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: EPD04060
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, 2004 through June 30, 2005
Project Amount: $224,832
RFA: Small Business Innovation Research (SBIR) - Phase II (2004) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , Ecological Indicators/Assessment/Restoration , SBIR - Monitoring
The goal of the research carried out under this Phase II contract was to demonstrate that a compact jet-REMPI instrument could be used for the real-time detection of aromatic hydrocarbon hazardous air pollution vapors at sub-ppb levels. For this important class of hazardous air pollutants, there is no currently available instrumentation 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 technique also has the ability to tackle industrial applications such as process monitoring, combustion diagnostics and waste remediation.
The jet-REMPI technique has already been shown to be highly effective for the real-time measurement of complex mixtures of hydrocarbon vapors in the laboratory, but to date has required both a large time-of-flight mass spectrometer (TOF-MS) and a large, complex and delicate high-resolution tunable ultraviolet (UV) laser system. By reducing the size and increasing the ruggedness of both the laser and mass spectrometer components, Opotek Inc. has been able to demonstrate the initial performance of a prototype field-portable instrument package. This work leverages upon the experience that Opotek’s subcontractors and research collaborators at SRI International have gained, characterizing such a laboratory jet-REMPI system.
Jet-REMPI technology combines the principles of optical spectroscopy and mass spectrometry to provide a “2-D” 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, without the need for pre-concentration or additional separation, as is the case, for example, in gas chromatography-mass spectrometry (GC-MS) analysis.
The overall goal of the Phase I and Phase II project components was development and demonstration of a field-portable jet-REMPI system, capable of detection limits in the 100-ppt range for a broad range of compounds, including small aromatics such as the BTEX family (benzene, toluene, ethylbenzene and xylene), phenols and chlorobenzene. These specifications were met and demonstrated in initial measurements of field air samples containing ambient traces of these volatile organic compounds, as well as for real-time analysis of combustion sources. 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 instrument has unique capabilities that make it attractive in industrial process monitoring, allowing real-time monitoring of trace species for process optimization, combustion diagnostics, and highly sensitive monitoring of contaminants in the clean-room environment.
To achieve Opotek’s Phase II goals of building, characterizing and field-testing a field-capable jet-REMPI instrument, the project was divided into four phases:
- Laser design, construction and testing
- Design and construction of the portable jet-REMPI instrument
- Laboratory instrument characterization
- Field sample analysis
The laser development phase involved improvements to the tunable-wavelength UV optical parametric oscillator (OPO) laser system, in order to increase the stability of the output power, especially in varying ambient temperature environments. The primary system components that were addressed were the pump-laser harmonics generators and the UV-doubling module. New hardware and software components were developed to allow for automated, real-time optimization of the three non-linear crystals in these sub-systems. The effectiveness of these controls for improving laser output stability was assessed for the laser system prior to its integration into the jet-REMPI instrument.
The Phase II prototype jet-REMPI instrument was based on two primary system components: the compact laser system and the compact TOF-MS. These two components were integrated into a single, moveable system frame, along with support hardware for vacuum, data acquisition, laser control and optics.
The jet-REMPI instrument underwent a number of performance tests in the laboratory prior to field sample analysis. A reference library of jet-REMPI spectra was recorded for a number of pure compounds, and the detection limit for the various compounds was obtained. Relative signal for the various components with respect to an external calibrant gas mixture was measured to allow for absolute concentration measurements to be obtained from the instrument. Signal stability over time was also measured.
The performance of the jet-REMPI instrument was then tested under two separate field-sampling trials: direct real-time sampling of combustion gases and direct real-time sampling of ambient air at a single location.
The tunable laser system is a key component of the jet-REMPI system, providing the optical spectral selectivity that contributes to the high overall chemical selectivity, and providing the high sensitivity through efficient ion generation. For the Phase II jet-REMPI instrument, the tunable UV laser source utilized is an OPO, which yields a broad and flexible wavelength tuning range from a solid-state optical device. The bulk of the laser development phase at Opotek involved the development of active feedback control electronics for stabilization of the laser output power via continuous optimization of three harmonics conversion stages: the second- and third-harmonics generation stages for the OPO pump beam, and the tunable UV-doubling stage.
After testing a number of options for active stabilization schemes, a full-time optimization scheme for the harmonics generation stages (second harmonic generator, SHG, and third harmonic generator, THG) was chosen, with continuous phase-matching adjustments made based on output beam asymmetry measurements. A dedicated custom hardware and software package was built to detect asymmetry in the SHG and THG output beams, and a closed-loop linear motor system was implemented to provide active control of the respective crystal angles.
Both beam asymmetry and peak-finding schemes were tested for optimization of the UV-doubling stage. In this case, it was found that the beam asymmetry was too unreliable, being influenced by multiple factors from the previous optical stages that contribute to the beam asymmetry. A peak-finding algorithm was found to be more stable, and was incorporated into the existing laser-control software to periodically provide optimization of the crystal angle prior to each wavelength scan.
The effect of these active stabilization schemes on the UV output level was measured over time, and ambient temperature was varied over ± 8 C. The output was found to be constant, within the ± 10 percent pulse-to-pulse fluctuation inherent in the OPO output. This level of stability is much greater than the initial system used for the Phase I experiments, which required frequent optimization by the user in order to stabilize the output power.
jet-REMPI System Integration
The actively stabilized tunable UV laser system was combined, along with a compact TOF-MS package and associated support hardware, in a single system frame that can be rolled on integrated wheels to different sampling locations. Hardware included on the system frame included computers for data acquisition and processing, computer and signal-processing hardware for automated laser harmonics control, pulsed valve support electronics, and mass spectrometer power supply. The final system dimensions—48 inches wide, 38 inches tall and 24 inches deep—were larger than optimum to allow for flexibility in introducing the novel hardware of the Phase II project, particularly the laser diagnostics and control hardware systems. In future deployments, the size of the system may be reduced to approximately two-thirds of the volume of the current prototype system.
Laboratory Instrument Characterization
In preparation for the field sample analysis, the performance of the jet-REMPI instrument was characterized, and a reference library of jet-REMPI spectra from pure compounds was generated. The system characterization involved detection limit and linearity measurements, relative detection sensitivity measurements versus the calibration gas mixture, and signal stability over time. Reference jet-REMPI spectra were measured from pure samples of many of the species expected to be detected in the actual field samples, primarily including the substituted single-ring aromatics and some multi-ring species. These reference spectra facilitated resolution of the individual components in the complex ambient mixtures found in the field samples.
Detection-limit characterization was performed by measuring varying calibrated dilutions of pure compounds prepared by a conventional permeation-tube system. Absolute detection limits were measured for several compounds, including toluene and benzene, and relative detection limits were then derived from the relative measured signals from a calibrated TOC-14 gas mixture. Initial detection limits were higher than expected, at approximately 1 ppb, due to contamination in the sample inlet system. This performance level was worse than the previously demonstrated performance of approximately 300 to 500 ppt for benzene and toluene, but was sufficient to perform detection demonstrations for field samples from the diesel combustion source. A cleaned sample inlet system was later installed, including a new pulsed valve, which allowed sub-ppb level detection for the ambient urban air-sample measurements.
Reference jet-REMPI spectra were prepared for a range of single- and multiple-ring aromatic species expected to be present in the combustion and ambient field air samples. Both mass and wavelength scans were recorded across a broad range in order to uncover possible interference between individual components, and to generate a reduced list of scan parameters for efficient and fast measurements during the actual field sample analysis. 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 Amap@ for later chemical specificity tests using complex mixtures.
Field Sample Analysis
The primary focus of the Phase II project was to develop a prototype jet-REMPI instrument capable of demonstrating performance on actual field samples, and to provide the first demonstrations of its field performance. Two field sample demonstrations were to be performed: direct real-time sampling of combustion gases and direct real-time sampling of ambient air at a single location. For these tests, the combustion source was a diesel generator, and the direct ambient air measurements were made for ambient urban air in the Menlo Park, Calif. area, on the campus of SRI.
The most impressive demonstration of the unique capabilities of the jet-REMPI instrument for trace aromatic detection came from the combustion diagnostics demonstration. A diesel generator was made available on the SRI campus, and air samples were measured in real time at various distances from the exhaust. Even at a 5-foot distance from the exhaust, concentrations in the 80 to 400 ppb range could readily be measured by the sub-ppb sensitivity demonstrated by the system. The unique real-time detection and quantification was demonstrated during time-dependency measurements for several components as the exhaust makeup developed from its cold-start to steady-state composition after engine warm-up. These real-time, second-by-second measurements could not be obtained by existing competing technologies such as GC/MS. Technologies such as surface acoustic wave devices, which can provide rapid detection, cannot yield the high chemical selectivity that is critical for detection in complex exhaust streams.
The final field sample demonstration, which occurred in the Menlo Park, California area, was for direct, real-time sampling of unprocessed urban air. This type of real-time analysis of trace species, at the ppb level, is important for understanding the time-varying human exposure to such species, and cannot currently be obtained by existing instrumentation. The direct sampling of ambient urban air was able to demonstrate detection and quantification for aromatic species at much lower concentrations: 500 ppt for toluene and 50 ppt for benzene, which is at the detection limit demonstrated in the laboratory characterization. As a demonstration of the real-time monitoring capability of the instrument, a time-dependent concentration of toluene was obtained. The level sensitivity and high temporal resolution demonstrated in this experiment will be key to assessing the time-dependent exposure levels of these toxic aromatic pollutants, and cannot be obtained with any other existing technologies.
During the course of the Phase I and II SBIR project, Opotek and its subcontractor and research collaborators at SRI have transitioned the jet-REMPI technology from a large laboratory-based device to a relatively compact prototype instrument capable of direct, real-time trace analysis of aromatic species.
The Phase I project initially demonstrated the abilities of a more compact laser system component to provide sensitivity in the 100-ppt range, even for complex mixtures. This performance level compared very well with Opotek’s initial of 300 ppt goals prior to initiation of the Phase I project. These levels 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 was quite good, with very high (>100-to-1) isomer selectivity demonstrated. The Phase I project demonstrated a unique capability for real-time mass spectrometric detection that would otherwise require both lengthy pre-concentration and lengthy separation by additional means such as GC.
The Phase II project continued the transition of the technology into an actual instrument package, integrating the compact and ruggedized laser into a complete compact instrument package, including both compact laser and mass spectrometer, and demonstrated the prototype=s real-time trace detection capability in measurements on actual field air samples. The system provided real-time quantification of species for both exhaust sources and ambient air that currently cannot be obtained via any other experimental method.
Based on Opotek’s initial marketing research assessing the commercial interest for this type of instrument, demand for a flexible, real-time and highly sensitive method of quantifying a broad number of species is important for not only the environmental monitoring market, but also for industrial process control, combustion diagnostics and contaminant monitoring. Opotek has initiated discussion with industrial end users to assess the viability of this technology for both specific combustion analysis applications and process contaminant issues. Since 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.