TO-15 HSS (High Sensitivity/Selectivity)
Measuring Trace Level VOCs in High Concentration Soil Gas Matrices – A New Tool to Meet Risk-Based Screening Levels
Vapor intrusion (VI) investigations often rely on volatile organic compounds (VOCs) measurements in soil gas to evaluate potential inhalation risks to occupants of overlying/nearby buildings. While EPA Method TO-15 can be configured to achieve the required sub-ppbv risk-based screening levels, soil vapor may contain high levels of matrix requiring dilution to minimize gross contamination of the analytical instrumentation. The resulting elevated reporting limits often yield non-detect values for key risk drivers at reporting limits several magnitudes higher than the risk-based screening levels for soil vapor samples. High concentrations of co-eluting matrix peaks can also confound the identification and accurate quantification of these target chemicals. To effectively evaluate potential risk and support decisions regarding site closure and effectiveness of remediation activities, new analytical techniques are required to measure trace levels of VOCs in these challenging soil gas matrices.
A novel solution was developed to measure trace concentrations of selected VOCs in high concentration matrices utilizing conventional TO-15 canister sample collection and a standard TO-15 air concentrator. To remove interfering peaks and isolate selected VOCs, a custom gas chromatographic system was designed using a sequence of chromatographic separations and timed concentration steps. Compound detection was achieved using a Time-of-Flight mass spectrometer (TOF-MS) which provided improved sensitivity and selectivity as compared to a conventional quadrupole mass spectrometer. The enhanced sensitivity of the TOF-MS allowed the lab to reduce the volume of soil gas required to meet the screening levels, thereby minimizing contamination of the air interface. A schematic of the analytical system is shown in Figure 1.
Figure 1. Schematic of modified TO-15 analytical configuration
The modified TO-15 method was applied to two common challenges encountered in vapor intrusion investigations and remediation activities:
- Measuring Ethylene Dichloride (EDC) and Ethylene Dibromide (EDB) at risk-based screening levels (RBSLs) in the presence of high levels of petroleum hydrocarbons; and
- Measuring 1,4-Dioxane in the presence of high concentrations of chlorinated solvents such as Tetrachloroethene (PCE)
The TO-15 applications were developed using a sample volume of 50 mL. Deuterated analogues of the target VOCs were used as the associated internal standards for compound quantification.
The two applications were validated against EPA TO-15 performance criteria, demonstrating compliance with all requirements including:
- BFB Tune Check mass ion ratios;
- Calibration Linearity (≤30%RSD) as shown in Figures 2a, 2b, and 3 and summarized in Table 1;
- Accuracy (70-130%) and precision (±25% RPD) demonstrated by independent calibration verifications and daily lab control samples; and
- Internal standard recoveries within ±40%difference of the daily continuing calibration verification.
|Figure 2a. EDC Initial Calibration Curve||Figure 2b. EDC Initial Calibration Curve|
|Figure 3. 1,4-Dioxane Initial Calibration Curve||Table 1. Initial Calibration Curve Performance Summary|
To evaluate the effectiveness of the sequential chromatographic separations and concentration steps in isolating the selected VOC(s), trace concentrations of the target compounds were prepared in a highly impacted matrix. For Application 1, two test standards were prepared with EDB and/or EDC spiked at concentrations at or near their respective risk-based screening levels (RBSLs) in a 0.1%v/v gasoline matrix. The test standards were screened by loading an aliquot onto a conventional GC/MS. The total ion chromatogram of the 0.1% petroleum standard is shown in Figure 4 with the retention times (RT) of EDC and EDB labelled.
Figure 4. Conventional GC/MS Total Ion Chromatogram of 0.1%v/v TPH standard spiked with sub-ppbv EDB and EDC.
Figure 5. Conventional GC/MS Total Ion Chromatogram
Compound Identification and Recovery
The 0.1% petroleum standards spiked with EDB and EDC and the ppmv-level PCE/BTEX Test Matrix spiked with 1,4-Dioxane were analyzed undiluted using full 50 mL sample loads. Each application was configured with a sequence of appropriately timed heart cuts and concentration steps in coordination with optimal GC parameters to remove interfering peaks and isolate the selected VOC(s) for detection by the TOF-MS. All compound identification requirements were easily met for EDB and EDC (Application 1) and 1,4-Dioxane (Application 2) including:
- Relative Retention time within window (± 0.06 RRT)
- Full spectral information generated, matching NIST Reference Spectrum (Figures 6a, 6b and 7)
- Ratio of extracted primary (I°) and secondary (II°) ion areas matching daily CCV (Tables 2a, 2b, and 3)
Figure 6a. Sample Spectrum for EDB in 0.1%TPH compared to Reference Spectrum
Table 2a. Ion Area Ratio Comparison of 0.16 µg/m3 EDB in 0.1% TPH to CCV
Figure 6b. Sample Spectrum for EDC in 0.1%TPH compared to Reference Spectrum
Table 2b. Ion Area Ratio Comparison of 0.40 µg/m3 EDC in 0.1% TPH to CCV
Figure 7. Sample Spectrum for 1,4-Dioxane in PCE/BTEX matrix compared to Reference Spectrum
Table 3. Ion Area Ratio Comparison of 5.0 ppbv 1,4-Dioxane in PCE/BTEX matrix to CCV
Additionally, recoveries of the target compounds at or below the risk-based screening levels or near the project specified reporting limit were unaffected by the high concentration test matrices (Table 4.)
Table 4. Modified TO-15 GC/TOF-MS Target VOCs recoveries in highly impacted test matrices
Table 5 compares the performance of the modified TO-15 GC-TOF application with the expected performance of the conventional TO-15 SIM/Scan option for the test samples evaluated. While the test matrices were analyzed on the custom GC-TOF with no analytical dilution, analysis on the conventional TO-15 unit would require significant analytical dilutions of approximately 2500 for 0.1%v/v petroleum matrix and 50 for the PCE/BTEX test matrix. Additionally, selective removal of interfering peaks using the GC-TOF application provided definitive identification and accurate quantitation of the key risk drivers and target VOCs with no evidence of matrix interference.
a. Assuming canister pressurization yields TO-15 sample dilution of approximately 2.
b. Clean Matrix reporting limits multiplied by estimated Analytical Dilution Factor required for sample analysis
Table 5. Performance comparison of Modified TO-15 GC-TOF/MS and conventional TO-15 SIM/Scan for test matrix samples
Subsequent analyses of a number of field samples using both the conventional and the modified TO-15 methods further demonstrated the advantages of the custom GC-TOF application for a range of matrix profiles and concentrations ranging from ppmv to percent level hydrocarbons. Despite the presence of high concentrations of non-target compounds in the soil vapor, no analytical dilutions were required and reporting limit objectives were routinely achieved.
The modified TO-15 application using the customized GC to isolate the compounds coupled with the sensitivity of the Time-of-Flight mass spectrometer provides a significant improvement in sensitivity and resolution from potential interferences that may confound identification and/or accurate concentration measurements of trace level VOCs. The application is robust and readily customizable by modifying the timing of the heart cuts and concentration steps and/or the specific GC column phases used for separation.