fluorescence interferences in raman scattering from combustion products

detector are illustrated in Fig. 2, for butter yellow dye separated from Stahl's solution. The increase in concentration gained through the use of PMD is shown by the increased absorbance of the bands, while the advantage of using the MCT detector is reflected in the reduced measurement time required to achieve a given noise level.

It is noteworthy that when separation is effected by PMD and an MCT detector is used for the infrared measurement the total scan time required to attain submicrogram sensitivity is less than 5 s (10 scans at 8 cm -1 resolution), even for such weakly absorbing samples as the chlorinated pesticides. These results suggest that it might be feasible to construct a device to measure infrared spectra automatically across an entire chromatoplate developed by PMD. Spectra could be measured after moving the plate by intervals as small as 1 mm in a total measurement time less than the time taken to develop the plate. The only unfavorable factor for such a device would be the difference between the shape of the infrared beam at its focus (circular) and the shape of a spot on a chromatoplate developed using PMD, which is a very elongated ellipse, typically 1 by 6 mm. However, it might be possible to use a toroidal mirror rotated 90 ° through its optical axis to deform the

beam to the desired shape as suggested by Low. s Such a device would be very useful for rapidly screening the components of complex mixtures.

The TLC-ir technique described above is able to be used for the identification of quantitites of strong infrared absorbers as low as 10 ng, as shown in Fig. 3. It may be noted that since the diameter of the infrared beam is larger than the width of the spot on the chromatoplate, a little over half the beam misses the sample completely. Thus, if the beam could be distorted to better match the shape of the spots produced by PMD, the detection limits should be further reduced by approximately a factor of 2.

ACKNOWLEDGMENT

This work was supported by Grant R804333 from the Environmental Protection Agency.

1. C. J. Percival and P. R. Griffiths, Anal. Chem. 47, 154 (1975). 2. M. M. Gomez-Taylor, D. Kuehl, and P. R. Griffiths, Appl. Spectrosc. 30,

447 (1976). 3. J. A. Perry, K. W. Haag, and L. J. Glunz, J. Chromatogr. Sci. 11, 447

(1973). 4. J. A. Perry, T. H. Jupille, and L. J. Glunz, Anal. Chem. 47, 65A (1975). 5. J. A. Perry and L. J. Glunz, J. Assoc. Off. Anal. Chem. 57, 832 (1974). 6. J. A. Perry, J. Chromatogr. llO, 27 (1975). 7. D. Kuehl and P. R. Griffiths, Anal. Chem. in press (1977). 8. M. J. D. Low, J. Ag. Food Chem. 19, 1124 (1971).

Fluorescence Interferences in Raman Scattering from Combustion Products

R. E. SETCHELL and D. P. AESCHLIMAN Combustion Research Division 8351, Sandia Laboratories, Livermore, California 94550 and Plasma and Fluid Physics Division 5217, Sandia Laboratories, Albuquerque, New Mexico 87115

Spectral interferences to Raman bands due to laser-induced fluorescence are examined in exhaust gases from an internal combustion engine operating on different fuels and fuel/air mixtures. A broad- band fluorescence spectrum is observed in all cases, but the relative fluorescence intensity depends strongly on the particular engine operating condition. Elimination of the fluorescence background by various exhaust conditioning devices confirmed the fact tha t aerosols of hydrocarbons or other organic compounds are the principal source.

Index Headings: Raman spectroscopy; Combustion products; Fluo- rescence.

INTRODUCTION

The ability to obtain local temperature and concentration measurements using laser Raman spectroscopy has been demonstrated in relatively nonluminous, particulate-free laboratory flames. 1-4 The application of this technique to experimental studies of practical com-

Received 20March 1977; revision received 15 Ju ly 1977.

bustion systems, such as gas turbine combustors, fur- naces, and internal combustion engines, is currently of considerable interest. Unfortunately, most practical combustion devices introduce the possibility of spectral interferences to Raman bands due to broad-band flame luminosity and emission resulting from laser absorption. Eckbreth ~ used a pulsed dye laser operating at 588 nm to generate nitrogen Raman spectra for temperature measurements in particulate-laden flames, but his efforts were hindered by incandescence from laser- heated particulates. Using a pulsed nitrogen laser operating at 337 nm, Bresowar and Leonard" and Leonard and Rubins 7 observed an intense, broad-band, laser- induced fluorescence from the exhaust gases of a gas turbine combustor. These observations prompted Roquemore and Hodgson" to investigate in detail the fluorescence spectra of exhaust gases from a turbine combustor operating on different fuels, inlet pressures and inlet temperatures. Exhaust collected with a sampling probe was bubbled through a hexane solvent, and the resulting solutions were examined with a commer-

530 Volume 31, Number 6, 1977 APPLIED SPECTROSCOPY

cial spectrofluorometer. Under excitation at 255 nm, all solutions showed a single broad band with a maximum near 325 nm. The peak fluorescence intensity varied considerably between samples corresponding to different fuels. For particular fuels, a correlation was observed between the relative fluorescence intensity and the total hydrocarbon concentration measured during exhaust sampling by an on-line flame ionization detector. By comparing composition studies of gas turbine exhaust with known fluorescence characteristics of hydrocarbon compounds, Roquemore and Hodgson concluded that aromatics, particularly multiple-ring compounds, were probably the principal contributors to the observed fluorescence. By examining the effects of a 0.03-tLm glass filter placed in the exhaust sample line, they further concluded that the fluorescing hydrocarbons were in the gas phase.

In a previous study 9 the authors investigated laser- induced fluorescence in the exhaust gases of an internal combustion engine operating on gasoline. Excitation with the 488 nm output of an argon ion laser produced a broad-band fluorescence spectrum, and various exhaust conditioning devices were used to examine the source of this fluorescence. Contrary to the gas turbine results of Roquemore and Hodgson, 8 the total gas-phase hydrocarbon concentration measured with a flame ionization detector did not vary significantly as the conditioning devices were used to eliminate the fluorescence. The apparent source of the fluorescence was thus identified as aerosols of hydrocarbons or other organic compounds. More detailed information, such as the relative fluorescence contributions from liquid hydrocarbon aerosols or from solid soot particulates, was not obtained. In addition, conclusions from this study were restricted by the fact that only a single fuel and engine operating condition were used. In the present investi- gation, an argon ion laser is used to examine fluorescence in the exhaust gases of an internal combustion engine operating on different fuels and fuel/air mixtures. A series of improved exhaust conditioning devices are used to test the previous conclusion that aerosols are the principal fluorescence source. In addition, liquids condensed from the exhaust gases in a refrigerated cold trap are examined for Raman and laser-induced fluorescence spectra.

I. EXPERIMENTAL SYSTEM

The experimental system is similar to that used previously, 9 and is shown schematically in Fig. 1. Exhaust gases from an air-cooled, single-cylinder, L- head engine are passed through an optical scattering cell within the cavity of an argon ion laser. Light scattered perpendicular to the intracavity beam is directed through a 3/4-m single spectrometer and detected using commercial photon-counting equipment. A col- ored glass filter and an image rotator are positioned in front of the spectrometer entrance slit. The filter im- proves rejection at the laser wavelength, while the rotator orients the laser beam image parallel to the slit. A polarizer can be added to the collection optics when desired. All tests were performed with the laser operating at 488 nm at constant output power, with the

spectrometer slits set to give a spectral resolution of 0.5 n m .

An unheated Teflon exhaust line from the engine to the scattering cell included various conditioning devices which could be used singly or in any combination. These devices included a refrigerated (2°C) cold trap, a CaSO4 drying unit, a 0.3-~m fiberglass filter and a 1-m length of stainless steel tubing which could be heated to 340°C. The exhaust gases could also be directed into a commercial gas analysis system. This system included a flame ionization detector for total hydrocarbon concentrations and a nondispersive infrared analyzer for carbon monoxide concentrations. The refrigerated cold trap and the 0.3-tam filter mentioned previously were, in fact, preconditioning components within the commercial analysis system.

The engine was operated at lean and rich fuel/air ratios using either propane or methane. The air and fuel flow rates were measured using linear mass-flow meters. A shaft encoder was used to observe engine speed, and a water-brake dynamometer was used to control engine speed during lean operating conditions.

II. RESULTS

A. Unconditioned Exhaust Spectra. The four nominal engine operating conditions used for this study are shown in Table I. Typical total hydrocarbon and carbon monoxide concentrations observed with the commercial analysis system are also listed. All conditions showed relatively high hydrocarbon levels, while the carbon monoxide readings were quite sensitive to the fuel/air mixture ratio. These observations demonstrate that carbon monoxide is principally a result of oxygen insuf- ficiency in chemical processes during and after flame propagation throughout the combustion chamber,

STAINLESS-STEEL PROBE ~ ^ R . . . . . . . . . . . . . . . . . ~ uaSO 4 D YING UNIT 7 O. 3-pm FIBERGLASS FILTER7

REFRIGERATED 2°C COLD TRAP ~ ROEAMEIER-L~-J J INTRA- CAVITY CELL ~ I I \ \ ~

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FIE-. 1. Schematic drawing of experimental system. Laser, Spectra- Physics 170; spectrometer, Spex 1800; photomultiplier, Bendix Chan- nel tron 7501-5103; photon-counting electronics, PARC 1120 Ampli- fier/Discriminator, 1105 Ratemeter.

TABLE I. Nominal engine operating conditions.

Fuel/air Case equiva- rpm

lence ratio

Exhaus t analysis data

Total HC CO (%) (ppm)

Rich propane 1.7 1870 >7700 >7 Lean propane 0.8 1410 5100 0.1 Rich methane 1.4 1900 >8000 >3.7 Lean methane 0.7 1820 4500 0.1

APPLIED SPECTROSCOPY 531

whereas unburned hydrocarbons are primarily a result of flame quenching on the relatively cool chamber surfaces. Fig. 2 shows scattered light spectra in the 515 to 580 nm region obtained from unconditioned exhaust gases for each operating condition. The volumetric flow rate of exhaust gases through the optical scattering cell was approximately the same for each spectrum. A broad-band, laser-induced fluorescence was observed in every case. The relative fluorescence levels varied considerably between cases, however, as shown by the different intensity scales used in the figure. Rich propane combustion (Fig. 2a) gave the most intense fluorescence background, followed by the rich methane (Fig. 2c) and lean propane (Fig. 2b) cases. Lean methane (Fig. 2d) gave a finite but small background. The Raman Q-branch bands of the principal combustion products are clearly visible in the lean methane case, but in the rich propane case even the strong nitrogen band is obscured. It should be noted that nitric oxide concentrations measured with a chemiluminescence analyzer were too low (<500 ppm) in all operating conditions to produce observable Raman spectra.

Identification of general features in the fluorescence spectra was hampered by slow variations in the engine operation, which caused the background level at any wavelength to vary during time intervals of several minutes. This variation was most noticeable in the rich propane runs, where the background level drifted as much as _+50%. A second spectrum is shown in Fig. 2a to illustrate that the apparent features are not repeata- ble. In the rich methane runs the engine variations

were less noticeable, and the fluorescence background was quite flat over the recorded spectral region (Fig. 2c). The relative spectral response of the detection system, obtained using a calibrated tungsten-halogen lamp, is also shown in Fig. 2c.

In order to obtain a better record of the spectral features of the fluorescence in the rich propane case, an exhaust gas sample was collected in a heated optical cell and analyzed with a Raman spectroscopy system in an adjacent laboratory. This system included an argon ion laser operating at 488 nm and a double spectrometer. Fig. 3 shows the broad-band spectrum that was observed, with the peak fluorescence intensity occurring at a wavenumber shift of approximately 1400 cm-L The relative spectral response of the double spectrometer system is also shown in Fig. 3, as well as a curve showing the fluorescence background corrected for this response. The notation "C--H" indicates the spectral region where propane and other hydrocarbons have Raman bands corresponding to C--H stretching vibra- tions.~° The lack of a strong Raman band for propane is somewhat surprising, particularly in comparison with the strong methane bands in Fig. 2c and 2d. Methane has a larger quenching distance under comparable conditions, u however, and should be more readily pre- served in the quench layers near the combustion chamber surfaces.

B. Effects of Conditioning Devices. The anticipated effects of the exhaust conditioning devices were outlined in the previous note2 Briefly, the refrigerated cold trap and the CaSO~ drying unit eliminate water vapor from

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Fro. 2. Laser-induced spectra from unconditioned exhaust a t room temperature. Single-cylinder L-head engine operating on (a) rich propane (dashed curve is repeat spectrum), (b) lean propane, (c) rich methane (dashed curve is relat ive spectral response of detection system), and (d) lean methane. Peaks labeled "Ar II" are laser plasma radiat ion at 529 nm which has been Mie-scattered from aerosols under fuel-rich operation.

5 3 2 Volume 31, Number 6, 1977

the exhaust gases, while the 0.3-/zm filter eliminates aerosols. The porous CaSO4 granules also have some mechanical filtering properties. The cold trap may eliminate some hydrocarbons as well, either directly through condensation or by hydrocarbon aerosols serv- ing as nucleation centers for water condensation. The heated stainless steel tubing is intended to vaporize liquid components, although changes in chemical composition may also result. It should be noted that some water condensation occurs within the unheated sampling line from the engine, making it necessary to blow dry air through the line periodically. Although a heated

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FIG. 3. F luorescence s p e c t r u m from a s ta t ic e x h a u s t ga s s amp le a t 200°C obta ined d u r i n g r ich p ropane opera t ion. R a m a n spectroscopy s y s t e m inc luded a Spex 1402 double spec t romete r (0.5 n m spect ra l resolut ion) , Spec t ra -Phys ics 170 argon- ion l ase r (488 n m output) .

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sampling line would have eliminated this difficulty, the use of an unheated Teflon line is consistent with typical procedures used in the analysis of exhaust gases from spark-ignition engines. The sampling line used in the experiment was provided with the commercial exhaust analysis system for this purpose.

Fig. 4 shows spectra recorded when exhaust gases from rich propane combustion were passed through the various conditioning devices. Spectra were also recorded using exhaust gases from rich methane combustion, but the results were quite similar to those in Fig. 4 and are not reproduced. All of the devices were effective to some degree in reducing the fluorescence level of unconditioned exhaust (Fig. 2a). The refrigerated cold trap alone reduced the background by a factor of approximately 5 (Fig. 4a). However, the addition of the 0.3-~m filter in series with the cold trap nearly eliminated the background altogether (Fig. 4b). The CaSO4 dryer alone reduced the fluorescence level by an order of magnitude relative to unconditioned exhaust (Fig. 4c), while the heated stainless steel tubing alone produced an e v e n greater background reduction (Fig. 4d).

A simple signal/background parameter can be defined as the ratio of the nitrogen Raman Q-branch band intensity to a representative intensity of the laser- induced background. Such a representative intensity should be measured at a wavelength near the peak of the fluorescence spectrum where other spectral features are absent. Table II lists typical values of this parameter that were observed for the various engine operating conditions and with different exhaust conditioning devices. A value for gasoline exhaust gases from the previous study 9 is included for comparison.

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FIG. 4. Effect of va r i ous e x h a u s t cond i t ion ing devices on observed f luorescence for r ich p ropane operat ion: a, r e f r ige ra ted 2°C cold t rap only; b, wa te r t rap and 0 .3 -~m f iberg lass filter; c, ca lc ium su l fa te d rye r only; and d, s t a i n l e s s s teel t u b i n g h e a t e d to 340°C only.

APPLIED SPECTROSCOPY 5:33

The numbers listed in Table II are those obtained without consideration of background contributions due to light scattered at the laser wavelength. This light is not completely rejected by the single spectrometer and can also cause fluorescence in the collection optics• A high intensity of scattered light was observed when examining aerosol-laden exhaust products• A careful examination of these background sources during the previous study 9 concluded that no more than 15% of the observed background levels could be due to scattered light at the laser wavelength. Finally, since Raman Q- branch bands are strongly polarized while a fluorescence background is unpolarized, the signal/background values in Table II could be improved by simply adding a linear polarizer to the collection optics. For example, a polarizer was added while unconditioned exhaust gases from rich methane combustion were examined, and the observed signal/background ratio increased by 80% over the value shown in Table II.

C. Examinat ion of Cold Trap Condensates. Because the refrigerated cold trap was effective in reducing fluorescence levels, the liquids which condensed in the trap were examined with a separate Raman spectroscopy system which utilized a double spectrometer (the same system used to obtain the static exhaust spectrum in Fig. 3). Fig. 5a shows the spectrum produced by the liquid which condensed during rich propane combustion. In addition to the anticipated Raman bands of water, a broad fluorescence background is evident. This background is absent in a similar spectrum recorded from distilled water (Fig. 5b). Fig. 5c shows the net fluoresence spectrum obtained by subtracting the distilled water spectrum from the cold-trap condensate spectrum• Also shown in Fig. 5c are the net fluorescence spectra for condensates obtained during rich methane combustion, and during rich propane combustion with the exhaust gases passing through the heated stainless steel tubing. The spectra in Fig. 5c are corrected for the relative spectral response of the detection system. The apparent structures in the region from 570 to 590 nm are simply errors introduced in subtracting large water Raman signals. The relative fluorescence levels for the different cases shown in Fig. 5c roughly correspond to the levels observed directly in the exhaust gases (Figs. 2 and 4). The general shape of the net fluorescence spectrum for the condensate produced by rich propane combustion is similar to that observed in the static gas

TABLE II. Signal/background comparisons for different fuels, fuel-air mixtures and exhaust conditioning devices.

Fuel/air Signal/ Fuel equiva- Exhaus t conditioning backgrounda

lence ratio

Propane 1.74 None 0.11 Propane 0.76 None 16 Methane 1.41 None 3.3 Methane 0.73 None 190 Propane 1.66 CaSO4 dryer only 0.95 Propane 1.66 H~O trap only 0.54 Propane 1.66 H20 trap and 0.3-tLm il l- >500

ter Propane 1.66 Heated tube only 160 Gasoline (Rich) None 3.9

" Signal /background N2 Raman Q-branch band intensity =

laser-induced background at 532.4 nm "

534 Volume 31, Number 6, 1977

WAVE NUMBER SHIFT (CM - I )

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FIG. 5. a: fluorescence spectrum from liquid cold trap sample obtained from unconditioned exhaus t dur ing rich propane operation; b: s imilar spectrum for distilled water (dashed curve is relative spectral response); and c: - - , net fluorescence spectrum obtained by subtracting 5b from 5a; . . . . . , net fluorescence spectrum for cold trap sample from unconditioned rich methane exhaust; - - - , net fluorescence spectrum for sample from rich propane exhaust which has passed through the heated stainless steel tubing.

sample (Fig. 3), with a broad peak occurring at a wavenumber shift of approximately 2300 cm -1.

The effect of laser wavelength on the relative fluoresence intensity was also investigated using the condensate from rich propane combustion. The fluorescence intensity at a fixed shift of 1870 cm -1 from the laser wavelength was compared with the net intensity of the strongest Raman water band during separate scans using laser wavelengths of 514.5, 488.0, and 457.9 nm. The Raman water signals were adjusted for the wavelength dependence of the scattering cross section, and all signals were corrected for the relative spectral response of the detection system. The ratio of the Raman water signal to the background signal, normalized by the ratio obtained at 514.5 nm, was found to be 0.43 using 488.0 nm and 0.14 using 457.9 nm.

Finally, the cold-trap condensates were analyzed by several additional methods in an at tempt to identify

components which could be contributing to the observed fluorescence. The presence of water as the principal consitituent, however, severely limits the identification of small concentrations of other species. Infrared absorption spectra in regions not dominated by the water (for example, between 1000 and 1500 cm-') showed a great number of weak bands, but many classes of hydrocarbons have overlapping absorption bands in these regions. Gas-liquid chromatography yielded neg- ative results, with no organic compounds observed within the instrument sensitivity (10 to 100 ppm, de- pending on the particular compound). It should be noted that the Raman spectra of the condensates (Fig. 5) also showed no identifiable Raman bands that could be associated with components other than water.

III. CONCLUSIONS

Under excitation at 488 nm, the exhaust gases produced during all of the engine operating conditions showed a broad-band fluorescence. As found in the studies of fluorescence from gas turbine exhaust, ~ the general shape of the fluorescence spectrum does not appear to be sensitive to the operating condition. The relative intensity of the fluorescence background, however, is quite sensitive to the choice of fuel and fuel/air mixture. Rich propane combustion resulted in the most intense fluorescence excitation, followed by rich methane, lean propane, and lean methane. The previous study using rich gasoline found Raman signal/background values that compare with those for rich methane combustion in the present work.

Fluorescence spectra from the liquid which condensed from the exhaust gases in a refrigerated cold trap were similar to spectra obtained directly from the exhaust gases. Excitation at different wavelengths showed that the relative fluorescence intensity increases rapidly with decreasing laser wavelength. Attempts to analyze the cold trap condensates for particular hydrocarbon components were not successful.

The exhaust conditioning devices having the largest effect on the observed fluorescence levels were the 0.3- ~m filter and the heated stainless steel tubing. The virtual elimination of the fluorescence background when the 0.3-~m filter was added to the exhaust sampling line (Fig. 4a and 4b) is strong evidence that aerosols of hydrocarbons or other organic compounds, rather than gas-phase constituents, are the principal source of laser-induced fluorescence. This conclusion is the same as that of the previous study using exhaust gases from gasoline combustion,* but differs from the

* In the previous s tudy a 5-tem filter was found to have no effect on observed fluorescence levels. This suggests t ha t the size distribu-

observations in the gas turbine studies. Such disagree- ment is not necessarily disturbing, since the mecha- nisms resulting in unburned fuel hydrocarbons in gas turbine combustors can be quite different from those in internal combustion engines, TM and the fuels used are obviously different. Unburned hydrocarbons from lubri- cation oils have also been identified as a possible source of fluorescence in gas turbines, 7 but in the present experiment the strong dependence on engine operating condition indicates that engine oil is not a principal source. In addition to probable differences in chemical composition, particular fluorescing compounds may be present in either liquid or gas phases at the point of observation, and differences in phase composition can affect observed fluorescence levels. The elimination of the fluorescence background by the heated stainless steel tubing may have been due, in part, to reduced number densities of fluorescing molecules as a result of the vaporization of fluorescing aerosols. The tube temperature of 340°C, however, was sufficiently high to affect the chemical composition as well.

The conclusions from this study are strictly applicable only to relatively cool exhaust products from internal combustion engines. However, the results emphasize the need to consider the potential masking of Raman bands by laser-induced fluorescence in any new application of Raman spectroscopy to studies of combustion processes. Further investigations into the composition of fluorescing flame products need to be performed. In studies of periodic or transient processes such as in internal combustion engines, the spatial and temporal dependence of the production of fluorescing species will strongly influence what measurements are possible using Raman spectroscopy.

1. M. Lapp, in Laser Raman Gas Diagnostics, M. Lapp and C. M. Penney, Eds. (Plenum Press, New York, 1974), p. 107.

2. S. Lederman, M. H. Bloom, J. Bornstein, and P. K. Khosla, Int. J. Heat Mass Transfer 17, 1479 (1974).

3. R. E. Setchell, Western States Section]The Combustion Institute Paper 74-6 (1974).

4. R. E. Setchell, AIAA Paper 76-28 (1976). 5. A. C. Eckbreth, AIAA Paper 76-27 (1976}. 6. G. E. Bresower and D. A. Leonard, AIAA Paper 73-1276 (1973). 7. D. A. Leonard and P. M. Rubins, ASME Publication 75-GT-83 (1975). 8. W. M. Roquemore and F. N. Hodgson, Paper 27-4, International Confer-

ence on the Sensing of Environmental Pollutants, Las Vegas (September 1975).

9. D. P. Aeschliman and R. E. Setchell, Appl. Spectrosc. 29, 426 (1975). 10. D. A. Stephenson, J. Quant. Spectrosc. Radiat. Transfer 14, 1291 (1974). 11. R. M. Fristrom and A. A. Westenberg, Flame Structure (McGraw-Hill,

New York, 1965), Chap. 2. 12. P. S. Myers, O. A. Uyehara and H. K. Newhall, in Engine Emissions, G.

S. Springer and D. J. Patterson, Eds. (Plenum Press, New York, 1973), Chap. 1.

tion of fluorescing aerosols, at least for the conditions examined, is between 0.3 and 5 p.m.

APPLIED SPECTROSCOPY 535

fluorescence interferences in raman scattering from combustion products

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