Articles Anal. Chem. 1997, 69, 286-293 Real-Time Quantitative Analysis of Combustion-Generated Polycyclic Aromatic Hydrocarbons by Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry Christopher M. Gittins, ² Marco J. Castaldi, ‡ Selim M. Senkan, ‡ and Eric A. Rohlfing* ,² Combustion Research Facility, Sandia National Laboratories, Mail Stop 9055, Livermore, California 94551, and Department of Chemical Engineering, University of California, Los Angeles, California 92004 We have combined resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry with on-line flame sampling to determine the centerline con- centrations of naphthalene, fluorene, and anthracene in a pure methane + oxygen/argon (1:5) diffusion flame. Naphthalene concentrations between 100 parts per billion by volume (ppbV) and 6 parts per million by volume (ppmV) and fluorene concentrations below 50 ppbV are determined using one-color REMPI on jet-cooled samples extracted from the flame; anthracene concentrations in the 5-40 ppbV range are determined using two-color REMPI. The REMPI ion signals are converted to absolute concentrations in real time by performing gas-phase standard additions to the flame sample. Isomer-selective detection of larger polycyclic aromatic hydrocarbons, such as perylene and benzo[a]pyrene, is possible using the two- color REMPI approach. Risk assessment studies consistently point to the need to regulate the emissions of polycyclic aromatic hydrocarbons (PAHs) and their derivatives formed at trace levels in a variety of combustion devices. Recent legislative actions on air toxics, for example as stipulated in the 1990 Clean Air Act Amendments, target the emission of such trace combustion byproducts with increasing stringency. The average concentration of PAH in the exhaust from a typical 250 000 barrel/day petroleum refinery or 300 MW fossil-fuel-fired power generation plant under normal operating conditions is in the parts per billion by volume (ppbV) or lower range. 1,2 However, controlled experiments using a research-grade, industrial style diffusion flame burner show that emissions of PAH precursors increase by orders of magnitude during substoichiometric operation. 3 (PAH concentrations were not measured in ref 3.) This observation implies that PAH emissions could be dramatically reduced through active control of the combustion process that avoids conditions favorable to their formation. To implement effective control strategies for environ- mentally benign combustion processes, a highly sensitive tech- nique that can provide emissions data in real time and at a moderate cost is needed. The detection sensitivities of commercial analytical equipment dictate that all current PAH determinations be made via the “preconcentration” method. 4-6 In this approach, gases bearing trace PAHs are withdrawn from combustion processes using sampling probes and concentrated on a variety of organic absorbents. The adsorbed PAHs are recovered by solvent extraction and concentrated. Their concentrations are quantified by addition of internal standards and subsequent analysis using gas chromatography/mass spectrometry and liquid chromatog- raphy/fluorescence. The conventional analytical approach only provides evidence for PAH emission long after the sampling process and is clearly unsuited for use as a continuous emissions monitor. We address the need for real-time methods for the speciation, characterization, and quantification of gas-phase PAHs at the ppbV level by coupling two well-known analytical techniques: standard additions, commonly used to compensate for sample matrix effects when making absorbance-based concentration measurements, and resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS). We demonstrate the effective- ness of our approach on an atmospheric pressure, coflowing diffusion flame of methane and oxygen, which provides a prototypical source of various PAHs in the ppbV to ppmV range. ² Sandia National Laboratories. ‡ UCLA. (1) Stockdale, R.; Lev-On, M.; Meeks, N.; Reheis, C. H. Western States Petroleum Association (WSPA) Pooled Source Test Program. Industry Specific Air Toxics Emission Factors. Paper presented at the Air and Waste Management Association Meeting, Pittsburgh, PA, March, 1991. (2) Bjorseth, A., Ramdahl, T., Eds. Handbook of Polycyclic Aromatic Hydrocarbons, Vol. 2; Marcel Dekker Inc.: New York, 1985; Chapter 2. (3) Edwards, C. F.; Goix, P. J. Combust. Sci. Technol., in press. (4) Bjorseth, A., Ed. Handbook of Polycyclic Aromatic Hydrocarbons; Marcel Dekker Inc.: New York, 1983. (5) Otson, R.; Leach, J. M.; Chung, L. T. K. Anal. Chem. 1987, 59, 1701- 1705. (6) CARB, California Air Resources Board. Determination of Polycyclic Aromatic Hydrocarbon Emissions from Stationary Sources. Method 429; Sept 12, 1989. 286 Analytical Chemistry, Vol. 69, No. 3, February 1, 1997 S0003-2700(96)00969-9 CCC: $14.00 © 1997 American Chemical Society