Performance Enhancement of Traditional Membrane Interface Probe (MIP) Detection Using an Integrated HAPSITE® Portable Gas Chromatograph/Mass Spectrometer (GC/MS).
Background: MIP technology has revolutionized initial site assessments and has made it possible to determine instantly the location and extent of site contaminants. MIP is a system manufactured by Geoprobe Systems® for the detection and measurement of Volatile Organic Compounds (VOCs) in the subsurface. A heated probe carrying a permeable membrane is advanced to depth in soil. VOCs in the subsurface cross the membrane, enter into a carrier gas stream, and are swept to gas phase detectors (typically FID/PID/ECD) at ground surface for measurement. In spite of these advancements, MIP technology has encountered three limitations: 1. inadequate sensitivity, 2. inability to detect methyl tert-butly ether (MTBE), 3. inability to identify individual contaminants in-situ.
System Development: In order to solve these shortcomings, Sentinel Mobile Laboratories, LLC, and Vironex®, Inc. in cooperation with Geoprobe®, Inc. and INFICON®, Inc. have successfully integrated the MIP and HAPSITE® portable Mass Spectrometer, two cutting-edge ESA technologies. The resulting service allows the site investigator to gain 100 times the sensitivity of traditional in-line FID/PID/ECD detectors, to detect MTBE, and - using GC/MS National Institute of Standard and Technology (NIST) library search technology - to 100% identify contaminants in-situ.
The initial work was performed at the Geoprobe® corporate headquarter in Salina, KS. Geoprobe® furnished technical resources/personnel, MIP equipment/probe and a direct push rig. INFICON®, Inc. supplied HAPSITE® instrumentation and technical reach-back. Sentinel Mobile Laboratories, LLC and Vironex®, Inc. provided the technical expertise to integrate the MIP with the HAPSITE®.
System Improvements: The HAPSITE® offers system improvements in a number of ways. First, the HAPSITE® utilizes an integrated three-phase carbon trap which concentrates the MIP effluent allowing for increased sensitivity. (Note: this trapping concept is already being used with traditional detectors in an attempt to increase MIP sensitivity). Second, the mass spec is a universal detector that is able to detect and confirm the presence of MTBE. Traditional PID/FID detectors are unable to reliably detect MTBE due to their inherent design. The FID is prone to many false positives and the PID has insufficient ionization potential to detect MTBE at all but the highest concentrations. The third, and most important improvement that the integrated MIP-HAPSITE® system offers is the ability for 100% in-situ identification of subsurface contaminants. This is accomplished by comparing the mass spectrum of an unidentified component against a reference library such as the National Institute of Standard and Technology (NIST) Mass Spectral Database.
Laboratory Testing Results: The first phase of laboratory testing was to determine the sensitivity of the integrated MIP-HAPSITE® system using standards prepared in 500 mL of water. Geoprobe® uses this technique to simulate real world MIP field response. The goal was to see to what extent the sensitivity could be increased in the typical 45-second exposure time that is used for the FID/PID/ECD response testing outlined in Geoprobe’s® MIP Standard Operating Procedure (SOP).
The MIP carrier gas flow was split 50:50 and sent equally to traditional in-line PID/DELCD detectors and to the HAPSITE®. This approach allows for real time contaminant logging data as well as for incremental (typically 5-10 ft interval) GC/MS trapping/speciation data collection (see figure 1 for example).
Figure 1. Example of traditional MIP data integrated with incremental GC/MS data.
Real Time Conductivity Logging Data
(Lithology of Subsurface)
Real Time PID Logging Data
(Note response @ 23-30 feet)
Incremental HAPSITE® GC/MS Data
(In-Situ identification of contaminates)
After some initial experimentation to add make-up Nitrogen flow to the HAPSITE® MIP carrier flow, allowing seamless entry of MIP carrier gas onto the HAPSITE® three-phase carbon trap, testing began. Trichloroethane (TCE) was chosen for the sensitivity testing because it is a common chlorinated contaminant encountered in the field. The Geoprobe® MIP SOP uses a TCE standard at 1000 ppb prepared in 500 mL of water to perform low level response testing. This concentration was sequentially cut in half until the HAPSITE® GC/MS no longer responded. It was determined that a 10 ppb TCE standard was well within the sensitivity range of the HAPSITE® GC/MS and would be used for subsequent response testing (see Figure 2). This represents a 100-time increase in sensitivity over the traditional FID/PID/ECD in-line detectors.
Figure 2: 100 Times Increase in Sensitivity with HAPSITE® Integrated MIP System (Usual MIP challenge is @ 1000 ppb TCE)
The second phase of the laboratory testing was to determine if MTBE could be detected using the integrated MIP-HAPSITE®. An initial high-level 1000 ppb standard was prepared in 500 mL of water. It was determined that MTBE easily passed across the membrane and was trapped by the HAPSITE®. The subsequent laboratory work with MTBE determined a workable response test level. It was determined that a conservative reproducible response test level would be at 100 ppb in 500 mL of water (See Figure 3).
Figure 3: Ability to Detect MTBE (Current MIP Configuration is Unable to Detect MTBE)
Field Testing Results: A real world test was then needed to verify the laboratory results. Geoprobe® had done previous MIP testing at a field within their facility, where it was known that a large PID response could be obtained at a depth of 25-30 feet.
A series of MIP pushes was performed with the integrated MIP-HAPSITE® configuration. As in the lab testing the MIP flow was split 50:50 being sent equally to the traditional PID/DELCD detectors and the HAPSITE®.
As expected a large PID response (indicating aromatic hydrocarbons) was encountered at 25 ft. The HAPSITE® was able to concentrate the MIP flow from 25-30 feet and speciate the aromatic hydrocarbons that caused the PID response (see Figure 4). Previous to this test Geoprobe® had not known what compounds were causing the PID response. It may have been large concentration of a highly regulated compound such as benzene or as it turns out large concentration of hexanes and pentanes, which are not highly regulated. This site demonstrated the value of 100% identification of contaminants in-situ.
Figure 4: Example of Output from Contaminated Zone (No DELCD response/Large PID Response)
Real World Case Studies: California EPA's Department of Toxic Substances Control (DTSC) has certified the analytical capabilities of the HAPSITE portable gas chromatograph-mass spectrometer (GC-MS) system as a field-based analytical method as well as a laboratory instrument for measuring volatile organic compounds (VOCs) in water, soil and soil gas. This 2004 certification report evaluates the performance of the HAPSITE instrument based on a detailed review of data packages submitted by the technology proponent, field data generated by independent parties, and of new data collected under the oversight of the California Environmental Technology Certification Program.
Full Report (March 2004, 75 pages).
Written By: Kenneth Dockery, HAPSITE® Operations Manager, Sentinel Mobile Laboratories, LLC.
