Use of On-Site GC/MS Analysis to Distinguish between Vapor Intrusion and Indoor Sources of VOCs

Dr. Thomas McHugh | GSI Environmental, Inc.

ER-201119

Objectives of the Demonstration

Distinguishing between vapor intrusion (VI) and indoor sources of volatile organic compounds (VOCs) is a significant challenge in site assessments, greatly increasing the cost and complexity of investigations. Rapid on-site analysis of indoor air samples using a portable gas chromatograph/mass spectrometer (GC/MS) allows users to understand the distribution of VOCs in real-time, supporting identification of the source while in the field. The objective of this demonstration was to develop and validate a step-wise investigation procedure using commercially available off-the-shelf (COTS) on-site GC/MS analysis with real-time decision making as a tool to distinguish between vapor intrusion and indoor sources of VOCs.

Back to Top

Technology Description

Use of on-site GC/MS analysis to distinguish between vapor intrusion and indoor sources of VOCs requires a field-portable analytical instrument with sufficient sensitivity to measure VOC concentrations in indoor air within the concentration range of regulatory concern (i.e., low µg/m3). A high degree of precision is also required because the protocol relies on measuring concentration gradients within a building to identify sources of VOCs. For the demonstration, a HAPSITE portable GC/MS instrument was utilized. Although specific procedures in the investigation protocol were developed using the HAPSITE, any on-site instrument with sufficient sensitivity and precision may be used.

Back to Top

Demonstration Results

The field investigation included application of the on-site GC/MS analysis protocol at four Department of Defense (DoD) sites. To evaluate the validity of the protocol, conventional vapor intrusion and compound-specific stable isotope analysis (CSIA; ESTCP project ER-201025) investigations were conducted at the study sites. Results from the three investigation methods were compared to evaluate the relative effectiveness of the different approaches.

The on-site analysis protocol performed as well as or better than the conventional investigation approach at all seven buildings evaluated. At six of the seven buildings, the results from the on-site analysis protocol were consistent with the overall evaluation of the vapor intrusion condition based on the results from all three of the investigation methods combined. At one building, the on-site results were consistent with the conventional program results, which suggested trichloroethylene (TCE) vapor intrusion; however, for this building, the CSIA result provided strong evidence of an indoor source. The scenario that best fits the results from all three investigation methods combined is that TCE was recently used in the building, but that the indoor source was removed prior to sampling.

In addition to this demonstration program, the on-site protocol has been used by the project team at a number of other sites for indoor source and vapor entry point identification. Overall, the on-site GC/MS analysis protocol has performed well under a wide variety of building conditions. The protocol includes an option to conduct on-site analysis while the building is pressurized or depressurized. This option can be used to get a better understanding of the VOC source as well as temporal variability and the susceptibility of a building to vapor intrusion.

Back to Top

Implementation Issues

This project developed and validated an on-site GC/MS analysis protocol to distinguish vapor intrusion from indoor sources of VOCs. The protocol can be used as a standalone investigation method or can be used within a larger investigation program.  

Advantages of the protocol include:

  • Real-time results. The key advantage of the on-site analysis protocol is the ability to measure indoor air VOC concentrations and determine the primary sources (i.e., indoor vs. subsurface) in real time during the course of the field investigation. Because of the short analytical method run times, many samples can be collected while on site, resulting in a large volume of data available for interpretation while still in the field. This allows the investigators to more readily react to building-specific situations and make decisions (e.g., rule out vapor intrusion, determine potential vapor entry points, find primary VOC sources, etc.).
  • Definitive data. Although the method focuses on on-site analysis, a small number of air samples are collected for off-site laboratory analysis to confirm key findings. These confirmation sample results are supported by standard laboratory quality assurance/quality control (QA/QC) and can be used for regulatory decision-making.
  • No sub-slab sample points. The protocol eliminates the need to drill through the building foundation.
  • Reduced sampling requirements. Because indoor sources of VOCs can be identified and removed during the investigation, the on-site analysis protocol will more frequently yield clearer results compared to the conventional investigation approach. When used in conjunction with building pressure manipulation, the need for further sampling to characterize temporal variability may also be reduced or eliminated. 

Potential limitations on the use of the on-site GC/MS analysis protocol include:

  • Equipment availability and reliability. The HAPSITE Smart Plus or alternate instrument for on-site analysis is less common than the equipment used for the conventional investigation approach. As a result, equipment availability, procurement, and scheduling may be more complex. Reliability, sensitivity, and other QA/QC requirements should be considered when selecting the on-site GC/MS instrument for use in the protocol.
  • Staff suitably trained in interpretation of vapor data. The field team should include a senior staff member with the knowledge, skills, ability, and authority to make field decisions based on the on-site measurements. The team should also include at least one experienced GC/MS operator.
  • Target compounds. Specific target compounds should be sufficiently volatile to be detected at concentrations similar to the applicable indoor air screening concentration. Less volatile compounds such as naphthalene may not be good candidates for on-site analysis because it is difficult to calibrate the on-site instrument for analysis of low concentrations of these compounds. Additionally, accurate identification may be problematic with certain VOCs. This issue may be addressed by fine-tuning the analytical method or interpreting chromatograms and ion mass ratio data in the field.
  •  Building construction. For the building pressure control option to be effective, the building cannot be too large (>20,000 square feet) or too leaky (e.g., presence of built-in ventilation slats).

Back to Top

Share