, K. Environ. Sci. Technol. 2006, 40, 6110.
A method was developed to determine the vapor concentration of tetrachloroethylene (PCE) at and below its equilibrium vapor-phase concentration, 168 000 µg/L (25 °C). Vapor samples were drawn by vacuum into a sixport sampling valve and injected through a jet separator into an ion trap mass spectrometer (MS). This on-line MS can continuously sample a vapor stream and provide vapor concentrations every 30 s. Calibration of the instrument was done by creating a saturated stream of PCE vapor, sampling the vapor with the on-line MS and with thermal desorption tubes, and correlating the peak area response from the MS with the vapor concentration determined by automated thermal desorption gas chromatography mass spectrometry. Dilution of the saturated stream provided lower concentrations of PCE vapor. The method was developed to monitor the vapor concentration of PCE that was sparged from a twodimensional flow chamber and for determination of the total PCE mass removed during each sparge event. The method has potential application for analysis of gas-phase tracers.
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10.1021/ac701859n CCC: $40.75
© 2008 American Chemical Society Published on Web 01/19/2008
Figure 1. Diagram of on-line sampling system consisting of a GC oven, six-port sampling valve, valve actuator, jet separator, needle valve for vacuum flow control, and vacuum pumps.
PCE, 18.2 mmHg at 25 °C,7 was used to estimate the saturated vapor concentration. Assuming an ideal gas, an estimate of the vapor concentration of PCE at this vapor pressure and temperature is calculated to be 168 000 µg/L, 24 900 µL/L (ppm), or 2.49% v/v. Calibration of an instrument at or near this concentration is difficult because commercially available, pressurized gas standards of PCE are not available above 500 ppm.8 As concentrations approach the saturation vapor concentration, temperature changes during shipping would cause condensation of the vapor. The presence of water in the sample vapor makes the preparation of a calibrated standard more difficult. Thus, to calibrate an on-line MS at high vapor concentrations we generated a controlled, dynamic PCE vapor stream that is equivalent to the stream that would flow from the 2D flow cell. This vapor stream was created by sparging PCE and water in a purge-and-trap vessel with carbon dioxide. Dilution of the saturated vapor stream with carbon dioxide saturated with water allowed preparation of lower vapor concentrations of PCE. Calibration of the response of the on-line MS was done externally by sampling the vapor stream with thermal desorption tubes and correlating the on-line MS response to the vapor concentration determined by automated thermal desorption gas chromatography mass spectrometry (ATD/GC/MS). A second requirement for the instrument and the method was that concentration measurements of PCE would be determined continuously at intervals of less than 1 min. This would provide sensitive, temporal profiles of the vapor concentrations as PCE mass was removed by carbon dioxide flow during the sparge cycle. (7) CRC Handbook of Chemistry and Physics, 77th ed.; Linde, D., Ed.; CRC Press: New York, 1996. (8) Technical Service, Scott Specialty Gases, Plumsteadville, PA. Personal communication, 2007.
The flow chamber had three ports at the bottom of the cell. As these were switched on and off, different regions of the cell were sparged with carbon dioxide and spikes of PCE vapor would exit the chamber as zones containing a higher or lower mass of PCE were sparged. Rapid and continuous determinations of vapor concentrations utilizing an on-line MS can be done using a multiport sampling valve9 or a vacuum/helium flow switching interface.10 With our system, a vacuum was required to draw the sample into a sampling valve since a back pressure was not allowed in the vapor stream (i.e., to simulate natural conditions, the top of the flow chamber was maintained at approximately atmospheric pressure). With this system, the carrier gas of the sample must be the same as the standard, since the analyte response of the MS changes when oxygen, nitrogen, or carbon dioxide are present compared to helium.11 This problem can be minimized if the carrier gas is separated from the analyte before the sample enters the MS, which can be done using a fixed-gap jet separator12 or a short, small i.d. capillary column connected directly to the MS.10 With a jet separator, helium, carbon dioxide, and water vapor diffuse instantaneously into the vacuum as the sample crosses the fixed gap. Heavier molecules with higher momentum cross the gap and enter the capillary connected to the MS source. For this study, a jet separator was used to interface a six-port sampling valve to an ion trap MS. (9) Richter, P.; Zunige, C.; Calderon, K.; Carrasco, R. J. Chromatogr., A 2006, 1102, 232. (10) Arnold, N.; McClennen, W.; Meuzelaar, H. Anal. Chem. 1991, 63, 299. (11) Cameron, D.; Hemberger, P.; Alarid, J.; Leibman, C.; Williams, J. J. Am. Soc. Mass Spectrom. 1993, 4, 774. (12) Cooks, R. Final Report on Proposal to Develop and Test a Membrane Sampling Module for the Extraction of Volatile Organic Compounds from Water. OSTI ID:10103275; USDOE, Washington, DC, 1993.
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Figure 2. Profile of PCE pulses for a vapor stream sampled every 30 s. A purge-and-trap vessel containing 1.5 mL of PCE and 1.5 mL of water was purged with carbon dioxide. A 90% dilution of the PCE stream was obtained by adding a second carbon dioxide stream saturated with water.
A method for the rapid and continuous measurement of the concentration of PCE near its saturated vapor concentration is presented in this paper. This method provides an effective tool for monitoring of DNAPL mass removal during laboratory experiments and can be extended to measure the elution of other volatile compounds in different sparging experiments as well as measuring the breakthrough curves of gas-phase tracers. EXPERIMENTAL SECTION On-Line Sampling Mass Spectrometer. PCE was sparged from the flow chamber with carbon dioxide flowing at 200 mL/ min through ports in the bottom of the cell. A fraction of the vapor flow, ∼4 mL/min, was drawn through a six-port Valco sampling valve (Figure 1) by a mechanical vacuum pump. The sampling valve located inside a GC oven was actuated by a Valco microelectric actuator residing outside the oven. The actuator was controlled by a laptop computer via an RS-232 serial interface. A “C” compiler program loaded on the PC was used to control the time between sampling and injection and the total time that the valve would continue to actuate. The sampling and injection times were each set at 15 s. A Key high-vacuum metering/needle valve was used to regulate the vacuum applied to the sampling valve. The needle valve was adjusted so that the pressure in the vacuum pump was 50 mTorr. After the sparge vapors filled a 50 µL sample loop, the valve was actuated so that helium flowing at ∼20 mL/ min via a flow controller pushed the sample through an empty, deactivated fused-silica capillary (0.53 mm i.d. × 300 cm length) into a jet separator (SGE, Melbourne, Australia). An Agilent 5890 gas chromatograph oven (Wilmington, DE) was used to heat the valve and jet separator at 200 °C. An uncoated, deactivated fusedsilica capillary (0.1 mm i.d.) was used as a transfer line between the jet separator and the MS. A Thermo-Electron Polaris Q ion 1330
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trap mass spectrometer (San Jose, CA) monitored the PCE vapor. PCE was determined by selected ion monitoring at m/z 166 with a scan time of 0.1 s. Figure 2 shows a profile of PCE pulses separated near baseline between each pulse. The time between each pulse was 30 s. On-Line MS Calibration. Calibration of the on-line MS system was done externally by creating a PCE vapor stream and by sampling the vapor with the on-line MS system and with thermal desorption cartridges. The calibration curves were prepared by correlating the PCE peak area response from the on-line MS with the vapor concentration determined using the ATD/GC/MS. The PCE vapor stream was generated by passing carbon dioxide through a purge vessel which contained equal volumes of PCE and water (Figure 3). Most of the PCE vapor stream was directed through stainless steel tubing to a granular activated carbon trap. A fraction of the flow was drawn from this stream into a second six-port sampling valve that contained a 100 µL sample loop. This unheated sampling valve was located on top of the GC oven. This sampling valve was not heated. A temperature sensor located at the valve indicated that its temperature was a few degrees above room temperature. At the appropriate time the valve was manually switched, and the PCE in the sample loop was flushed by helium into a thermal desorption tube containing Chromosorb 106. The flow stream continued from this valve into the on-line sampling valve located in the GC oven. To obtain diluted concentrations of the PCE vapor, a carbon dioxide stream from a second purge vessel containing only water was combined with the saturated PCE vapor stream. The carbon dioxide flow in the saturated PCE stream was decreased by the flow of the carbon dioxide stream added so that the total flow was always 200 mL/min. Flow meters (Alltech) and manual flow controllers were used to set the carbon
Figure 3. Diagram of the on-line sampling MS system with a thermal desorption cartridge sampling valve and PCE vapor generation/CO2 dilution manifold; 98% of the PCE stream is trapped in the carbon trap.
Figure 4. Polaris Q ion profile of PCE peaks created by dilution of saturated PCE vapor with increasing flows of CO2. The purge vessel contained 5.0 mL of PCE and 5.0 mL of water.
dioxide flows. Six to eight dilution steps were done with vapor sampling into the ATD tubes taken at each dilution. The Polaris Q MS profile of the dilutions is shown in Figure 4. Calibration of the Polaris Q was done by plotting the vapor concentration of PCE from the ATD/GTC/MS versus the PCE peak area from the Polaris Q on-line MS. Figure 5 shows a typical calibration curve. ATD/GC/MS Calibration. The ATD/GC/MS consisted of a Perkin-Elmer TurboMatrix automated thermal desorber (Shelton,
CT), a Thermo-Electron Focus gas chromatograph (San Jose, CA), and Thermo-Electron DSQ quadrupole mass spectrometer (San Jose, CA). Typical parameters were used for operation of the ATD, GC, and MS systems.13,14 Thermal desorption tubes containing (13) TurboMatrix Thermal Desorber, Instrument Manual; Perkin-Elmer: Shelton, CT, 2000. (14) Focus Gas Chromatograph and DSQ Mass Spectrometer User Manuals; Thermo Fisher Scientific: Austin, TX, 2003.
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Figure 5. Calibration curve of PCE vapor concentration from ATD/GC/MS plotted vs PCE peak area from the on-line MS.
0.32 g of Chromosorb 106 were obtained from Supelco (Belefonte, PA). Calibration of the ATD was accomplished by injecting 4 µL of PCE (Aldrich) standards in methanol and 4 µL of 1250 ng/µL fluorobenzene (Aldrich) in methanol into the thermal desorption tubes. The mass of PCE added to the cartridge ranged from 500 to 80 000 ng. To accommodate these high sample masses, a split of 1/200 was used in the ATD. Second-source PCE calibration standards were analyzed to confirm accuracy of the calibration. The standards were injected through a Swagelok tee fitted with a septum at the top into a nitrogen stream that flowed through the thermal desorption tube. The nitrogen flow was set at approximately 60 mL/min and was allowed to flow through the tube for 5 min after the calibration and internal standards were injected. Calibration curves were prepared by plotting the peak area ratio of detected PCE and fluorobenzene against the injected mass (nanograms) of PCE. The calibration curves were linear with R-squared values of 0.995 or better. The concentration of PCE sampled from the vapor stream was calculated from the mass of PCE in the cartridge and the volume of the sampling loop, 100 µL. Extraction of PCE Adsorbed on Granular Activated Carbon. PCE in the sparge stream not captured by the MS sampling system was trapped in a glass cylinder containing ∼160 g of granular activated carbon (GAC) (Calgon Filtrasorb-400, Pittsburgh, PA). After a sparge experiment, the GAC was transferred to a bottle and placed on a roller mixer overnight to thoroughly mix the carbon. Three 1.0 g subsamples were then sonicated in 20 mL of methylene chloride using a Fisher Scientific ultrasonic dismembrator (model 500, Pittsburgh, PA). After sonicating for 3 min with 50% duty time and at 50% amplitude, the extracts were centrifuged for 10 min at ∼1000g. Four microliters of the methylene chloride extract was injected into thermal desorption tubes along with the internal standard, fluorobenzene. The tubes were then analyzed for PCE by the ATD/GC/MS system. The mass of PCE in the samples of carbon and the total weight of the 1332
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carbon were used to determine the total mass of PCE collected in the carbon. Control experiments were conducted by purging known volumes of PCE into glass cylinders containing weighed amounts of GAC. RESULTS AND DISCUSSION Instrument Development. With the on-line MS system it was found that the carrier gas used for purging PCE during calibration needed to be the same as the gas used to sparge the flow chamber.11 A comparison of helium and carbon dioxide as sample purge gases revealed that the response of PCE decreased ∼37% when helium was replaced with carbon dioxide. Water also affects the response of PCE and must therefore be present in the standard. However, the presence of water vapor in the carbon dioxide caused the PCE response to increase by 17% compared to that in carbon dioxide alone. A difference in MS response for PCE in the presence of helium versus carbon dioxide would be expected since carbon dioxide like methane can be used as buffer gas for chemical ionization.15 Likewise the presence of water is also expected to affect the ionization of PCE. Maintaining the same conditions in the standard gas stream and in the sample stream was found to be critical in obtaining reproducible results. The correlation between the peak area from the on-line MS and the vapor concentration from the ATD/GC/MS was very linear over six dilutions (Figure 5) with an R-squared value for the least-squares fit of 0.9987. The concentration of undiluted PCE vapor, 163 000 µg/L was close to the expected value. From the temperature at the purge-and-trap vessel (25 °C) and the vapor pressure of PCE at this temperature, the expected vapor concentration is 168 000 µg/L indicating a relative percent difference between the expected and measured concentrations of 3%. The concentration of PCE in the vapor phase is very dependent on temperature. A drop in temperature from 25 to 20 °C causes the (15) Knighton, W.; Sears, L.; Grimsrud, E. Mass Spectrom. Rev. 1995, 14, 327.
Figure 6. Breakthrough purge experiment for 1.5 mL each of PCE and water. The PCE concentration is obtained from the peak areaconcentration calibration curve. Open boxes indicate vapor concentrations from ATD cartridges. The cumulative volume of PCE mass purged from the vessel was determined from the Polaris Q peak area.
PCE saturated vapor concentration to decrease from 168 000 µg/L to 131 000 µg/L. The temperature at the purge-and-trap vessel was monitored during the experiments described here and remained constant with an average temperature of 25.3 °C (SD 0.6 °C). Recovery Check-Breakthrough Curve Using a Purge Vessel. A quality control check was conducted after calibration to determine the accuracy of the method. This involved the complete purge of a known volume of PCE (1.5 mL of PCE and 1.5 mL of water) from the purge vessel and determination of the percent recovery of the total volume from the on-line MS concentration measurements. A flow of 200 mL/min of carbon dioxide was used to purge the PCE. Figure 6 shows the profile of individual concentration points obtained using the calibrated online MS. Numerical integration using the trapezoidal rule was utilized to estimate the total volume of PCE removed by the carbon dioxide:
V)
Q
n-1
∑(t
CFF i)1
i+1
- ti)(Ci+1 + Ci)/2
where, F is the PCE density (assumed to be 1.623 g/mL), Q is the carbon dioxide flow rate (L/min), C is the PCE vapor-phase concentration (mg/L), t is time (min), and CF (conversion factor) is 1000 mg/g. A total volume of 1.41 mL or 94% of the volume added to the purge vessel was recovered using the on-line MS concentration measurement. During this purge experiment most (98%) of the vapor flow was collected in the carbon trap. Triplicate samples of the carbon were extracted and the total PCE determined in the GAC was 1.45, 1.31, and 1.48 mL. The average percent recovery of PCE
trapped in the carbon was 94% (SD 6.1%). The agreement between the two methods, on-line MS and GAC extraction, is good. To evaluate the continued calibration of the on-line MS system, during the breakthrough test, ATD cartridge samples of the vapor were also taken at four different times. The relative percent differences between the concentrations determined from the ATD/GC/MS and the on-line MS were 3.7%, 3.5%, 6.4% and 5.0% with an average of 4.6% (SD 1.3%.) Two-Dimensional Flow Cell Experiments. The on-line MS system was used to measure the PCE mass removed during a number of sparging events during two experiments. A volume of 8.00 mL of PCE was added to the flow chamber at the beginning of each experiment, and an initial flux test conducted. A series of sparge events and flux tests were conducted with each experiment. Figure 7 shows the vapor concentration profiles obtained on days three, four, and five of the second experiment. Two sparge events were completed on days three and four and one sparge event was completed on day five. The volumes of PCE sparged on each of the 3 days were 0.86, 0.20, and 0.12 mL. The cumulative volume over the 3 days, 1.18 mL, agrees quite well with the volume of PCE determined from extraction of the GAC trap, 1.15 mL. The agreement between the two measurements was 97%. Recovery data for these and additional sparging events for the two experiments are provided in Table 1. The agreement between the volume of PCE determined by the on-line MS and the extraction of the carbon traps ranged from 87% to 111%. Figure 7 also includes data points for PCE concentrations determined from ATD cartridges taken during the sparge experiments and are represented in the figure with open boxes. During this segment of the experiment, 16 vapor samples were collected in ATD cartridges. Additional ATD cartridges were collected during the other segments of both experiments. The average Analytical Chemistry, Vol. 80, No. 4, February 15, 2008
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Figure 7. Profile for a flow chamber sparge experiment containing initially 8.0 mL of PCE. Five sparge events were done over 3 days. The time for each sparge event is presented in the x-axis. Open boxes represent PCE concentrations obtained from ATD cartridges taken during the sparge experiments. Table 1. Summary of PCE Volume Measurements for CO2 Sparging Events for Two Cell Sparge Experiments
*One GAC trap was used to collect PCE during episodes 2, 3, and 4 in expt 1. **One GAC trap was used to collect PCE during episodes 3, 4, and 5 in expt 2.
relative percent difference between the ATD/GC/MS and on-line MS vapor concentrations was 14% (SD 10%.) This data demonstrates that the on-line MS system maintained calibration during the three days of sparging. The analysis of second source PCE standards were always included in the ATD sample queues. For this quality control check, two replicates of 20 000 ng of PCE were injected into ATD cartridges and analyzed. The recovery of PCE in these cartridges was 105% and 96%. CONCLUSIONS With the use of an on-line MS instrument, a method was successfully developed and applied to determine very high vapor concentrations of PCE present in a gas stream. The instrument was calibrated by creating a vapor stream of saturated PCE and 1334
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water in a carbon dioxide carrier and diluting the PCE vapor with a secondary carbon dioxide stream saturated with water. The calibration curve for PCE was constructed by correlating the PCE peak areas from the on-line MS against vapor concentrations determined using an automated thermal desorption GC/MS. The on-line MS instrument was calibrated between high vapor concentrations near 168 000 µg/L, the saturated vapor concentration of PCE at 25 °C, and diluted concentrations as low as 3000 µg/L. Measurements of PCE in a vapor stream were made continuously every 30 s for sampling events as long as 200 min during a study involving removal of PCE by sparging from a 2D flow chamber. The instrument maintained calibration over five days during which time 1700 vapor concentrations measurements were obtained. The
vapor stream was split between the on-line system (∼2% flow) and a GAC trap (∼98% flow.) Good agreement was obtained between the total PCE determined by the on-line MS and that extracted from GAC traps. This method is one of the few in the published literature that quantifies very high concentrations at volatile vapors in the tens of thousands of parts-per-million. Given the rapid sampling time of the on-line MS, the method could easily be extended to measure the elution of other compounds in different sparging experiments and could also be used to measure the breakthrough curves of gas-phase tracers.
Environmental Research and Development Program (SERDP), a collaborative effort involving the U.S. Environmental Protection Agency (EPA), the U.S. Department of Energy (DOE), and the U.S. Department of Defense (DoD). It has not been subjected to Agency review and, therefore, does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation of use. We gratefully acknowledge, the analytical support provided by Shaw Environmental (Contract No. 68-C-03-097).
ACKNOWLEDGMENT The work upon which this paper is based was supported by the U.S. Environmental Protection Agency through its Office of Research and Development with funding provided by the Strategic
Received for review September 4, 2007. Accepted October 30, 2007. AC701859N
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