Final Report: Evaluation of a Proton Transfer Reaction Mass Spectrometer for Analysis of Mobile Source Air Toxics in Engine ExhaustEPA Contract Number: 68D02073
Title: Evaluation of a Proton Transfer Reaction Mass Spectrometer for Analysis of Mobile Source Air Toxics in Engine Exhaust
Investigators: Grossenbacher, John
Small Business: Griffin Analytical Technologies
EPA Contact: Manager, SBIR Program
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $92,193
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)
Under the Clean Air Act, the U.S. Environmental Protection Agency has identified 21 chemical compounds as Mobile Source Air Toxics. Because no on-vehicle, real-time method exists for determination of these compounds in motor vehicle engine exhaust, there is an opportunity for application of a miniature mass spectrometer (MMS) used in conjunction with a proton transfer reaction (PTR) inlet/ionization source to address this problem. This research project was intended to determine the technical feasibility of an analytical instrument package including sample inlet, PTR ionization source, and control/data acquisition software for use with the miniature cylindrical ion trap mass spectrometer and deliver a detailed design of a beta-version prototype. While the Phase I effort demonstrated the feasibility of the proposed system, Phase II will be aimed at construction and development of a prototype instrument system. This will result in an on-vehicle MMS-based instrument system housed in a rugged package, direct gas-phase sample inlet port(s), and robust software for instrument control and data acquisition/manipulation.
During Phase I, the critical components of the PTR-MMS system were designed and evaluated in terms of their technical feasibility and anticipated analytical performance in such areas as reproducibility, sensitivity, sampling frequency, and limit(s) of detection. The majority of the compounds listed as Mobile Source Air Toxics are known to have proton affinities that make them amenable to ionization via proton transfer reaction according to the literature, so sensitive and selective detection was expected through the innovative combination of PTR inlet/ionization source and MMS.
The use of commercial components requiring only slight modifications provides the basis for the various structures throughout the inlet/ionization source system. The components include tubing and fittings capable of maintaining reduced pressures that assemble in a standard modular fashion. For proper operation of the flow drift tube portion of the inlet/ionization source, additional pumping is required at the PTR/MMS interface region to maintain the necessary sample flow rate at a sufficiently reduced pressure. A pumping system consisting of a rugged turbo pump with a backing pump in the form of a diaphragm pump has been identified that will maintain a relatively small overall instrument size for on-vehicle applications. It is important to note that the addition of this pumping system will significantly increase the power requirements for the entire PTR-MMS instrument. This will require additional power in the form of batteries to be carried with the instrument during operation, particularly during startup when the power draw of the instrument, specifically the pumps, is highest.
Both laboratory experiments and computer simulations were conducted to evaluate the design of PTR system components. Specific tests were conducted to evaluate the discharge ion source within the proposed PTR setup. Although not identical to the proposed hollow cathode discharge source, the glow discharge arrangement tested as part of the Phase I evaluation provided valuable insight as to the ion production and transport efficiencies of the instrument. It was found that the discharge is capable of producing copious amounts of ions that subsequently are transported to the analyzer with reasonable efficiency. The electric fields generated within the flow drift tube portion of the inlet/ionization source were modeled in silico, and it was determined that under the proposed operating conditions, reactant and product ions were effectively delivered from the source to the ion optics and, subsequently, the analyzer. Based on these results, the design for the PTR system components likely will provide good analytical performance, which will be further optimized through testing/refinement of the prototype instrument.
Software is another important area of development/evaluation during the Phase I effort. Griffin Analytical Technologies, Inc., has worked closely with Elm Street Software, LLC, to develop innovative software for instrument control/data acquisition for use with the PTR-MMS instrument. The instrument user controls the instrument by means of a Windows-based graphical user interface. The user can adjust voltages, calculate and apply waveforms to the ion trap, and set timing events such as gating of ions, switching of valves, etc. Mass spectral data that are acquired can be displayed in real time or in a user-defined averaged mode. The total ion chromatogram information (i.e., the total signal at the detector) also is acquired and can be displayed. Data being displayed in any or all of these formats can be saved at any time during an analysis (data manipulation is another important component of the developed software package). A calibration routine also is included that allows for mass calibration. The innovative combination of a powerful yet intuitive instrument control interface, data display, and data manipulation window makes the PTR-MMS instrument a useful analytical tool.
Overall, the instrument components designed as part of the Phase I project allow for a relatively small instrument footprint with commercial applications in other areas that can benefit from rugged, miniaturized instrumentation. Significant commercial applications exist in the areas of air quality monitoring, respiratory toxicology, process monitoring, and homeland defense in which qualitative and/or quantitative chemical information is needed at the point of sample collection. Because many applications for miniaturized, real-time analyses exist, Griffin Analytical Technologies, Inc., is pursuing the commercialization of MMS-based instrumentation, including the PTR-MMS system evaluated in this work. An important commercial objective is to develop innovative sample introduction/ionization techniques that when used in conjunction with the MMS provide potentially in-situ solutions to a wide range of analytical problems. The PTR system represents a major step forward in this direction.
As a result of the Phase I work, it was found that the unique and innovative combination of the proposed PTR inlet/ionization source and MMS instrument represents a feasible approach to monitoring the mobile source air toxics in engine exhaust. One area of concern, which will be addressed in Phase II, is the power requirements of the pumping system (e.g., turbo pump and backing pump) connected to the flow drift tube portion of the PTR system. Although an area of concern, the power requirements are mitigated by the fact that the PTR-MMS system is not intended to be operated as a portable (i.e., carried by an operator) instrument, rather as a vehicle-based sampling platform for real-time monitoring of components of engine exhaust. The work performed in Phase I provides a solid starting point for construction and testing of a prototype PTR-MMS instrument in Phase II with a high likelihood of success.