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Dec 10, 1998 - Single-photon photoionization mass spectrometry (SPPI-MS) with a ... In recent years, two-dimensional maps of species concentrations, ...
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Anal. Chem. 1999, 71, 364-372

Mapping of Trace Hydrocarbon Concentrations in Two-Dimensional Flames Using Single-Photon Photoionization Mass Spectrometry Charles S. McEnally,*,† Lisa D. Pfefferle,† Rahima K. Mohammed,† Mitchell D. Smooke,† and Meredith B. Colket‡

Yale University Center for Combustion Studies, New Haven, Connecticut 06520-8286, and United Technologies Research Center, East Hartford, Connecticut 06108

In recent years, two-dimensional maps of species concentrations, obtained with planar laser Raman or fluorescence techniques, have proven useful in validating and improving computer simulations of multidimensional combusting flows.1-3 Unfortunately, the most important species in many aspects of combustion are nonfluorescent trace hydrocarbons for which laser imaging is not possible. Although such species can usually be measured with extractive gas sampling followed by gas chromatography, infrared spectroscopy, or mass spectrometry,4 the necessity for tracking a large number of samples and the long measurement

times associated with these techniques have made multidimensional mapping impractical. A significant example of a combustion situation where trace hydrocarbons are important is the formation and growth of aromatic species in flames. This process accounts for most of the ambient concentrations of many air toxics such as benzene and benzo[a]pyrene.5 Perhaps more importantly, continued growth of aromatic species leads to formation of soot. Soot production can greatly affect the performance of combustion devices by altering their radiation heat-transfer characteristics, by clogging flow passages, and by eroding exposed surfaces such as those on turbine blades.6 Also, soot comprises a significant fraction of the sub-2.5-µm-diameter particulates in urban air; these fine particulates are the subject of a recent ambient air quality standard due to their possible toxicity.5,7 During combustion of most fuels, the formation of the initial aromatic species is a key rate-limiting step to soot formation,6,8,9 which therefore must be modeled correctly to accurately predict the final soot concentrations.10,11 We have recently developed a technique for analyzing combustion gas mixtures with extractive microprobe sampling followed by on-line single-photon photoionization mass spectrometry (SPPIMS); it can measure most C3-C12 hydrocarbons simultaneously at part-per-million concentrations with total measurement times of less than 1 min, and without caching of samples.12 The goal of the work reported here was to examine whether this technique could be used to generate two-dimensional images of hydrocarbon concentrations suitable for comparison with computational simulations of aromatic formation and growth. Measurements were made in an axisymmetric nonpremixed methane/air jet flame which has

* Corresponding author: (phone) (203)-432-4059; (fax) (203)-432-7232; (email) [email protected]. † Yale University. ‡ United Technologies. (1) Smooke, M. D.; Lin, P.; Lam, J. K.; Long, M. B. Computational and Experimental Study of a Laminar Axisymmetric Methane-Air Diffusion Flame. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 575-582. (2) Smooke, M. D.; Xu, Y.; Zurn, R. M.; Lin, P.; Frank, J. H.; Long, M. B. Computational and Experimental Study of OH and CH Radicals in Axisymmetric Laminar Diffusion Flames. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 813-821. (3) Smooke, M. D.; Ern, A.; Tanoff, M. A.; Valdati, B. A.; Mohammed, R. K.; Long, M. B. Computational and Experimental Study of NO in an Axisymmetric Laminar Diffusion Flame. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1996; pp 2161-2170. (4) Fristrom, R. M. Flame Structure and Processes; Oxford University Press: New York, 1995; pp 142-150.

(5) Koshland, C. P. Impacts and Control of Air Toxics from Combustion. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1996; pp 2049-2065. (6) Glassman, I. Combustion; Academic Press: Orlando, FL, 1987; pp 360-375. (7) Kaiser, J. Science 1997, 277, 466-469. (8) McEnally, C. S.; Pfefferle, L. D. Combust. Sci. Technol. 1997, 128, 257278. (9) McEnally, C. S.; Pfefferle, L. D. Combust. Flame 1998, 112, 545-558. (10) Smooke, M. D.; McEnally, C. S.; Pfefferle, L. D.; Hall, R. J.; Colket, M. B. Combust. Flame, in press. (11) McEnally, C. S.; Schaffer, A. M.; Long, M. B.; Pfefferle, L. D.; Smooke, M. D.; Colket, M. B.; Hall, R. J. Computational and Experimental Study of Soot Formation in a Coflow, Laminar Ethylene Diffusion Flame. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, in press. (12) McEnally, C. S.; Pfefferle, L. D. Combust. Sci. Technol. 1996, 116-117, 183-209.

Single-photon photoionization mass spectrometry (SPPIMS) with a vacuum ultraviolet laser beam at 118 nm has been used to quantify trace hydrocarbons sampled with a quartz microprobe from an axisymmetric nonpremixed methane/air jet flame. More than 20 C3-C12 hydrocarbons were detected, including linear species, single-ring aromatics, and two- or three-ring polynuclear aromatic hydrocarbons. For each of these species, high-resolution two-dimensional concentration maps were obtained that are suitable for comparison with detailed computer models of aromatic hydrocarbon formation and growth. Preliminary comparison with such a model indicates that the sampling process accurately captures the spatial structure of the flame. In general, the results show that the broadband sensitivity, part-per-million detection limits, negligible ion fragmentation, and rapid data acquisition rate of SPPI-MS make it an ideal technique for studying the complex hydrocarbon chemistry that occurs in flames.

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© 1999 American Chemical Society Published on Web 12/10/1998

Figure 1. Schematic diagram of the burner, sampling system, and portions of the mass spectrometer.

been used to validate detailed flame structure models through comparison of model predictions with laser images of temperature, major species, and chain-carrying radicals.1,2 Maps of more than 20 trace hydrocarbons were obtained; representative examples are presented and discussed below. The models are currently being extended to include formation of aromatic hydrocarbons and soot,10,11,13 and some preliminary comparisons with the measured species maps are also presented to assess the viability of the microprobe sampling process. A more extensive discussion of the model results, including reaction pathway analysis, etc., will be presented elsewhere.14 EXPERIMENTAL METHODS Burner and Flame Conditions. The two-dimensional axisymmetric flame was generated with a burner in which the fuel, a mixture of 65 mol % methane in nitrogen, flowed from an uncooled 0.4-cm-diameter vertical brass tube and the oxidizer, air, flowed from the annular region between this tube and a 5.0-cmdiameter concentric tube (see Figure 1). Nitrogen was added to the methane to suppress soot formation, since doing so simplified the measurements and eliminated uncertainties in the model due to soot radiation, “scrubbing” of hydrocarbons by soot, etc. The total fuel flow rate was 260 cm3/min (at STP) and the air flow rate was 41 000 cm3/min, which produced a football-shaped flame with a length of roughly 3.2 cm. The fuel tube of this burner is open, such that its outlet velocity distribution is parabolic (average velocity 35 cm/s), and the air annulus is filled by a honeycomb, such that its outlet velocity is uniform (velocity 35 cm/s). The absence of a honeycomb in the fuel tube makes this burner slightly different from that used in earlier work.1 The flame is unconfined and the flow is laminar. The combination of nitrogen dilution of the fuel, a narrow fuel tube, and a high air flow rate caused the flame to be lifted several millimeters above the burner surface. This prevented heat transfer from the flame to the burner and consequent preheating of the (13) Mohammed, R. K.; Tanoff, M. A.; Smooke, M. D.; Schaffer, A. M.; Long, M. B. Computational and Experimental Study of a Forced, Time-Varying, Axisymmetric, Laminar Diffusion Flame. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, in press. (14) Mohammed, R. K.; Smooke, M. D.; McEnally, C. S.; Pfefferle, L. D.; Colket, M. B., in preparation.

reactants. Thus, the thermal boundary conditions at the burner surface are well-definedsthe reactant temperature equals room temperature at all radiiswhich is crucial to obtaining good agreement between the model and experiments.10,11 Gas Sampling. Gas samples were extracted from the flame with a quartz microprobe and transmitted directly to the vacuum chamber through a sample line (see Figure 1). The microprobe has an outer diameter of 0.9 cm, but narrows over its last 1.5 cm to a diameter of less than 1 mm at the tip. The orifice has a diameter of ∼240 µm. The stainless steel sample line is roughly 4 m long and its walls were heated to 140 °C with heat tape. It enters the high-vacuum chamber of the mass spectrometer and terminates in a plate containing a 430-µm-diameter orifice; flow of a portion of the sample gases through this orifice produces the molecular beam that intersects the ionizing laser beam. The flow rate through the microprobe orifice, and therefore the flow rate into the mass spectrometer, will vary due to changes in the gas temperature and, in soot-containing flame regions, changes in the microprobe orifice area caused by soot deposition. To minimize the latter effect, the microprobe was cleaned in the oxidizing layer on the outer edge of the flame between each measurement. We factored out remaining changes in the sample gas flow rate by using an internal standard: argon was added to the fuel at a concentration of 1 mol %, which matched its natural concentration in the oxidizer, and then the relative sample gas flow rate was determined by assuming that the argon mole fraction remained 0.01 throughout the flame. In soot-free regions of the flame the temperature profile can be recovered from the argon flow rate data, indicating that this assumption is reasonable. The flow rate varies by up to 70%, so this correction is important. An essential function of the microprobe is to rapidly quench reactions in the sampled gases so that their composition is not altered from that in the flame. Previous studies have demonstrated that hydrocarbons can be successfully sampled from atmosphericpressure flames if the backing pressure is low, on order of 1050 Torr,15-18 and if the microprobe walls are cooled.19 In our experiments, both of these conditions are met: the backing pressure was roughly 1 Torr, and the microprobe was aligned horizontal to the burner surface so that its walls were maintained near room temperature by convective cooling to the coflowing air stream (see Figure 1). Two observations indicate that our sampling procedure produces effective quenching of reactions. First, in earlier measurements in a slightly different flame, we found that hydrocarbon concentrations were insensitive to the backing pressure.12 Since the residence time of the sample gas in the microprobe tip is proportional to the backing pressure, this insensitivity implies that microprobe reactions are not significant.15,17 Second, we have compared measurements of benzene and acetylene in a coflowing ethylene/air flame with model calculations and obtained good quantitative agreement.11 (15) Schoenung, S. M.; Hanson, R. K. Combust. Sci. Technol. 1981, 24, 227237. (16) Colket, M. B., Chiappetta, L.; Guile, R. N.; Zabielski, M. F.; Seery, D. J. Combust. Flame, 1982, 44, 3-14. (17) Kaiser, E. W.; Rothschild, W. G.; Lavoie, G. A. Combust Sci. Technol. 1984, 41, 271-289. (18) Reference 4, pp 139-142; pp 177-179. (19) Kassem, M.; Qun, M.; Senkan, S. M. Combust. Sci. Technol. 1989, 67, 147157.

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After reactions have been quenched, the next concern is to convey the sample gases to the mass spectrometer without net losses to the walls of the sample line. At the sample manifold temperature of 140 °C, the vapor pressure of pyrene, a four-ring aromatic species, is roughly 1 Torr,20 which is comparable to the total pressure in the sample line; thus condensation will not occur for aromatics at least this large (or for water). However, when the microprobe is moved to a new flame location, hydrocarbons flowing through the sample line will still experience net adsorption or desorption until the adsorbed concentration is in equilibrium with the new gas-phase concentration. In these experiments, a 15-s equilibration time was allowed before each measurement. As will be discussed in the Results and Discussion section, this proved to be sufficient for measurements of C1-C12 hydrocarbons, which is the mass range of primary interest for studying formation of aromatic species and their initial growth. Nonetheless, the mass spectrometer is capable of measuring much larger species, so adsorption is the factor that limits the upper end of the mass range. This limit can be extended by allowing longer equilibration times and/or by heating the sample line to higher temperatures; for example, Castaldi and co-workers have used on-line sampling with wall temperatures of 300 °C to measure species as large as benzo[a]pyrene (five rings).21 Locations in the flame are described by the Cartesian XYZ coordinate system shown in Figure 1, which has its origin at the center of the fuel tube outlet and its X axis aligned with the microprobe center line. The burner is moved relative to the microprobe by three translation stages, and its relative location along these axes is indicated by scales on these stages. Determining the absolute location of the measurements with respect to the coordinate system is more difficult due to the likelihood that the “center of gravity” of the sampled gas is some distance in front of the physical location of the orifice.17,22 We assume that this displacement is entirely along the microprobe axis; therefore, since our microprobe is horizontal, the height of the measurement is the same as that of the orifice. The measurements were located in the horizontal XY plane through a two-step process. First, selected profiles were measured in the X direction, and X ) 0 was placed at the symmetry axis of these profiles. Then the final data were acquired as a series of Y profiles at X ) 0 and a range of Z, and Y ) 0 was placed at the symmetry axis of each of these profiles. Overall, we estimate that the relative and absolute uncertainties in the coordinates are (0.01 and (0.05 cm. Mass Spectrometer. The gas samples were analyzed with a custom-built single-photon photoionization mass spectrometer. The basic idea of SPPI is to ionize the target gases by exposing them to a laser beam whose photon energy (EP) exceeds their ionization energy (EI). Since absorption is effectively continuous beyond the ionization threshold, almost any molecule for which EI < EP will absorb the incident light and subsequently ionize. Thus, SPPI can ionize large numbers of species simultaneously and SPPI mass spectrometers possess broad-band sensitivity, which is extremely useful for studying a complex reactive system such as a flame. At the same time, since the ionizing laser beam is monoenergetic and the difference EP - EI is generally (20) White, C. M. J. Chem. Eng. Data 1986, 31, 198-203. (21) Castaldi, M. J.; Vincitore, A. M.; Senkan, S. M. Combust. Sci. Technol. 1995, 107, 1-19. (22) Reference 4, pp 58-60.

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comparable to or smaller than bond strengths, the resulting ions suffer minimal fragmentation.23 This attribute is essential for studying the chemically complex fuel-rich flame samples considered here. Finally, photoionization cross sections are large enough, for the range of EP - EI encountered in a vacuum ultraviolet excitation of hydrocarbons, for SPPI mass spectrometers to have good sensitivity. In recognition of these advantages, at least one other group has used SPPI-MS in flames, to measure onedimensional profiles.24 Mass spectrometers based on SPPI can be usefully compared with those that employ resonance-enhanced multiphoton ionization (REMPI), which have also been used for analyzing hydrocarbons in flames.25-27 In REMPI, one or more photons with EP < EI raise the target molecules to an excited electronic state, and then additional photon(s) ionize the excited molecules. This technique offers much better selectivity than SPPI, since only one or a few specific compounds will have an electronic state at the right energy to absorb the incident photons. Also, REMPI-MS typically has better sensitivity since higher laser intensities can be produced at the relevant wavelengths, which are in the ultraviolet, than at those for SPPI-MS, which are in the vacuum-ultraviolet. On the other hand, since REMPI is selective, the excitation wavelength(s) must be changed to measure different species, making comprehensive analysis of a sample much more difficult. Furthermore, the spectroscopic information needed to choose the REMPI wavelengths can be difficult to obtain. In general then, SPPI and REMPI are highly complementary since they have contrasting strengths (broad-band sensitivity versus selectivity) and can be performed in the same apparatus simply by changing the laser source. In our SPPI spectrometer, the ionization source is a laser beam at 118 nm, which is generated by frequency-tripling the third harmonic of a Nd:YAG laser (Spectra Physics DCR-3G) in a xenon cell.28 Photons at 118 nm have an energy of 10.5 V, which exceeds the ionization energies of all C3 and larger hydrocarbons except propane and butane;29 thus, SPPI at this wavelength is well suited to analysis of trace hydrocarbons in combustion. The 118-nm light is focused through the molecular beam by a lens at the highvacuum side of the cell (see Figure 1). The ions are separated by space-focused time-of-flight mass filtering30 and detected with a dual-plate microchannel plate detector (R. M. Jordan C-701). The detector output is digitized and averaged over multiple laser shots by a digitizing oscilloscope (LeCroy 9410), and the results are stored on a personal computer. Finally, the ion signal in each mass (23) Arps, J. H.; Chen, C. H.; McCann, M. P.; Datskou, I. Appl. Spectrosc. 1989, 43, 1210-1214. (24) Werner, J. H.; Cool, T. A. The combustion of trichloroethylene studied with VUV photoionization mass spectrometry. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, in press. (25) Bernstein, J. S.; Fain, A.; Choi, J. B.; Cool, T. A.; Sausa, R. C.; Howard, S. L.; Locke, R. J.; Miziolek, A. W. Combust. Flame 1993, 92, 85-105. (26) Siegmann, K.; Hepp, H.; Sattler, K. Combust. Sci. Technol. 1995, 109, 165181. (27) Gittins, C. M.; Castaldi, M. J.; Senkan, S. M.; Rohlfing, E. A. Anal. Chem. 1997, 69, 286-293. (28) Kung, A. H.; Young, J. F.; Harris, S. E. Appl. Phys. Lett. 1973, 22, 301-302; 1976, 28, 294. (29) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry; American Chemical Society: New York, 1988. (30) Wiley: W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150-1157.

channel is numerically integrated using the midpoint rule and converted to a concentration by custom-written software. The efficiency of 355- to 118-nm conversion in pure xenon is quite low, on the order of 0.01-0.1%,28 so the 118-nm output is embedded in a much more intense beam of residual 355-nm light. Thus, there is the potential for the ions formed from one 118-nm photon to absorb one or more 355-nm photons and fragment, which would destroy one of the primary advantages of the SPPI technique. We minimize this phenomenon with differential focusing of the 118- and 355-nm beams. The lens on the output side of the xenon cell is made of MgF2, which has significantly different indices of refraction at 118 and 355 nm, and the geometry of the cell is chosen such that the transmitted 355-nm light is collimated while the 118-nm light is focused through the molecular beam. This differential focusing is essential for preventing ion fragmentation and is well worth the lower 118-nm transmittance of MgF2 compared with LiF. The benzene concentration at one height on the center line was calibrated by switching the gas flow into the mass spectrometer back and forth between flame gases and a known concentration benzene/nitrogen mixture. Then a vertical profile along the center line was measured and pegged to the value at the calibration location, and finally, radial profiles were measured at a range of heights and pegged to the appropriate center line values. With this procedure, every measurement occurs within 20 min of a calibration measurement; repeated measurements show that the sensitivity of the instrument changes very little over this time scale. Overall, the absolute uncertainty in the benzene concentrations is estimated to be 30%. For other selected hydrocarbons, the sensitivity of the instrument relative to that for benzene was determined from gas mixtures and used to calibrate their concentrations. However, many of the mass peaks observed in the flame cannot be directly calibrated either because they are composed of multiple isomers whose ratios are unknown (e.g., the C3H4 peak at 40 amu contains contributions from both propadiene and propyne)31 or because the corresponding hydrocarbons are too unstable to be commercially available (e.g., diacetylene which produces the C4H2 peak at 50 amu). In these cases, the sensitivity is assumed to be equal to that for benzene. Photoionization cross sections at 118 nm do not vary greatly for the species considered here,23,32,33 so the resulting concentrations are considered accurate to within a factor of 3. The relative uncertainty in all measurements is estimated to be 30%. COMPUTATIONAL METHODS The flame was modeled by solving the full set of elliptic equations for the conservation of total mass, momentum, energy, and individual species mass. Detailed transport coefficients and a finite-rate C6-chain kinetic mechanism with 140 chemical reactions and 39 chemical species were used in the calculations. The chemical kinetic mechanism, described in ref 34, is a modified version of that used by Smooke and co-workers,2 with additional (31) Saito, K.; Williams, F. A.; Gordon, A. S. Trans. ASME/J. Heat Transfer 1986, 108, 640-648. (32) Koizumi, H. J. Chem. Phys. 1991, 95, 5846-5852. (33) Arrington, C. A.; Ramos, C.; Robinson, A. D.; Zwier, T. S. J. Phys. Chem. A 1998, 102, 3315-3322. (34) Mohammed, R. K., Ph.D. Thesis, Department of Mechanical Engineering, Yale University, New Haven, CT, 1998.

C3-C6 chemistry derived from the mechanisms of Sun and coworkers35 and Wang and Frenklach.36 The system was closed with the ideal gas law and appropriate boundary conditions on each edge of the computational domain. We have also included an optically thin radiation model in our calculations and have assumed that for this essentially soot-free flame the only radiating species are water, carbon monoxide, and carbon dioxide.37 The governing equations were written in two-dimensional, axisymmetric form in primitive variables. The nonstaggered grid has been applied in this formulation by using a one-sided difference approximation for the pressure gradients in the momentum conservation equations and the velocity gradients in the continuity equation.38-40 The one-sided difference avoids oddeven pressure decoupling that occurs from the use of a central difference approximation. The reduced accuracy of the one-sided differencing scheme was offset by nonuniform gridding. The governing equations and boundary conditions were conservatively discretized by utilizing an implicit finite difference (nine-point stencil) technique on a nonstaggered, nonuniform grid. Diffusion terms were approximated by centered differences and convective terms by a monotonicity-preserving upwind scheme. The resulting system of nonlinear algebraic equations, written in residual form, was solved by a modified damped Newton’s method with a nested preconditioned (block Gauss-Seidel) BiCG-STAB iteration technique.41 RESULTS AND DISCUSSION Spectrometer Performance. Figures 2 and 3 show mass spectra measured in a room-temperature nonreacting gas mixture and at the center line of the methane/air flame 2.0 cm above the burner surface. They demonstrate several aspects of the mass spectrometer’s performance: Broad-Band Sensitivity. All five components of the nonreacting mixture (propadiene, propene, 1,3-butadiene, benzene, methylcyclohexane) are detected, and in the flame spectrum ∼20 different species are observed. The latter include linear species (propadiene/propyne, propene, diacetylene, vinylacetylene, butadiene, etc.), single-ring aromatics (benzene, toluene, phenylacetylene, styrene, diethynylbenzene, etc.), two-ring aromatics (indene, naphthalene, dihydronaphthalene), and three-ring aromatics (acenaphthylene, anthracene/phenanthrene). Thus, the SPPI-MS measurements in the flame provide a comprehensive picture of the chemistry that produces single-ring aromatics and causes their growth to two- and three-ring species. Furthermore, given the broad-band sensitivity of SPPI, we can be confident that those species that generate little or no signal in Figure 3, such as all C5 species and linear species larger than C6, are present, if at all, at low concentrations in this part of the flame. The elemental formula for each mass peak in Figure 3 was identified by assuming that pure hydrocarbons dominated oxygenated hydrocarbons at each mass and that formulas with a larger (35) Sun, C. J.; Sung, C. J.; Wang, H.; Law, C. K. Combust. Flame 1996, 107, 321-335. (36) Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173-221. (37) Hall, R. J. J. Quant. Spectros. Radiat. Transfer 1993, 49, 517-523; 1994, 51, 635-644. (38) Abdallah, S. J. Comput. Phys. 1987, 70, 182-192; 70, 193-202. (39) Armfield, S. W. Comput. Fluids 1991, 20, 1-17. (40) Babu, V.; Korpela, S. A. Comput. Fluids 1994, 23, 675-691. (41) van der Vorst, H. A. SIAM J. Sci. Stat. Comput. 1992, 13, 631-644.

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Figure 2. Composite mass spectrum produced by adding individual mass spectra of two nonreacting gas mixtures, one containing 132 ppm of propadiene (C3H4; 40 amu) in nitrogen and the other 1000 ppm each of propene (C3H6; 42 amu), 1,3-butadiene (C4H6, 54 amu), benzene (C6H6, 78 amu), and methylcyclohexane (C7H14; 98 amu) in nitrogen. The propadiene spectrum was multiplied by 1000/132 so that its peak height would be directly comparable to the other hydrocarbons.

Figure 3. Mass spectrum measured on the center line of the methane/air flame 2.0 cm above the burner surface.

C/H ratio dominate when two pure hydrocarbon formulas are possible (i.e., 128 amu is C10H8, not C9H20). The first assumption is justified by the fact that little or no signal is observed at masses that correspond only to oxygenates (e.g., C3H6O at 58 amu), that regular hydrocarbons exist at every mass where signals were observed, and that the signal in every mass channel peaks well to the fuel-rich side of the flame front. The second assumption follows from the relative thermodynamic stability of different hydrocarbon classes at high temperatures.42 The main species corresponding to each elemental formula are identified by comparison with previous gas chromatographic studies of hydrocarbons in methane/air nonpremixed flames.31,43 Rapid Measurements. All of the results presented here were obtained by averaging 259 single-shot spectra, which requires 33 (42) Stein, S. E.; Fahr, A. J. Phys. Chem. 1985, 89, 3714-3725. (43) Prado, G.; Garo, A.; Ko, A.; Sarofim, A. Polycyclic Aromatic Hydrocarbons Formation and Destruction in a Laminar Diffusion Flame. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1984; pp 989-996.

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s, for a total measurement time of 48 s when the equilibration time of the sample line is included. The averaging rate is limited by the processing rate of the oscilloscope (8 Hz) and the repetition rate of the YAG laser (10 Hz). Low Detection Limits. The signal-to-noise ratios in the flame spectrum (Figure 3) demonstrate the parts-per-million sensitivity of the spectrometer: the clearly distinct C5H6 peak at 66 amu has an intensity corresponding to ∼1 ppm. The sensitivity is limited by the low intensities of the 118-nm laser beam, the short averaging times, and the low density in the molecular beam. We have demonstrated sensitivity close to 0.1 ppm using 2000 shot averages elsewhere.12 In any case, for a highly reactive system such as the flame probed here, measurements of lower concentration species are increasingly suspect due to possible interferences from radical recombination reactions in the microprobe. Mass Resolution. Each hydrocarbon mass peak in Figures 2 and 3 is accompanied by a satellite peak at 1 amu greater mass due to one of the carbons being 13C instead of 12C. These isotope peaks are clearly separated from their companion peak, showing that the mass resolution is sufficient to distinguish species 1 amu apart throughout the mass range of interest. The mass resolution is limited by translational velocity components perpendicular to the time-of-flight axis in the molecular beam and the 5-ns temporal width of the laser pulse.44 Minimal Fragmentation. The most prominent mass peaks in the nonreacting mixture spectrum (Figure 2), aside from the parent ion and isotope peaks, are at 39, 53, 56, and 94 amu. The 39 and 53 amu peaks are likely C3H3+ and C4H3+ fragments, while the 56 amu peak is most likely a butene impurity in the gas mixture and the 94 amu peak a background gas in our vacuum system from other experiments. In any case, the intensities of these peaks are all less than 0.3% that of the parent ion peaks. Similarly, in the flame spectrum (Figure 3), the most prominent mass peaks that do not correspond to a stable hydrocarbon are at 39, 117, and 123 amu. Again their intensities are all very low, equivalent to less than 1 ppm of a species in the flame. These results can be compared to electron-impact spectra from methane/ air nonpremixed flames, where ion fragment signals are much more prominent.45 Uniform Sensitivity. The nonreacting gas mixture spectrum gives an indication of the species-to-species variation in the instrument response. The intensities of the five mass peaks agree to within 20%, despite the wide variation in the structure of these hydrocarbons. These results are consistent with our estimate that using the benzene calibration factor for all measured species produces absolute concentrations that are accurate to within a factor of 3. Note that in the results presented here we have not assumed that mass discrimination occurs at the orifices, as we had done in earlier work.12 Species Maps. Species maps were obtained for each of the 20 or so hydrocarbons appearing in Figure 3; some examples are shown in Figures 4 and 5. In these figures, the horizontal and vertical directions correspond to the horizontal and vertical coordinates in the flame, and the concentration at each flame location is represented by a color, as indicated by the color scale to the right of each map. Measurements were performed on a (44) Opsal, R. B.; Owens, K. G.; Reilly, J. P. Anal. Chem. 1985, 57, 1884-1889. (45) Hamins, A.; Anderson, D. T.; Miller, J. H. Combust. Sci. Technol. 1990, 71, 175-195.

Figure 4. Maps of diacetylene (left) and vinylacetylene (right) mole fractions experimentally measured in the methane/air flame.

rectangular grid with a spacing of 0.05 cm horizontally and 0.1 cm vertically. This grid encompassed the entire hydrocarboncontaining portion of the flame except the lowest 0.7 cm, where measurements were impossible due to attachment of the flame to the microprobe instead of the burner. A frame approximately indicating the boundary of the data grid is overlaid on the flame in Figure 1; for clarity, this frame is shown in the XZ plane instead of the YZ plane. The 462 data points acquired for each species were plotted in the maps without any smoothing, and then the software (Amtec Corp. Tecplot 7.0) used linear interpolation to fill the pixels between them. The data grid and artifacts from the interpolation strategy are somewhat apparent in the toluene map on the left-hand side of figure. In these maps, the burner surface is at Z ) 0, with the fuel tube outlet between Y ) -0.2 and +0.2 cm, and the air outlet at larger and smaller Y. Thus the hydrocarbons are present in the fuel-rich region centered over the fuel tube, as one would expect. The bullet-shaped surface where the hydrocarbon concentrations drop to zero roughly coincides with the high-temperature oxidizing flame front, whose location was determined by visual observations of blue luminosity from the flame and temperature measurements with a thermocouple. These maps clearly demonstrate that the combination of extractive gas sampling and SPPI-MS is able to furnish detailed information about the spatial structure of the flame. For example,

comparison of the two panels in Figure 4 shows that the maximum diacetylene (C4H2) concentrations occur significantly later in the flame than those of vinylacetylene (C4H4) and that the region of high vinylacetylene concentration extends over a larger region than that of diacetylene. These observations are consistent with the ideas that in nonpremixed methane flames vinylacetylene and diacetylene are formed from butadiene in the sequence C4H6 f C4H4 f C4H246 and that high temperature thermodynamic stability favors less saturated species.42 In this two-dimensional coflowing flame, the vertical distance from the maximum of C4H4 to that of C4H2 is 0.4 cm, which is easily resolved with a microprobe. In comparison, in opposed-jet nonpremixed flames, which have often been studied since they are effectively one-dimensional, the peaks of all intermediate hydrocarbons coincide to within the resolution of the sampling process.47 The maps in Figure 5 indicate the range of our measurements in terms of sensitivity and molecular weight. Toluene, shown on the left-hand side, is a good test of sensitivity since it has a maximum concentration of only 6 ppm. In fact, since toluene (46) Leung, K. M.; Lindstedt, R. P. Combust. Flame 1995, 102, 129-160. (47) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Lutz, A. E.; Vincitore, A. M.; Senkan, S. M. Chemical Kinetic Modeling of a Methane Opposed Flow Diffusion Flame and Comparison to Experiments. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, in press.

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Figure 5. Maps of toluene (left) and naphthalene (right) mole fractions experimentally measured in the methane/air flame.

produced one of the weaker mass peaks in Figure 3, even though this spectrum was acquired at the location of maximum toluene concentration, it is something of a worst-case scenario among the species visible in that figure. Nonetheless, while the toluene map is somewhat noisy, spatial information can still be inferred from it. In particular, toluene is formed significantly earlier than naphthalene and roughly at the same time as vinylacetylene. The naphthalene map on the right-hand side of Figure 5 shows that species as large as two-ring aromatics can be effectively mapped. The measurements were organized in the form of radial profiles from left to right at each height; therefore the horizontal symmetry of the naphthalene map indicates that the 15-s equilibration time before each measurement was sufficient for the adsorbed naphthalene concentration to relax into equilibrium with the new gas-phase concentration. Otherwise, net adsorption would have occurred on the left side of the center line, when the microprobe was moving toward higher concentrations, and net desorption on the right side, when the microprobe was moving toward lower concentrations, so the profiles would have been skewed to the right. The only other maps of nonfluorescent trace hydrocarbons in multidimensional flames appear to be those measured by Miller and co-workers using extractive sampling and electron impact 370

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mass spectrometry.45,48 Their maps cover only a portion of their flame and are restricted to species with higher concentrations than most of those measured here. Preliminary Comparison with Modeling. Although one primarily thinks of measurements as testing computational simulations, in this case, comparison between the experimental and modeling results also indicates whether the microprobe sampling technique is viable. This is an important question given that the flame is only 0.8 cm wide, which may be too small considering the finite spatial resolution of the probing process, and that the flame is lifted from the burner surface, making it relatively susceptible to microprobe disturbances. Figure 6 shows the predicted concentrations of diacetylene and vinylacetylene in maps that are directly equivalent to the experimental results in Figure 4. Since the calculations extend to the burner surface, they clearly show that the flame is lifted such that a substantial nonreacting zone exists between it and the burner surface. The spatial distribution of the calculated and measured concentrations is quite similar, except in two respects. First, the calculated liftoff height is ∼0.6 cm longer than the measured liftoff height, causing the calculated profiles to be displaced upward by 0.6 cm compared to the measured profiles. The lengths of the (48) Smyth, K. C.; Miller, J. H. Science 1987, 236, 1540-1546.

Figure 6. Maps of diacetylene (left) and vinylacetylene (right) mole fractions calculated for the methane/air flame.

reaction zone are similar, indicating that the discrepancy is primarily a shifting and not a stretching of the profiles. The liftoff height is extremely sensitive to the boundary conditions, and we believe the difference in liftoff heights is due to small differences between the idealized and real boundary conditions. For example, the air velocity profile is assumed to be flat, but in reality the honeycomb near the fuel tube is somewhat squashed, such that the velocity is probably lower near the tube, and experiments showed that decreasing the overall air flow rate noticeably decreases the liftoff height. In a slightly different burner, with better defined boundary conditions, the model predicts the liftoff height much more accurately.1-3 This difference in liftoff heights indicates the value of full twodimensional maps of species concentrations. If only radial profiles at a few heights were compared, then the agreement between the model and experiment would appear to be very poor. In contrast, it is immediately clear from the two-dimensional maps that the measurements and predictions are similar except for the vertical displacement. The second discrepancy is that the experimental profiles are somewhat wider than the calculated profiles, which is probably attributable to the finite spatial resolution of the microprobe. The

spatial resolution of microprobes are about five orifice diameters,49 or 0.1-0.2 cm in our case. This is consistent with the difference in the widths of the measured and computed species profiles, which is ∼0.2 cm. However, the overall agreement between the spatial structure of the measurements and calculations is quite encouraging and indicates that the microprobe technique delivers results adequate for testing model predictions. This is particularly significant given that the flame was chosen with laser diagnostics in mind and is something of a worst-case scenario for microprobe measurements due to its liftoff from the burner surface and small dimensions. In the case of the C4 species, comparison shows that the model correctly predicts the displacement of diacetylene to larger heights compared with vinylacetylene, its more rapid decrease near the tip, and its narrower vertical extent. In terms of absolute concentration, diacetylene is predicted well, but vinylacetylene is significantly underpredicted. CONCLUDING REMARKS All practical combustion systems are multidimensional; and are in fact turbulent flows, which are intrinsically three-dimen(49) Smyth, K. C.; Miller, J. H.; Dorfman, R. C.; Mallard, W. G.; Santoro, R. J. Combust. Flame 1985, 62, 157-181.

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sional. Therefore, an important goal of combustion science is to extend one-dimensional flame structure models that incorporate detailed chemistry to higher dimensions. We have shown here that, with the techniques of microprobe sampling and on-line single-photon photoionization mass spectrometry, hydrocarbon concentrations in steady multidimensional flames can be mapped with spatial resolution adequate for comparison to such models. ACKNOWLEDGMENT We appreciate assistance with the experiments from Elanor Williams and partial financial support from the United States Air

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Force (Grant F49620-94-1-0085), the Environmental Protection Agency (Grant R821206-01-0), the National Science Foundation (CTS-9714222), and the Department of Energy (Grant DE-FG0288ER-13966).

Received for review July 24, 1998. Accepted October 27, 1998. AC980818R