Here and Now
Gas chromatography has long been accepted as a method to speciate and quantify unknown organic compounds. One of the important uses for gas chromatography is for site assessment and characterization of contaminated field sites.
Technology advances have made accurate identification and quantification of numerous contaminants possible in the field. End-users, however, remain skeptical of field data, except as a screening tool. As a result, many field samples are still collected, preserved in the field and sent to a laboratory for analysis. While this is an accepted practice, problems associated with sample handling and storage can lead to erroneous results. Advantages of field sampling and analysis are an immediate answer to questions about the presence and concentration of contaminants, reduced sampling costs and the ability to quickly respond with remediation techniques appropriate to the field site.
Over the past few decades the number of sites requiring remediation and the cleanup cost per site has risen significantly. This has resulted in additional pressure to expedite site clean ups and return land to productive use with minimal cost impact. Use of real-time ambient air monitoring in the field allows these objectives to be met.
In the past, the primary approach to expedite site remediation has been to bring the lab to the field. This was most often accomplished by fitting a mobile trailer with laboratory equipment. While this approach cuts down on sample transport time, it does have several disadvantages. These disadvantages include: expense of the analytical instrumentation and trailer facility, necessity for skilled operators to run instrumentation and a reliable power source for the trailer. Laboratory instruments can be difficult to maintain in the field due to sensitivities to field conditions, such as ambient temperature fluctuations and dust. For these reasons mobile trailers have seen limited use at field sites over the years. An alternative to the mobile field trailer is the portable gas chromatograph.
The rapid field turnaround of samples allow site managers to make quick, informed decisions to reduce overall site cleanup costs. |
Portable gas chromatographs (GC) were introduced in the late 1970s to provide the ability to run samples on site. These early portable GCs had limited capabilities because they had no oven or software. Sample run time was limited by ambient temperature methods. In 1985, a portable GC was introduced that provided an isothermal oven and personal computer (PC) software. These improvements allowed for decreased run times and the ability to store and analyze GC runs on a PC. In 1991, a dual photoionization detector (PID) was developed that provided for a total VOC indication, as well as a detailed chromatogram for speciation and quantification.
Sample analysis by GC consists of several parts: accurate calibration, the sample introduction, separation of the sample inside the GC and detection of the desired compounds. Each of these steps must be optimized to achieve the desired performance goals. The overall performance of the field GC will be markedly improved by this optimization.
Calibration of field portable GC is generally performed every eight hours. A calibration standard can be a cylinder of gas purchased from a commercial supplier. The gas supplier will certify the composition of the cylinder contents so the user is assured a reliable reference standard. The GC user can also make their own calibration standard from purchased neat standards or from permeation tubes. During the eight-hour operating period, samples of the cal standard are introduced into the GC to verify the drift of the calibration. These periodic calibration verification runs assure the user that the GC is functioning correctly and provide confirmation to a regulatory agency auditing field data that accuracy of the instrument is maintained during sampling. Instrument accuracy can be optimized by insuring that the volume of the sample introduced into the GC is identical to the volume of the standard used in calibration. A field GC can be as accurate as +/- 5 percent of the reading.
Samples can be introduced into the field portable GC by either of two methods -- syringe mode or loop mode. In syringe mode, a gas tight syringe is filled with a sample from headspace over water or soil samples and injected into the GC. Or a user can pump in a sample directly from ambient air. This is called loop injection. The sample port in both syringe and loop mode is heated to eliminate sample condensation. Normally the portable GC is in the field and the sample is taken directly into the GC via loop injection. If a sample bag is used to take a sample, the bag sample should be introduced into the field GC as soon as possible to reduce degradation in the bag. Syringe samples should also be quickly injected to reduce sample degradation.
Since field personnel are not typically experienced chromatographers, it is important that a field portable GC is easy-to-operate. In order to simplify set up and operation of the portable GC, the equipment is programmed at the factory with a standard assay configuration. An assay is a standard library of compounds that can be detected by the portable GC along with an optimized column temperature and flow rate for achieving the best retention times and detection limits for the compounds of interest. The end user can choose between one of several preprogrammed assays. Other compounds that are not part of the standard assay can be added to the library at any time.
A field GC can be as accurate as +/- 5 percent of the reading. |
Once a field sample has been run on the portable GC, the results are stored in the internal datalogger and displayed on the GC's LCD screen. The information provided includes the name of the compound, the concentration of compounds that are detected and when it was detected during the sample run. Stored chromatograms can be downloaded later to a PC for further analysis and printing. The stored chromatograms include all compounds in the assay library, whether they were detected in a sample, their concentration if detected, the method used for a particular analysis and a graph of the detector output.
Portable gas chromatograph user benefits include low ppb sensitivity to a wide range of VOCs, elimination or substantial reduction in lab costs, elimination of sample degradation caused by sample storage or transportation, and extremely rapid turnaround time of field samples. The rapid field turnaround of samples allow site managers to make quick, informed decisions to reduce overall site cleanup costs.
Recent Advances
Portable GC technology continues to evolve. The fourth generation of portable GCs, the Photovac Voyager, was introduced in 1996. The Voyager portable GC uses a three column design that allows the separation of compounds on one of three built-in columns. The three built-in columns allow determination of a wide range of compounds without having to physically change columns. The device is equipped with a dual detection system. Most volatile organic compounds are detected using a PID equipped with a 10.6 eV lamp. Some chlorinated VOCs such as carbon tetrachloride are detected on an electron capture detector (ECD) with a 15-millicurie Nickel-63 source. Samples are injected or pumped into a heated injection port and then introduced onto the isothermally heated column set. The user-selectable temperature range of the injection port and oven is 30 to 80 degrees Celsius. Ultra-high purity nitrogen is used as the carrier gas. The portable GC is completely self-contained and weighs 15 lbs.
The Voyager can be programmed for operation in the field using the built-in LCD screen with hard and soft keys. These keys control the portable Voyager assay specific method for column selection, column temperature and carrier gas flow rate. A Windows® compatible software package, Sitechart LX, is provided to program the Voyager method from a PC. Sitechart can be used to add additional compounds to the portable GC library, modify the standard column temperature and pressure, view instrument operation in the office or lab and review stored chromatograms that are downloaded from the Voyager. Typical run times for a field portable GC to run one sample are between five and 15 minutes. The total analysis time is dependent on the type of compound to be detected, the number of compounds to be detected, and the assay configuration.
Landfill Monitoring Project
During the 1950s and 1960s, a landfill in Ohio accepted thousands of drums containing mixed industrial waste. In the 1990s, an environmental consulting company was hired to remediate the landfill so the land could be redeveloped. Since the landfill was immediately adjacent to a neighborhood, a monitoring program was developed to ensure that neighborhood residents were not exposed to hazardous compounds that might become airborne during the process of digging up, removing, repackaging and transporting the waste for proper disposal or destruction. Five portable Voyager GCs were deployed at the fenceline for this purpose. Compounds of interest included benzene, toluene, vinyl chloride, trichloroethylene, tetrachloroethylene, ethylbenzene, para-, meta- and ortho-xylene.
A standard operating procedure was agreed to by federal and state regulatory agencies, the property owner, environmental consulting company and neighbors. The portable GCs were placed in small shelters. The equipment automatically sampled ambient air at the cleanup site for the compounds of interest. Results were collected at the end of each day, compiled and reported to all interested parties on a regular basis. If a compound exceeded the action limit, changes were made in the cleanup process to ensure that the action limit for a particular compound would not be exceeded again. The portable GCs were able to provide real time data that allowed all parties involved to make immediate decisions while keeping the project on schedule.
Sample analysis by GC consists of several parts: accurate calibration, the sample introduction, separation of the sample inside the GC and detection of the desired compounds. |
Manufactured Gas Plant Cleanup
Throughout the United States there are thousands of sites where manufactured gas was produced from a variety of raw materials for distribution to local homes and businesses. With the widespread development of pipelines that allowed distribution of natural gas captured at well heads, these manufactured gas plants became obsolete. Most of these sites were abandoned or converted to other users. In the last five years, a number of sites have been remediated. During remediation it is important to monitor the ambient air around the site perimeter to protect the heath of the surrounding community.
A joint venture between a utility company and an environmental consulting company has developed a real-time monitoring system for use during cleanup operations. The system uses a Voyager portable GC to monitor total volatile organic compounds at the site. The total VOC readings from up to nine GCs are reported back via radio link to a trailer at the site. If any total VOC reading exceeds a preset action limit, the portable GC will automatically switch to GC mode, run an analysis and then report the presence and concentration of benzene, toluene, xylenes and 1,2,4-trimethylbenzene. If the level of these compounds exceeds a preset limit, cleanup work is stopped and measures are taken to control release of these compounds in the air. This system has been used at multiple locations and meets the regulatory agency's requirements for a continuous ambient air monitoring system.
This article originally appeared in the May 2002 issue of Environmental Protection, Vol. 13, No. 5, p. 28.
This article originally appeared in the 05/01/2002 issue of Environmental Protection.