Final Report: Pneumatic Focusing Gas Chromatography: A Continuous, Automated, Ambient, Fenceline and Fugitive Emissions Monitoring InstrumentEPA Contract Number: EPD04020
Title: Pneumatic Focusing Gas Chromatography: A Continuous, Automated, Ambient, Fenceline and Fugitive Emissions Monitoring Instrument
Investigators: Hard, Thomas M. , O'Brien, Thomas J.
Small Business: VOC Technologies Inc.
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2004 through August 31, 2004
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , SBIR - Air Pollution , Small Business Innovation Research (SBIR)
The goal of this research project was to construct two pneumatic focusing gas chromatographs and apply them to the analysis of atmospheric pollutants, volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) or air toxics. One instrument would be deployed at an Oregon Department of Environmental Quality (ODEQ) monitoring site for field testing and demonstration. The second instrument would be used for laboratory studies of sensitivity, precision, resolution, and durability.
VOC Technologies has developed patent-pending new technology for analysis of VOCs by compressing an air sample to high pressure before injecting it into a specially designed gas chromatograph. Compression both concentrates the sample and removes water vapor by condensation. This procedure is called pneumatic focusing gas chromatography (PFGC). This highly automated new technology has the potential to lower the cost of a VOC analysis by more than a factor of 10.
In PFGC, an air sample with a volume of 0.1-10,000 cc is pressurized to 600 psi (or higher) and injected directly on-column in a specially designed gas chromatograph, which operates at high pressure. The pressurization or pneumatic-focusing step both concentrates the sample by a factor of 40 (or more) and removes water by condensation. Nonretained chromatographic peaks get 40 times narrower and 40 times higher at constant area. This improves both resolution and sensitivity. Retained chromatographic peaks are further narrowed on the column head. In this process, the back end of the peak “catches up” with the front end as it enters the column and slows because of retention. At some point, compounds “stop” upon entering the column and are in essence “focused” at the column head. Upon oven heating, these compounds elute as well- formed peaks. The first compound to “stop” becomes the narrowest in the chromatogram; subsequent peaks widen normally as a result of diffusion. Thus, PFGC has the nontypical peak profile of wide, narrowing, narrowest, widening. Wide peaks at the start of a chromatogram are less of a problem because fewer isomers occur for low molecular weight compounds.
VOC Technologies’ PFGCs were constructed entirely within a personal computer case (patent-pending), as illustrated in Figures 1 and 2. PFGC models vary, but the one shown in Figure 1 shows a 6-inch integral monitor mounted in place of the CD drive. Figure 2 is a side view with the cover removed. Figure 1 displays the electronics, including the motherboard, the hard drive, the controller circuitry, and the proprietary 20bit A/D electrometer-amplifier. Line voltage and the standard computer power supply provide all needed power. Figure 2 displays a 300 cc sample loop, the chromatography oven, and the sampling pump.
Figure 1. VOC Technologies’ pneumatic focusing gas chromatograph housed within a personal computer.
Figure 2. Side view.
Two pneumatic focusing gas chromatographs were outfitted with dual photoionization-flame ionization detectors (PID-FID) in a series and outfitted with the proprietary VOC columns. One of these went into the field at an ODEQ monitoring site; the other remained in the laboratory for additional testing and calibration. The study was conducted in cooperation with the ODEQ. The ODEQ used the U.S. Environmental Protection Agency Standard Method TO-14. The thrust of the pilot study was to compare hourly PFGC data to traditional 24-hour-integrated sample collection, shipment, and gas chromatography/mass spectrometry analysis.
In this study, VOC Technologies experienced and fixed several instrumental difficulties either through component exchange or by redesign of faulty systems. More than 20 individual VOCs successfully were monitored at the field site for a period of several months.
In concurrent laboratory experiments with the second PFGC, calibration, precision, and resolution experiments were carried out. It was discovered that ambient methane (present in almost any air sample at constant 1.8 ppmV concentration) can serve as an internal standard for the FID in every chromatogram. Aromatic hydrocarbons (e.g., benzene) give virtually identical signals on both the PID and the FID. Benzene thus can serve as a cross calibration for the PID, which suffers from lamp deterioration and decreased response with time. The FID is a virtually constant-carbon-response instrument that requires no maintenance, while the PID is far more selective in its response, which also depends upon the ionizing lamp wavelength.
In calibration experiments with chlorobenzene, VOC Technologies determined PFGC precision of 1 part per trillion by volume for concentrations in the sub-ppb range for both the PID and the FID (see Figure 3) using a 300 cc on-column injection.
Figure 3. Exponential dilution of chlorobenzene with room air in 1m 3 teflon chamber. Standard error in concentration is 1.02 parts per trillion by volume.
The PFGC performed with only minor problems over a period of several months at the ODEQ field site. A typical day’s chromatic data for the FID detector are shown in Figure 4. Notice the pollution “spike” around midday, which would be completely “washed out” in a 24-hour canister sample. Note that there is significant variation in the benzene/toluene ratio, typically 0.6 for auto exhaust, indicating periodic, nonautomotive sources. These would be impossible to detect with the 6-day, single, 24-hour integrated sample TO-14 protocol. A week’s worth of PFGC data will cost less than the single TO-14 canister analysis.Figure 5 illustrates a continuous record of toluene and benzene over a 10-day period.
Figure 4. One-day set of continuous, hourly chromatograms showing midday spike in concentrations of several VOCs. Ambient methane (1.8 ppm) peak area attenuated 10-fold and serves as an internal FID standard. The PID chromatogram was similar, but showed fewer peaks resulting from a lack of response for many VOCs.
Figure 5. Continuous record of toluene and benzene for a 10-day period.
Once minor design problems and difficulties were solved, the constructed PFGC instruments were found to be sensitive, precise, rugged, and portable. Further work will address remaining technical issues and demonstrate long-term performance.
The advantages of VOC Technologies’ PFGC over conventional analysis methods include significantly higher automation, lower cost, internal calibration on every sample, as well as a continuous record of emissions, which invites correlation with weather patterns and emission sources. Being fully portable, the PFGC can be easily transported between monitoring sites. With sample sizes ranging from 0.1 to 10,000 cc, PFGC operates equally well in indoor, outdoor, process, fugitive, industrial, and stack monitoring situations. On several occasions, extraordinary single-hour pollution events were documented, which are totally washed out when a 24-hour integrated TO-14 canister sample is analyzed. The PFGC can be configured with a range of detectors and columns to suit almost any air monitoring situation. PFGC construction entirely within a Windows Operating System personal computer easily enables a variety of other computational functions, such as standard programs, networking, data communication, etc. This technology was developed at Portland State University. VOC Technologies is in the final stages of patent approval for this technology. Instruments are available for sale or lease. Contract monitoring also is possible.