Have Lab, Will Travel
Using portable high speed gas chromatographs for field monitoring
- By Jim Norgaard
- May 01, 2005
Even today, first responders, technicians, and professionals who need to monitor or evaluate volatile organic compounds (VOCs) in the environment have limited choices regarding gas measurement. Typically, the available options include using total VOC instruments, which provide a reasonable assessment of total VOC concentrations when used as a general survey type device, or collecting samples from the environment and taking them to a laboratory or facility, where more sophisticated instruments such as gas chromatographs can be used to provide a precise assessment of the VOC sample(s).
However, neither approach is satisfactory when such staff confront situations where personal safety may be at immediate risk and/or environmental harm is imminent. These scenarios are governed by statutory or industry imposed rules regarding VOCs encountered and appropriate responses. Permissible exposure levels (PELs) set by the U.S. Environmental Protection Agency (EPA) and the Occupation Safety and Health Administration (OSHA) for compounds such as acrylonitrile, 1,3-butadiene, benzene and phosphine are set at 1parts per million (ppm) or lower -- with industry imposed action levels often at a fraction of the official PEL.
What is essential, if not mandatory, are instruments that provide results quickly and with trustworthy accuracy. The focus of operators should be the job at hand, not their relative expertise with any particular analytical instrument.
Characteristics of Field Instrumentation
Over the last two decades, a variety of portable or transportable gas chromatographs have been specifically designed for field use. Each instrument has introduced innovations that have simplified the task of on-site air monitoring. Portable instrument users are well aware of the advantages of field sampling. These advantages include immediate and more representative results, rapid answers, and no sample degradation during transport. Results collected at the field site allow users to revise their sampling strategies in real time to possibly complete the sampling more efficiently and at a lower cost. What's more, in the case of an incident involving hazardous materials, rapid results can be crucial in determining the extent of a potential health hazard.
Naturally, a lighter, faster, and more sensitive field portable instrument is more beneficial for the end user. That being said, a portable instrument must be ergonomically designed for comfortable transport by a wide variety of users. High speed analysis is important, because it increases the number of samples that can be taken over the course of a day, better defining the boundaries of contamination. Low detection limits can also help accurately delineate the edges of a contamination plume or accurately assess compounds that have sub parts per billion (ppb) action levels.
There is significant discussion among professionals who deal with monitoring instrumentation regarding the very definition of field portable. There are many different definitions of portable that often depend on the user, the specific application, and both environmental and sampling conditions. Access to AC power in the field, the need to move the instrument around a site, sources of support equipment, intrinsic safety, etc. are important considerations at certain field monitoring sites. The general consensus regarding the definition of a field portable instrument is as follows:
- Combined weight below 15 pounds for the instrument and any required support equipment
- Size less than 1,000 cubic inches
- On-board consumables (e.g. carrier gas and batteries) for eight hours of field operation
- Built-in display and controls so a laptop is not required in the field to retrieve data or view results
- Rugged and weatherproof instruments with controls that are easy to manipulate while wearing thick gloves
Anecdotally, this definition seems to be reasonably well-accepted and puts some boundaries around what characteristics make an instrument portable.
Another important consideration is ease of use. A frequently heard comment is that an instrument is not very useful if no one can remember how to operate it. With staff reductions, inter-company transfers, and increased work loads, the expertise to effectively use an instrument is a valuable and limited resource. Recent advances in portable GC technology include "point and shoot" operation. This remarkably simple technique allows a user to utilize the analytical quality and precision of the GC with little more effort than pointing a probe at a source and pushing a "start" button. An easy-to-use instrument is a valuable addition to any air monitoring program.
Accurate and rapid screening of suspect volatile organic compounds (VOCs) in the field is another significant characteristic of a portable gas chromatograph. Measured against performance specifications provided by prominent TOVC instrument manufacturers, portable GCs easily surpass their performance, offering accuracy and precision variability well under 10 percent. Further, in keeping with the field-use criteria, a portable gas chromatograph should be self-contained, including battery power and carrier gas, and incorporate appropriate detector and valve technology to optimize the chromagraphic solution and speed of analysis.
Field Applications of Gas Chromatography
For decades, gas chromatography has been a well-established analytical technique, easily capable of identifying compounds in areas of interest. It is a comparative technique whereby compounds, introduced into and separated by the instrument's analytic column, are identified by matching their exit (elution) time from the column against library data stored in the instrument. The strength of the detection signal indicates the concentration level of the compound(s). Detection levels ranging from parts per million (ppm) to very low ppb levels are easily obtained with consistent accuracy and exceptional precision.
Signature compounds have been defined for a number of industries as well as for various environmental and law enforcement requirements in the United States and in other countries. These groups of compounds are effectively surrogates for a much broader range of compounds. Such compounds are often the most highly regulated for reasons of environmental hazard or for health and safety (e.g., general toxicity and/or cancer risk). These collectively monitored compound groups may number as few as two to four or as many as 10 to 12 compounds of interest.
For example, petroleum refining operations focus on benzene, toluene, ethyl benzene, and xylene (BTEX) detection to monitor their general quality of operation. In certain areas of plant operations, the decision to replace carbon canisters used for filtering process air focuses on the "leakage" of as little as 0.5 ppm of benzene. Decisions made on the basis of unreliable analysis techniques may expose a refinery to regulatory oversight and/or unnecessary expenses.
Similarly, environmental site cleanup organizations often set up perimeter monitoring equipment to detect BTEX as well as compounds such as 1,2,4-trimethylbenzene, and naphthalene. Ground disturbed during the cleanup activity may release these compounds into the air, necessitating immediate remedial action to prevent health risks impacting the communities surrounding the cleanup site.
Chemical plants concentrate on vinyl acetate, acrylonitrile, butadiene, and benzene (the VABB compounds) as they monitor their operations. Again, certain plant operations are governed by monitoring a single compound such as acrylonitrile or 1,3-butadiene, which like benzene, are known carcinogens and regulated to sub-ppm levels.
First responders approaching a clandestine drug lab may look for two particular compounds, (typically phosphine and ammonia), or as many as 12 particular compounds when assessing the risks and hazards of securing a clandestine drug lab operation. Statistics published by the Center for Disease Control point out that the most common injury suffered by first responders dealing with clandestine drug labs is respiratory damage, often attributed to poor site assessment and lack of personal protective equipment (PPE) usage. Further, the clean up of clandestine drug lab sites is increasingly measured by the elimination of certain select compounds. This work is the subject of pending federal legislation in the U.S. House of Representatives (H.R. 798), which directs EPA to determine which chemicals are present and detectable at clandestine drug labs and evaluate the effectiveness of cleanup techniques. Thirteen states so far have adopted legislation and/or executive guidelines regarding procedures for, and evaluation of site cleanup activity.
Field portable GCs, for example, such as the PetroPRO and other application specific GCs from Photovac, are designed to sample ambient air and be robust enough for use by operators in the field. Hydrophobic particle filters are used in these instruments to keep debris out of the sample inlet. Positive displacement diaphragm pumps draw samples into the sample loop. Run time for the pump may only be 10 to 20 seconds. The sample inlet path is be made from inert materials such as stainless steel and PEEK to maintain sample integrity, low loss at low concentrations, and low memory at high concentrations. Once the sample loop is filled, carrier gas sweeps the sample from the loop onto the precolumn; the compounds of interest are then gated onto the analytical column. No special sample preparation is required of the operator. This process is easily initiated by the operator via a single button activation.
Ultra zero air containing less than 0.1 ppm of total hydrocarbons is used as the carrier gas to push a sample through the portable GC. In order to increase the speed of analysis and eliminate sample condensation, a low temperature (below 80 degrees Celcius) heated isothermal oven is used to heat the valves, sample loop, pre-column, analytical column, and detector. The oven is low temperature for three reasons. First, high oven temperature would require a larger battery. Even with today's available battery technology, a serious weight penalty would be required to support a high temperature oven. Second, limited analysis (e.g., a short set of specific compounds) is well suited to a lower temperature oven. And third, keeping the oven at low temperatures is consistent with intrinsic safety standards (e.g., the required T4 rating specifies that no surface temperature in the instrument in contact with the sampling environment may be in excess of 135 degrees Celcius), which means the instrument can be safely used in locations where explosive atmospheres may be present. In most industries where VOCs are produced or used, the presence or potential presence of an explosive atmosphere is quite common.
VOC sample concentration values are measured by a photoionization detector (PID) in the GC. This detection method is optimized by the use of lamps whose wavelengths and intensity are matched to the industry or application VOCs targeted by the portable GC. While the 10.6eV lamp is most commonly used, a 10.0eV or an 11.7eV lamp may also be used. Photoionization is a time-proven technology that provides for very low detection limits for many volatile organic compounds. And, a proven technology such as photoionization is readily understood and acceptable to users and regulatory agencies that may be overseeing a project.
Whether conducting routine monitoring, making fiscally responsible decisions regarding capital equipment maintenance, or assessing situations involving immediate personal risk, users of today's innovative new portable GCs are better able to make informed critical judgments. These results protect the safety of employees at field sites and help secure the quality of the environment in surrounding communities.
This article originally appeared in the 05/01/2005 issue of Environmental Protection.