Final Report: A New Compact Portable Field Instrument for Continuous Real-Time Measurement of Trace Organic Air Pollution Emissions Using Jet-REMPI Mass SpectrometryEPA Contract Number: 68D02034
Title: A New Compact Portable Field Instrument for Continuous Real-Time Measurement of Trace Organic Air Pollution Emissions Using Jet-REMPI Mass Spectrometry
Investigators: Barnes, Rhett James
Small Business: OPOTEK Inc.
EPA Contact: Manager, SBIR Program
Project Period: April 1, 2002 through September 1, 2002
Project Amount: $69,995
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)
For many important hazardous air pollutants there is no instrumentation currently
available that is capable of making sensitive, real-time measurements of concentration
levels in the field. The capability for real-time detection and identification
of these pollutants is an essential component for emissions and dispersion modeling,
source apportionment, and ultimately of human exposure modeling. The goal of
this Phase I research project was to demonstrate that a compact tunable ultraviolet
(UV) laser system could be used for the real-time detection of aromatic hydrocarbon
hazardous air pollution vapors at sub-ppb levels using the jet-resonance-enhanced
multiphoton ionization (jet-REMPI) technique. This technique already has been
shown to be highly effective for the real-time measurement of complex mixtures
of hydrocarbon vapors in the laboratory, but to date it has required both a
large time-of-flight mass spectrometer as well as a large, complex, and delicate
high-resolution tunable UV laser system. By reducing the size and increasing
the ruggedness of the laser component, OPOTEK, Inc., has taken the initial step
towards transitioning this powerful technology from the laboratory and developing
a field-portable instrument.
Jet-REMPI technology combines the principles of optical spectroscopy and mass spectrometry to provide a "two-dimensional" detection selectivity. Simultaneous detection by both mass and optical spectroscopy yields extremely high chemical selectivity that is crucial to identifying one trace compound in the midst of many other similar species. The high selectivity and sensitivity of the technique allow real-time measurements to be obtained within seconds and without the need for pre-concentration or additional separation, as in gas chromatography-mass spectrometry, for example.
The overall goal of the project was development of a field-portable jet-REMPI system, capable of detection limits in the 100 ppb range for a broad range of compounds, including small aromatics such as the benzene, toluene, ethylbenzene, xylenes (BTEX) family; phenols; chlorobenzene; etc. With typical urban air concentrations for the BTEX family on the order of 10 ppb, this type of instrumentation will provide meaningful real-time data on hazardous air pollutants at real-world levels. The real-time nature of the detection will facilitate critical measurements that cannot be obtained using current technology, including: time-varying concentration measurement of variable pollution sources, and characterization of transient pollution emissions; mapping of the spatial pollution distribution by making rapid measurements at multiple locations using a single portable instrument; and providing timely online data on the efficacy of process waste-stream remediation.
In addition to its role in environmental monitoring, the unique capabilities of the instrument make it attractive in industrial process monitoring as well, allowing real-time monitoring of trace species for process optimization, continuous monitoring of waste streams for rapid waste remediation, and highly sensitive monitoring of contaminants in the clean-room environment.
To achieve the primary Phase I goal of proving the viability of using a compact UV laser system for jet-REMPI detection of small aromatics, OPOTEK, Inc., has teamed with researchers at SRI International, who have been successfully demonstrating the unique capabilities of jet-REMPI in the laboratory on a wide variety of hazardous air pollutants. The project was divided into four main tasks:
- Design, assemble, and test a compact tunable UV laser system tailored for
- Interface the laser system with SRI's existing jet-REPMI time-of-flight
mass spectrometer platform.
- Obtain detection sensitivity and chemical specificity performance data for
the system with a variety of small aromatic compounds.
- Generate preliminary designs for a complete compact jet-REMPI instrument capable of taking data in the field.
For the primary task of assessing the effect of the laser's lower spectral resolution
on chemical specificity and detection sensitivity performance, OPOTEK, Inc.,
chose to focus on a small group of single-ring aromatics, including benzene,
toluene, the xylenes, and chlorobenzene. The initial performance tests involved
assessing the optical spectroscopy performance of the system, which is one key
in determining the chemical specificity of the system. Detection sensitivity
measurements then were taken for pure compounds, samples with complex mixtures
of non-isomers, and mixtures of isomers.
Because the jet-REMPI technique involves a "2-D" selection comprising both a mass-resolving step and an optical spectroscopy step, the resolution of the laser system affects the ability to distinguish between different molecules, especially between isomers, where the mass resolution step alone cannot provide any distinction. To study the effect of the laser resolution, long "survey" scans were taken over the spectral transitions of several aromatic species, including isomers of the same compound, to prove that the laser can isolate unique transitions for each species. These scans verified that unique spectral transitions can be found to distinguish isomers that could not otherwise be separated using mass spectrometry alone, and provided a "map" for later chemical specificity tests using complex mixtures. The initial survey scans showed that the spectral linewidth of the laser is sufficient for high chemical specificity for the smaller single-ring aromatics under study, based on the sparseness of jet-cooled spectra and the narrow spectral peaks.
Because sensitive detection in a complex mixture is much more difficult than in a pure compound, the detection limits for several pure compounds initially were measured. This was followed by more meaningful detection limit determinations from multicomponent (non-isomers) mixtures in which a varying concentration of one component was mixed with a fixed background representative of high urban air concentrations. Finally, detection limits were measured using a mixture of three isomers that cannot otherwise be selectively identified by mass-spectrometry alone without additional separation. Detection limits on the order of 200-500 ppt were found for both pure compounds and mixtures, including the resolution of isomers, with high (greater than100:1) chemical selectivity for individual species, even for mixtures of isomers.
Although the performance test during Phase II eventually will dictate the components of the final instrument design, a preliminary concept design has been generated for a complete compact portable jet-REMPI instrument based on a next-generation laser system design and compact mass spectrometer. Components were chosen for size as well as ruggedness in the field environment. This concept design can be used as a basis for evaluating the feasibility of deploying such a system for real-world applications. The entire unit will measure 18" (W) x 25" (D) x 38" (H), weigh approximately 200 lbs, be entirely air cooled, and require only 110V 15A electrical service. The small package size and limited external requirements make this type of system ideal for many portable measurements that can take advantage of jet-REMPI's unique real-time performance capabilities.
With respect to initial performance goals, the research presented in this report has been very successful, meeting or exceeding the performance goals outlined in the initial proposal. With detection sensitivity in the 100 ppt range, even for complex mixtures, the actual performance of the jet-REMPI system with compact UV laser system compares very well with calculated estimates of 300 ppt. These numbers are more than sufficient to provide accurate detection and quantification at real-world concentration levels in the ppb range. In addition, the chemical and isomer selectivity performance is quite good, with very high (greater than100:1) isomer selectivity in our initial tests. This provides a unique capability for real-time mass spectrometric detection that would otherwise require both lengthy preconcentration and lengthy separation by additional means such as gas chromatography.
These performance benchmarks established at this stage are very promising for the further development and miniaturization of the system in Phase II. Based on the preliminary integrated compact jet-REMPI instrument design developed during Phase I, the level of performance presented in this work should be obtainable even in the final configuration, which includes both compact laser and mass spectrometer. These performance parameters, as well as the projected size of the system, will provide a unique instrument for novel, real-time field measurements that currently cannot be made via any other experimental methods.
Based on initial marketing research assessing the commercial interest for this type of instrument, demand for a flexible real-time highly sensitive method of quantifying a broad number of species is important not only for the environmental monitoring market, but also for industrial process control and contaminant monitoring as well. OPOTEK, Inc., has initiated discussion with industrial end users to assess the viability of this technology for specific process contaminant issues. Because the financial incentive for obtaining solutions to these problems is high, such applications can provide an ideal source of funding the eventual development and commercialization of a product that, by the nature of its technology, is flexible enough to serve both industrial and environmental users.