Final Report: Miniature Membrane Inlet Gas Chromatograph for Cone Penetrometers

EPA Contract Number: 68D99040
Title: Miniature Membrane Inlet Gas Chromatograph for Cone Penetrometers
Investigators: Dvorak, Michael
Small Business: Dakota Technologies Inc.
EPA Contact: Richards, April
Phase: I
Project Period: September 1, 1999 through March 1, 2000
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (1999) RFA Text |  Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)

Description:

The major objective of this Phase I project was the design and construction of a customized high performance gas chromatograph (GC) that operates within the interior of a cone penetrometer. The cone penetrometer is equipped with a heated microporous inlet membrane (MPIM). Volatile organic compounds (VOCs) in the soil formation are vaporized at the surface of the hot (120 ?C) membrane and passed through it into a carrier gas stream. In a previous project, Dakota Technologies, Inc. (DTI) had adapted two conventional GC detectors, one a photoionization detector (PID) and the other a halogen specific detector (XSD), to monitor the VOCs in the carrier gas stream. The combination of the MPIM and the downhole detectors, in conjunction with continuous advancement of the cone penetrometer, yields profiles of chemical contamination as a function of depth below ground surface. The data are quantitative but field screening in nature because many different compounds could be contributing to the signal. The current research project complements and expands the technology to chemically specific measurements at concentrations lower than set by drinking water quality standards. This unique instrument enables quantitative measurement of subsurface chemical contaminants in either the vadose or saturated soil zones without the need to transfer vapor or water to the ground surface.

Transitioning from the field screening mode to a fully functioning downhole GC required DTI to: (1) develop an analyte trap to preconcentrate VOCs passing through the MPIM; (2) arrange for ballistic heating of the trap to launch the analytes on the column as a narrow band; and (3) arrange the column in a compact configuration. It was also necessary to integrate these components together within the pipe and to develop an umbilical cable to meet power, control, and communication requirements.

Small packed traps obtained from Professor Janus Pawliszyn (University of Waterloo) were adapted to meet the size requirements imposed by the small diameter pipe. The traps consist of short lengths (ca. 15 mm) of 1/16 inch tubing packed with Tenax resin. The GC column, 5 meters in length, was wrapped around a 1-inch diameter mandrel that was specially machined to hold the column without kinking it. A hole through the center of the mandrel allowed for cable bypass. The column was covered with insulation and its temperature controlled to within one degree. The column was operated isothermally in Phase I, but will be temperature programmed in the next phase. The detector used in Phase I was DTI's modification of a commercially available halogen specific detector (XSD).

The usual operating temperature of the XSD is between 900 and 1200?C. Such a high temperature made it imperative that all gas and electrical lines passing near the XSD were well insulated. Gas lines were constructed from 1/16 inch stainless steel tubing. The electrical lines have Teflon insulation and are embedded in fiberglass sheathing.

Controlling the gas flows to the MPIM membrane, the trap, and the analytical column is another key to successful implementation of the downhole GC. The gas flow to each of the above components can be bypassed independently of the gas flow to the other components with the aid of three electronically activated 2-way solenoid valves. The control signals to the valves are derived from a remotely activated microprocessor located downhole. The microprocessor also controls the heating of the GC column and the discharging of the ballistic trap heaters. The probe's entire length is 5 ft, including the MPIM, and is operated using a 60-foot umbilical cable, but longer umbilicals are readily accommodated.

Various laboratory studies were carried out to characterize the performance of the traps, column, and detectors, both individually and in combination. The repeatability of retention times and the efficiency with which material is trapped and desorbed were assessed. Material was presented to the MPIM in the laboratory by flowing solutions over the membrane, by pipetting small volumes of water onto its surface, and by placing soil on the membrane surface. The project culminated in a successful field test at a site contaminated with perchloroethylene (PCE) and minor amounts of trichloroethylene (TCE).

Summary/Accomplishments (Outputs/Outcomes):

The retention time stability was studied over an 8-hour period. The MPIM, trap, column and XSD were operated in series and no changes or adjustments in the helium flow rate (established with a rotameter) were made after the start of the experiments. Approximately once an hour a 0.5 mL aliquot of a stock TCE/PCE solution (20 mg/L in each component) was delivered via a pipette onto the hot (120 ?C) outer surface of the membrane interface. The water boils rapidly (a few seconds) and the helium carrier gas sweeps the portion of the volatiles that passes through the sampling interface into the trap. For these studies, ballistic heating of the trap was initiated 120 seconds after the water was introduced onto the membrane interface. The heating step was activated by a keystroke to the control computer. The time of the keystroke was also taken as time zero for the retention time determinations. The detector signal was recorded every second.

The mean and standard deviation of the retention time for TCE are 78.5 and 1.2 sec, respectively at a flow rate of 25 mL/min. The corresponding figures for PCE are 177.5 and 1.7 seconds. The retention time precision is better than the target figure of 2%. Increasing the carrier gas flow rate to 40 mL/min reduced retention times to about 40 seconds for TCE and 100 seconds for PCE.

Trap temperature was not measured explicitly, but is estimated to reach 150 ?C. Analyte breakthrough (saturation of trap) was not observed in any of the experiments performed for this contract, even when a 0.5 uL drop of neat TCE was placed on the MPIM surface. Desorption efficiency was assessed by repeat heating of the trap. For 500 uL aliquots of 20 ppm TCE/PCE mixtures placed on the MPIM, desorption efficiency was around 90%. Mass transfer efficiency of the MPIM depends on the analyte of interest and how analytes are presented to the interface. We have used liquid aliquots, soil aliquots, and flowing liquid streams. None of these is a completely faithful representation of downhole operation, but we estimate a limit of detection better than 1 µg/L for the analytes of interest and a one-minute trapping time.

The system was field tested at a local site with both solvent (primarily PCE) and fuel contamination. The cycle time, measured from when the probe reaches the depth of interest to the completion of the chromatographic elution is 3-5 minutes. The field test site is approximately five miles away from DTI's facility. Within the span of two hours, the downhole GC was driven to the site, a hole was pushed with GC data collected at five different depths, and the system was returned to DTI's facility.

Conclusions:

All the technical objectives as originally defined in the Phase I proposal were met, although approaches were modified in a few cases to incorporate improved information and new discoveries. The Phase I accomplishments provides a clear technical direction for the Phase II research, which will also better define the path for commercial exploitation. The Phase II proposal will provide complete details on the research plans and technical objectives, but the general goals are to reduce the size of the unit and make it more robust. LEMO-like connectors to make the connections between modules will be introduced. LEMO connectors, which we are using extensively for the field screening work with the downhole GC detectors behind the MPIM, make electrical and gas connections as two pieces are mated together with a linear motion. We envision a series of modules, including sampling interface, trap, GC column, detectors, and electronics that will be "snapped" together in this fashion. Eliminating the wires and gas lines running from one module to another will make for much more stable operation. At this point, the capacitors for ballistically heating the trap represent the largest element in the apparatus. In Phase II we will use several smaller capacitors. Pawliszyn has recently communicated to us that multiple capacitors that are "fired" in sequence a few ms apart gives more efficient desorption from the trap. Phase II will also see greater consideration given to studies of the column temperature (including temperature programming), flow rates, and trapping times.

We view the downhole GC as simply the forerunner of a whole series of instruments that will solve important chemical analysis problems in unique fashion. An even larger market is represented by applications in which the GC will be positioned at the surface to analyze vapors sampled from the soil formation with the MPIM. Although there are certain limitations in returning vapor to the ground surface, this mode offers fewer operational complications and is already coming into widespread use with non-selective detectors (e.g., flame ionization detector). Size and power considerations are not nearly as demanding if the GC is operated uphole. We anticipate having units with dual PID and XSD detectors and the capability to route gas either directly from the trap to the detectors or through the column. Very fast elutions (short column, rapid thermal ramp) are desirable in this case.

The tens of thousands of monitoring wells already installed represent another very large market. The size reduction of the probe we will achieve in Phase II will give access to 2" wells. Certain modifications to the technology must be made for the "in the well" scenario. The outer casing (the pipe) does not have to be nearly as robust in this case as the probe can simply be lowered into the well. The umbilical cord can be shorter as well for most applications. It will also be possible to use alternative membrane sampling interfaces with better mass transfer characteristics since soil abrasion is no longer an issue. However, attention must be made to the standard well- purging procedure. Usual practice is to remove 3-10 casing volumes from the well before it is sampled in order to ensure that "fresh" water from the formation is sampled. It would be especially interesting to introduce packers into the well to isolate small intervals over which the well is screened. Total VOC concentration will be measured by bypassing the column and going straight from the trap to the detector(s) as water is drawn from the zone isolated by the packers. Once the total VOC signal was stable, the trap will be connected with the column for the full chromatographic elution. Similar strategies to minimize the amount of water purged and the generation of investigation-derived waste are commonly implemented with temperature, conductivity, or pH as the parameter measured for the purged water.

Yet other markets for a very compact and rugged GC relate to indoor air quality, industrial hygiene and occupational exposure, medical diagnostics, and process analyzers for many different industries.

Supplemental Keywords:

Gas chromatograph, cone penetrometer, in-situ, volatile organic compounds (VOCs), site characterization, PCE, TCE, membrane., Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, National Recommended Water Quality, Contaminated Sediments, Chemistry, Contaminant Candidate List, Monitoring/Modeling, Analytical Chemistry, Engineering, Groundwater remediation, 33/50, Engineering, Chemistry, & Physics, chromatography, Methyl tert butyl ether, soil , Toluene, soil sediment, detect, MTBE, VOCs, gas chromatography, benzene, Trichloroethylene, soil, measurement, groundwater contamination, Volatile Organic Compounds (VOCs), groundwater

SBIR Phase II:

Integrated Downhole Gas Chromatograph and Automated Sampler for Direct Push  | Final Report