Resonance ionization detection limits for hazardous emissions

Twenty-Fourth Symposium(International)on Combustionffl'heCombustion Institute, 1992/pp. 1587-1596

RESONANCE

IONIZATION

DETECTION

LIMITS

FOR HAZARDOUS EMISSIONS BRADLEY A. WILLIAMS, TONY N. TANADA AND TERRILL A. COOL School of Applied and Engineering Physics CorneU University, Ithaca, New York 14853 USA

Continuous emission monitoring of the stack gases of municipal and hazardous waste incinerators requires on-line instrumentation capable of the selective detection of a wide variety of toxic emissions at part-per-billion concentrations. The capabilities of a molecular beam/ resonance-enhanced multiphoton ionization/time-of-flight (MB/REMPI/TOFMS) apparatus proposed for such measurements are demonstrated. Detection limit and selectivity measurements are presented for the resonance ionization detection of thirteen hazardous species, commonly present in incinerator stack gas emissions. The species selected for study may serve as useful surrogates for monitoring wide classes of chlorinated aromatic and olefinic hydrocarbons. In particular, benzene, toluene, chlorobenzene, and tetrachloroethylene are found to be detectable in sub-ppb concentrations with virtually no interference from a wide variety of chemically similar compounds. The unique attributes of the MB/REMPI/TOFMS method for rapid, ultrasensitive, and selective detection of toxic organics make it a promising new tool for quantitative evaluation of incinerator performance.

Introduction The development of reliable methods for the realtime continuous monitoring of toxic emissions at the part-per-billion level in the stack gases of municipal and hazardous waste incinerators may help address current public health concerns and may produce more effective and simplified licensing procedures for incinerator facilities. Recent research efforts have focused on the development of ultrasensitive laserbased detection methods 1-7 and on the appropriate selection of monitored species to ensure that adequate incinerator operating conditions are maintained, s-16 Surrogate species, primarily aromatic hydrocarbons and chlorinated hydrocarbons (CHCs), often observed in trace amounts in incinerator effluent, have been suggested as appropriate candidates for continuous emission monitoring,s-16 Many of these compounds are chosen to be resistant to destruction under several potential incinerator failure modes or "upset conditions. "'s-l~ Post flame thermal degradation, under oxygen starved conditions, of species that penetrate the primary flame zone is thought to be critical to the achievement of high efficiencies for the destruction of many toxic species. 17 Resonance-enhanced multiphoton ionization (REMPI) offers detection sensitivities at the partper-billion (ppbv) level for several surrogates suitable for continuous monitoring of a wide variety of potentially hazardous emissions, t'2'5-7,ls-2~ Moreover, the method is highly selective, enabling the

detection of a given species in the presence of a large background of chemically similar compounds. 5'7'1s The analytical capabilities of REMPI detection of chlorinated organics in jet-cooled molecular beams with time-of-flight mass spectrometry have been extensively studied by Lubman and coworkers. 1 Syage, et alJ s measured detection limits and selectivities for organophosphonate and organosulfide molecules with this technique. Rohlfing and coworkers5'19'z~have demonstrated species and isomeric selectivity in the detection of chlorobenzenes and chloronaphthalenes and have discussed the capabilities of REMPI for continuous emission monitoring of CHCs. In previous work we have reported REMPI detection limits for the chloroethylenes and selectivity measurements for tetrachloroethylene. 7 In this paper we present measurements of detection limits for several aromatics, extending the survey of REMPI detection to thirteen potentially useful surrogates. Measurements are discussed of selectivities for the detection of benzene, chlorobenzene, toluene, and tetrachloroethylene exceeding 103 against a background "soup" of over a dozen potential interferant molecules.

Experimental The molecular beam/REMPI/time-of-flight mass spectrometer (MB/REMPI/TOFMS) used in this work has been described elsewhere. 7'zl The apparatus itself is a prototype of a potentially portable

1587

1588

DIAGNOSTIC METHODS

sampling and detection system envisioned for online real-time monitoring at an incineration facility. For detection limit measurements, trace concentrations of the molecule under study, mixed with a carrier gas at near atmospheric pressure, are fed through a heated Teflon transfer line to a stainless steel vacuum chamber. This sample is jetted into the chamber through a pulsed valve that delivers short duration (0.5 ms) gas pulses, at a 10 Hz repetition rate. The expanded jet from the 0.5 mm orifice of the pulsed valve is intercepted by a 2 mm diameter skimmer, 25 mm downstream of the orifice, to form a molecular beam. Measured rotational temperatures for the molecular beam range from 5 to 50 K, depending upon the specific heat ratio of the carrier gas. 7 The beam passes through the acceleration region of a TOFMS of the classic Wiley-McLaren design, z2 REMPI ionization is produced with a laser directed in a horizontal plane at right angles to the horizontal molecular beam axis and the vertical axis of the TOFMS. An electron multiplier detector records the sequential arrival of REMPI ions of successively greater mass, accelerated along the 1.3 m TOFMS flight tube. Details of the apparatus and measurement procedures used in this work have been previously discussed. 7 A Nd:YAG pumped tunable dye laser (QuantaRay DCR-3/PDL-2), was frequency-doubled to produce REMPI probe pulses in the 259-345 nm spectral region. For benzene, toluene, and the chlorinated benzene derivatives, typical output energies ranged from 2-3 mJ; the beam was unfocused with a spot size of 0.5 mm 2, giving an intensity of 40-60 MW/cm 2. For naphthalene pulse energies ranged from 4-6 mJ in a spot size of 2.5 mm , yielding an intensity of 20-25 MW/cm 2. For chloronaphthalene, the 248 nm output of a KrF excimer laser (Lambda Physik EMG 101) was used. The beam was attenuated and collimated through a telescope, giving 8-9 mJ in a 12 mm 2 spot size (7 MW/cm2). The laser intensities used in these experiments were comparable to those used in previous resonance ionization studies of aromatic molecules. 1,2,5,6

REMPI Detection Schemes

The selectivity of the MB/REMPI/TOFMS approach is primarily the result of the narrowing of the absorption resonances caused by rotational cooling. 5'7'18 The aromatic molecules studied here also have selectivities that are appreciably enhanced by the absence of parent ion fragmentation, which permits a high degree of mass selectivity. With the exception of chloronaphthalene, these aromatics have conveniently accessible vibronic levels of the first excited singlet states $1 at energies just exceeding half of the molecular ionization potential. Thus sin-

gle color, resonant two-photon ionization (R2PI) schemes (1 + 1 REMPI) work well for these species. This differs from the chloroethylenes, studied earlier, 7A2'21'23 with strong absorptions from the ~" orbital of the C-~-C double bond to Rydberg states situated closer to the ionization limit, which required a (2 + 1) three-photon REMPI detection scheme. An advantage of the R2PI approach is that "soft ionization" at relatively low laser intensities (typically using an unfocused beam), with minimal fragmentation of the parent ion, is feasible. 1.2,5,6 In contrast, the three-photon (2 + 1) REMPI approach used for the chloroethylenes required the use of higher laser intensities that result in significant parent ion fragmentation, 7 which may compromise selectivity for some molecules. Table I summarizes the REMPI detection schemes employed for detection of thirteen selected surrogate molecules in our studies to date. The spectral signatures for R2PI of benzene, toluene and chlorobenzene are presented in Fig. 1. These data are for trace samples mixed in a helium diluent with jet-cooling to less than 10 K. 7 The narrow spectral features used for the detection limit measurements discussed in the next section are indicated by the vertical arrows. These features, in most cases the same as used in previous studies, 1'2'5'6'7A2were selected with a preliminary consideration of likely interferences, although experience with on-line detection in the presence of interferant incinerator stack gases may dictate the use of other wavelengths. 2-Chloronaphthalene (2-CN) is an example of an aromatic compound with principal S1 ~-- So vibronic transition energies less than one-half its ionization potential. For such molecules, efficient R2PI requires two lasers, one for resonant $1 ~-- So excitation and a second laser of shorter wavelength for one-photon ionization. We investigated two-color R2PI with use of the broadband (200 cm -1) 248 nm KrF excimer laser for the ionization step. As it happened, the KrF laser generated a strong R2PI signal without the dye laser, even with the beam unfocused at low energy (<0.5 mJ; <5 MW/cmZ). Fortunately the fragmentation of the parent ion is small and thus mass selectivity is preserved even in the absence of wavelength selectivity; the broadband KrF laser may indeed be useful for selective one-color R2PI detection of chloronaphthalene.

Detection Limit Measurements

Detection limits, defined as that species concentration, in a helium carrier gas, at which the signalto-noise ratio is unity, were measured for benzene, toluene, chlorobenzene, 1,2- and 1,4-dichlorobenzene, naphthalene, and 2-chloronaphthalene to complement our previous measurements 7 for the chloroethylenes. These molecules are representa-

1589

DETECTION LIMITS FOR HAZARDOUS EMISSIONS TABLE I REMPI Detection Schemes for CHCs and Aromatics

Compound

Transition ~

)t (nm)

Linewidth b

VC 1,1-DCE

~r ~ 3p ~r ~ 3d

317.1 295.3

25 cm -~ 9 cm -l

c/s 1,2-DCE

Ir ~ 3d

303.9

8 cm -~

tr 1,2-DCE

~ ~ 4f

283.5

7 cm -~

TCE PCE Benzene Toluene Chlorobenzene 1,2-DCB 1,4-DCB Napthalene 2-CN

,rr ~ 3d ~" ---> 3p So---'> S~ So ~ S~ So "--> S~ So ~ Sl So ~ Sl So ---> S~ So --* ?f

310.7 343.8 259.0 266.7 269.8 272.6 274.1 301.6 248 (KrF)

30 5 3 5 9 6 8 8

cm -l cm -~ cm -l cm -I cm -l cm -1 cm -~ cm -~ --

Ion Fragment C2Hz + (mass 27)a CCI + (mass 47) or CzHzCI + (mass 61) e CCI* (mass 47) or C2HzC1+ (mass 61) ~ CC1 + (mass 47) or CzH2C1§ (mass 61) e CCI § (mass 47) CCI + (mass 47) parent (mass 78) parent (mass 92) parent (mass 112) parent (mass 146) parent (mass 146) parent (mass 128) parent (mass 162)

Detection Limit (ppbv) c (20) 5.6 15 30 40 0.5 0.09 0.05 0.6 30 10 5 (4)g

aResonanee in REMPI ionization process. hAs measured in molecular beam with rotational temperature of 5 K. CDefined as S / N = 1 (helium carrier gas). ~This mass channel is subject to interference; detection limit for this molecule is estimated (see ref. 7). eDepending on laser intensity (see ref. 7). fResonant state has not been assigned (see text). gEstimated (see text).

tive of a wide class of aromatics and their chlorinated derivatives of concern as potential effluent components. While each of these are subject to current EPA regulation and thus require careful monitoring, they may also be excellent surrogates for monitoring the presence of many other species including the polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs). Detection limits are given in Table I for thirteen potential surrogate molecules. The detection limit for 1,4-DCB may be compared with Rohlfing's previous forecast 5 of 10 ppbv. An excellent inverse relationship exists between the detection limits presented here for benzene, toluene, and naphthalene, and previous estimates ~ of the ionization efficiencies of these three species with a laser intensity of 10 M W / c m 2. Measurement Procedures:

A stainless steel and glass gas handling system was used for the preparation of sample mixtures of known concentration in helium (MG industries, zero grade, < 0.5 ppm THC). The effects of wall adsorption and desorption were minimized by maintaining the wall surfaces above 120 ~ C. The stain-

less steel and Teflon pulsed molecular beam valve (General Valve, Iota One) was heated to approximately 85 ~ C, and was connected to the manifold by a 1.2 m heated Teflon transfer line (Dekoron/ Unitherm), which was also maintained above 120 ~ C. Gas partial pressures were measured with a capacitance manometer (MKS Industries t22A, 0 1000 Torr). The manometer could not be heated and was closed off from the system except when samples were being prepared. The procedures used for the preparation of test samples, diluted in helium, at concentrations ranging from 20 ppb to 3 ppm (depending on the signal level obtained for a given species) have been described elsewhere. 7 With laser conditions fixed, the parent ion signal of a given species was recorded as each test sample was successively prepared and introduced to the molecular beam. Excellent linear plots of signal vs sample concentration were obtained, comparable to the data for toluene presented in Fig. 2, provided that wall desorption effeets were minimized by introducing the samples in a sequence progressing from the lowest to the highest concentrations. Signal-to-noise ratios used to determine detection limits could be measured in two ways. In the

1590

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FIC. 1. REMPI ionization signal as a function of laser wavelength for benzene, toluene, and chl()robenzene in the energy, region of the St state. The spectral feature used for the detection studies is indicated by arrows.

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Toluene Concentration (ppb) FIC. 2. Signal vs. sample concentration plot obtained during the detection limit determination for toluene. The solid line is the best fit of unity slope on a log-log scale, indicating a good linear relationship between signal and concentration.

FIG. 3. Determination of the detection limit of toluene using the sample "'off/on/off.' method (see text). The initial overshoot when the pulsed valve is turned on may be caused by degassing from interior valve surfaces during the sample off period. The ratio of the mean signal level to the RMS noise level with the signal off was 618 for a 28 ppb sample, yielding a detection limit (S/N = 1) of 45 pptv.

first method the integrated signal of the TOF mass peak is compared with the RMS noise of the boxcar with the molecular beam turned off. Fig. 3 displays a typical "off/on/off" recording for a 28 ppb mixture of toluene in helium. This sample, prepared with a nominal concentration of 31 ppb, was deduced to have an actual concentration of 28 ppb by comparison of the observed signal level with the concentration indicated by the best fit line of Fig. 2. Each data point connected by the solid curve of Fig. 3 is the (10 second) average signal, obtained with 100 laser shots, of the mass 92 parent ion signal excited at the 266.7 nm toluene origin band wavelength. The dotted line connects points, on a 100X magnified scale, of the baseline signals recorded with no sample flowing through the pulsed valve. The data of Fig. 3 are representative of four such (off/on/off) recordings. The RMS noise level is 56 units on the relative ion signal scale of the figure. With the sample on, the signal reaches a steady value of 34600 units of relative ion signal. This S/N ratio of 618 for the 28 ppb sample gives a detection limit of (28 ppbv/618) ~ 45 pptv. Another estimation of S/N level, less susceptible to fluctuations in the baseline of the recording system, is obtained directly from the TOF mass spectrum. Fig. 4 displays the (64 shot averaged) mass spectrum in the vicinity of the mass 92 parent ion peak for this 28 ppbv sample of toluene. The expanded baseline, shown in the inset, reveals three weak mass peaks at 106, 108, and 112 amu rising above the background noise level of about 2.25 units

DETECTION LIMITS FOR HAZARDOUS EMISSIONS 1200

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FIG. 4. Use of the TOF mass spectrum to determine the detection limit for toluene. The mass 91/ 92 doublet consists of the mass 92 parent ion and the mass 91 daughter ion formed with the loss of a hydrogen atom from the methyl group (ref. 25). The noise level, estimated from the variance of the signal between mass peaks about the mean baseline for an extended region (75 to 130 amu) of the mass spectrum, was 2.25 units. This value, compared to a peak height of 1000 units for a 28 ppb sample, yields a detection limit of 63 pptv. of relative ion signal. The mass 92 parent ion peak has a relative ion signal of 1000 units and thus a S I N ratio of about 450. This second method yields a detection limit, determined with this 28 ppbv sample, of (28 ppbv/450) ~ 65 pptv. The two methods gave comparable results for most molecules, but during the benzene and 2-CN tests the baseline varied erratically, leading to a value for the detection limit approximately four times higher for the signal (off/on/ofl~ method. For these two molecules the values given in Table I are from the TOFMS spectra. Because the observed noise level changed little from species to species and appeared invariant to signal level, a good reciprocal correlation existed between the response (ion signal/species concentration) and the detection limit for a given species. For 2-CN, wall degassing effects were found to be severe, even at temperatures above 100~ C. We were thus unable to obtain a satisfactory linear plot for signal vs concentration; the detection limit given is based on the TOFMS signal/noise ratio of a stock sample of 5 ppm, prepared by dilution of saturated room temperature vapor (about 20 ppm).

Selectivity Determinations In addition to the sensitivity limits determined above, a second important figure of merit of each

1591

species is its selectivity for detection against a background of interferant compounds. For a given pair of detected and interferant molecules, selectivity may be defined as the concentration ratio (interferant/ detected) for which the interferant and detected species make equal contributions to the detected signal. 7'Iv In stack gas monitoring the idealized definitions of detection limit and selectivity are less relevant than the effective detection limit obtained in the presence of a complex and unknown ensemble of interferants. The molecules of Table I with the lowest detection limits, benzene, toluene, ehlorobenzene and tetrachloroethylene, are also excellent surrogates for monitoring incinerator performance. Benzene is often observed in high concentrations as a "product of incomplete combustion" (PIC) formed in the decomposition of chlorobenzene and toluene, s'gA6 Chlorobenzene, tetrachloroethylene, and toluene have high thermal stabilities in the absence of oxidizing agents and are therefore useful in monitoring post flame thermal degradation efficiency under locally oxygen-starved conditions that may arise from incomplete mixing. 9'1~ For these reasons we have tested the detection of these molecules in the presence of complex "soup" mixtures of likely organic interferant compounds. Results are reported here for benzene, chlorobenzene, and toluene, using the procedures followed earlier for tetrachloroethylene. 7 The soup species include many common solvents in addition to a number of PICs frequently observed in incinerators, m Automobile exhaust was added to each mixture to achieve a realistic simulation of the gasdynamic characteristics and moisture fractions of incinerator effluent. Three different mixtures were used (Table II): Mixture A: Used for testing benzene and tol-

uene; contained all other species. Mixture B: Used for testing chlorobenzene in the presence of all other species. Mixture C: Used for testing benzene; contained no chlorobenzene, naphthalene or acetone (see below). The only observed interference arose when detecting benzene in the presence of chlorobenzene (mixture A). The REMPI fragmentation of chlorobenzene has been carefully studied by Durant, et al. 24 The mass 77 phenyl cation, formed in the fragmentation of a metastable state of the chlorobenzene cation, has a characteristically broad mass peak that overlaps the mass 78 channel of the benzene cation. This interference is significant only when the concentration of chlorobenzene greatly exceeds (>1000X) the benzene concentration, a condition unlikely to ever occur in practice. When a soup containing no chlorobenzene was used (mixture C), the benzene signal suffered no interference; neither

1592

DIAGNOSTIC METHODS TABLE II Soup Compositions

Except as noted below, all soups contained: Methylene Chloride (1000 ppm) Chloroform (1000 ppm) Carbon Tetrachloride (1000 ppm) 1,2-Dichloroethane (1000 ppm) 1,1,1-Trichloroethane (1000 ppm) 1,2-Dichlorobenzene (1000 ppm) 1,3-Dichlorobenzene (1000 ppm) 1,4-Dichlorobenzene (1000 ppm) Toluene (1000 ppm)

cis 1,2-Dichloroethylene (1000 ppm) trans 1,2-Dichloroethylene (1000 ppm)

Trichloroethylene (1000 ppm) Tetrachloroethylene (1000 ppm) 1,2,4-Trichlorobenzene (200 ppm) Chlorobenzene (1000 ppm) Naphthalene (100 ppm) Benzene (1000 ppm)

Soup "A'" contained the preceding compounds at one-half the concentrations listed above, minus benzene and toluene, plus the following: Methanol (1.3%) Automobile Exhaust (50%) Acetone (670 ppm) Balance Nitrogen Soup "'B'" contained the common interferants at the concentrations listed, minus chlorobenzene and the dichlorobenzenes, plus the following: Chlorobenzene (1 ppm) Methanol (5000 ppm) 1,2-Dichlorobenzene (5 ppm) Acetone (670 ppm) 1,3-Dichlorobenzene (20 ppm) Balance Auto Exhaust 1,4-Dichlorobenzene (5 ppm) Soup "C" contained the common interferants at the concentrations listed, minus benzene, chlorobenzene, and naphthalene, plus the following: Benzene (400 ppb) Balance Auto Exhaust

were any interferences found in the selectivity tests of chlorobenzene or toluene. The selectivity test results are illustrated for the case of toluene in Figs. 5 and 6. At the 266.7 nm toluene resonance, the mass spectrum of the soup (mixture A) displays prominent peaks attributable to chlorobenzene and naphthalene. Nevertheless, the mass 91/92 doublet characteristic of toluene (see caption for Fig. 4) may be easily distinguished as indicated by the dashed line of Fig. 5b. For reference, the mass spectrum of a calibration sample of 0.94 ppm toluene in nitrogen is displayed in Fig. 5a. The ratio of signals (100X) between Figs. 5a and 5b shows the presence of toluene, as an impurity, at about 10 ppb in soup A. This is further confirmed by the comparison of wavelength scans in the mass 92 channel presented in Fig. 6. The spectrum recorded for soup A has been multiplied by the (100X) factor of Figs. 5a and 5b to place it in proper scale with the 0.94 ppm toluene comparison spectrum. With account taken of the higher noise level for the weaker soup spectrum, the excellent spectral overlap proves that the mass 91/92 signals at this wavelength are indeed due to toluene, and not false signals attributable to one of the other

species present in the soup. Therefore none of the soup compounds produces any measurable interference for toluene detection even at concentrations exceeding 10a that of toluene. One issue important to practical application is whether the sensitivity of REMPI/TOFMS is likely to be different in an incinerator environment than in a helium carrier gas. A preliminary test was conducted comparing signal levels of benzene at concentrations of 350-400 ppb in three different diluents: helium, nitrogen and soup "C." Comparable signal levels were obtained with either the nitrogen diluent or the soup. The signal from the heliumdiluted sample was approximately twice that of the other two. More extensive tests are needed, but these results suggest that although the lower rotational temperatures (~5 K for helium compared with ~50 K measured for Nz)7 achievable with a monatomic carrier gas do indeed provide lower detection limits, the difference is not dramatic. Moreover, the presence of interferant compounds in relatively large amounts does not appear to have a significant effect on sensitivity relative to results obtained in a nitrogen carrier gas.

DETECTION LIMITS FOR HAZARDOUS EMISSIONS ' ' ' .......

T:lu'ene' 019'4 'ppm . . . . . . . . .

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:::::::::::::::::::::::::::::::

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90

100 110 Ion Moss (omu)

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.

.

120

.

130

FIG. 5. Comparison of (a) the mass spectrum obtained at 266.7 nm for a calibration sample of 0.94 ppm toluene in nitrogen with that (b) recorded for soup "A." The mass 92 signal of the calibration sample was 100• stronger than that for soup "A"; a concentration of =10 ppbv of toluene is thus present in the soup. (The toluene was not added expressly, but was present as an impurity among one or more soup components).

Conclusions

The detection limits and selectivities for the surrogate molecules selected for these studies document the capabilities of REMPI spectroscopy for

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1593

continuous emission monitoring. Where possible we have selected prototype apparatus and techniques that minimize the complexities of eventual on-line implementation. Because the detection limits are specific to the apparatus and methods employed, modest reductions in these limits may be expected with further refinements. Increases in sample reservoir pressure, for example, could provide more molecules within the laser focal volume and a corresponding increase in S / N ratio. The margin for improvement of the MB/REMPI/TOFMS method may be judged from the detection limits for tetrachloroethylene and toluene, which have the highest ionization efficiencies for single-color (2 + 1) REMPI and R2PI, respectively. With the assumption of an isentropic expansion to 5 K at the skimmer, followed by a free-molecule effusive beam expansion to the laser ionization region, the estimated average numbers of molecules present in the laser focal volumes at the detection limits are 300 for tetrachloroethylene and 750 for toluene. Assuming a TOFMS detection emciency of 10%, respective ionization efficiencies of 3% and 1.3% are indicated. Because the value for toluene approaches the known ionization efficiency of 6% for comparable laser intensities, 6 significant improvements in per~brmance of the present apparatus seem unlikely. The unique attributes of the MB/REMPI/TOFMS method for rapid, ultrasensitive, and selective detection of toxic organics make it an important new tool for quantitative evaluation of incinerator performance. The results of Table I for several aromatic and olefinie compounds encourage fizrther study of wider classes of proposed surrogates. Indeed, REMPI detection capabilities for specific compounds are expected to significantly influence strategies for the use of surrogates in emission monitoring.

Acknowledgements

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We thank Eric Rohlfing for helpful discussions. This work was supported by the National Science Foundation under NSF Grant CTS-8921649 and by a grant from the New York State Solid Waste Combustion Institute located at Cornell University.

266.75 266.70

Laser Wavelength (nm)

REFERENCES FIG. 6. Spectral scans taken in the vicinity of the toluene resonance for samples of Figure 5. When the soup spectrum is multiplied by the factor of 100 observed in the mass spectrum of Figure 5, the spectral scans show an excellent overlap, indicating that the observed mass 92 signal must be caused by a 10 ppbv toluene impurity in soup "'A'" and not an interferant species,

1. TEMBREULL, R., SlY, P. L., LI, P., PAN(-;, H. M. AND LUBMAN, D. M.: Anal9 Chem. 57, 1186 (1985). 2. DUNCAN, M. A., DIE'VZ, T. G. ANI) SMALLEY, R. E.: J. Chem. Phys. 75, 2118 (1981). 3. JEFFRIES, J. B., RAICHE, G. A. AND JUSINSKI, L. E.: Detection of Chlorinated Hydrocarbons via

1594

4.

5.

6. 7.

8.

9.

10. 11. 12.

13.

DIAGNOSTIC METHODS

Laser Atomization/Laser Induced Fluorescence, Appl. Phys. B, in press. LUCAS, D., KOSHLAND, C. P., MCENALLY, C. AND SAWYER, R. F.: The Detection of Chlorinated Hydrocarbons Using Photofragmentation: Combust. Sci. Tech., in press. ROHLF1NG, E. A.: Twenty-Second Symposium (International) on Combustion, p. 1843, The Combustion Institute, 1989. BOESL, U., NEUSSER, H. J. AND SCHLAG, E. W.: Chem. Phys. 55, 193 (1981). COOL, T. A. AND WILLIAMS, B. A.: Ultrasensitive Detection of Chlorinated Hydrocarbons by Resonance Ionization, Combust. Sci. Tech. 82, 67 (1992). TIREY, D. A., DELLINGER, B., TAYLOR, P. H. AND LEE, C. C.: Pyrolytic Thermal Degradation of a Hazardous Waste Incinerability Surrogate Mixture, presented at The 15th Annual EPA Research Symposium on Land Disposal, Remedial Action, Incineration, and Treatment of Hazardous Waste. Cincinnati, Ohio, April 1012, 1989 (unpublished). DELLINGER, B. AND HALL, D. L.: J. Air Pollut. Control Assoc. 36, 179 (1986). DELLINGER, B. : J. Air Polht. Control Assoc. 37, 1019 (1987). CHANG, W. D., KaaRA, S. B. AND SENKAN, S. M.: Environ. Sci. Technol. 20, 1243 (1986). COOL, T. A. AND WILLIAMS, B. A.: Hazard. Waste Hazard. Mater. 7, 21 (1990); and references therein. GRAHAM, J. L., HALL, D. L. AND DELLINGER, B.: Environ. Sci. Technol. 20, 703 (1986).

14. TSANG, W. AND SHAUB, W.: Detoxification of Hazardous Waste (J. H. Exner, Ed.), Chap. 2, p. 41, Ann Arbor Science, 1982. 15. Ln FOND, R. K., KRAMLICH, J. C., SEEKER, W. M. AND SAMUELSEN, G. S.: J. Air Pollut. Control Assoc. 35, 658 (1985). 16. OPPELT, E. T.: J. Air Pollut. Control Assoc. 37, 558 (1987). 17. TIREY, D. A., TAYLOR, P. H., KnSNER, J. AND DELLINGER, B.: Combust. Sci. Tech. 74, 137 (1990); and references therein. 18. SYAGE, J. A., POLLARD, J. E. AND COHEN, R. B.: Appl. Opt. 26, 3516 (1987). 19. ROHLFING, E. A. AND ROHLFING, C. M.: J. Phys. Chem. 93, 94 (1989). 20. ROHLFING, E. A. AND CHANDLER, D. W.: Advances in Laser Science II, Optical Science and Engineering Series 8 (M. Lapp, W. C. Stwalley, and G. A. Kenney-Wallace, Eds.), p. 618, American Inst. of Physics, 1987. 21. WILLIAMS, B. A., COOL, T. A. AND ROHLFING, C. M.: J. Chem. Phys. 93, 1521 (1990). 22. WILEY, W. C. AND McLaREN, H. I.: Rev. Sci. Instrum. 26, 1150 (1955). 23. WILLIAMS, B. A. AND COOL, T. A.: Resonance Ionization Spectroscopy of the Chloroethylenes, submitted to J. Phys. Chem., 1992. 24. DURANT, J. L., RIDER, D. M., ANDERSON, S. L., PROCH, F. D. AND ZARE, R. N.: J. Chem. Phys. 80, 1817 (1984). 25. BEYNON, J. H.: Mass Spectrometry and its Applications to Organic Chemistry, p. 341, Elsevier, 1960.

COMMENTS Prof. Frederick L. Dryer, Princeton University, USA. As practical matters for applied use of techniques, each must be robust in terms of use in industrial environments (mechanical, heat, humidity stressing, etc.) and in environmentally similar flows. Combustion exhausts poorly simulate the aerosol, and particulate loading often found in incinerator exhausts. Such materials also cause difficulties in maintaining expansion flows similar to your designs. Finally, economics are a substantial issue. Could you give us some indications as to how these issues might affect applied use of this technique? (I have no doubt of the valuable nature of the method for research purposes.)

Author's Reply. Our search for useful surrogates has thus far been directed toward volatile species that may be directly sampled with a conventional heated transfer line. Because aerosols and particu-

lates may be routinely removed by filtering before the sample is introduced to the R E M P I / T O F M S system, such condensed components present no difficulty in this approach to effluent monitoring. Of greater concern is the possible formation of clusters within the expanded flow. Cluster formation for expansions from atmospheric pressure is likely to occur for condensible species present at concentrations exceeding about 100 ppm. We have addressed this concern with the use of "soups" containing moist automobile exhaust and a variety of condensible interferants at concentrations near 1000 ppm (see Table I). Present trial b u r n testing procedures require that samples taken over an extended period be taken to a laboratory for G C / M S analysis. This approach is time-consuming and expensive. Indeed, the cost of a single test exceeds that of the present apparatus, valued at about $150,000.

DETECTION LIMITS FOR HAZARDOUS EMISSIONS

Edward R. Ritter, Villanova University, USA. Interfering species may be present both in terms of absorption wavelength (excitation) and the resulting mass spectra. To what extent do you feel this may influence actual detection thresholds when realistic mixtures, such as the effluents from typical hazardous waste incinerators are present? Such mixtures may consist of more than a hundred chlorinated aliphatics, olefins, benzenes, aromatics, and biphenyls, in addition to their products of incomplete combustion. Some of these may have broad absorbance bands, grossly different sensitivities to your technique, and may even produce mass fragments which overlap other species of interest. It is noted that for chloro-olefins, you needed a focussed laser to produce the (2 + 1) REMPI, while for aromatics, an un-focussed laser was used for (1 + 1) REMPI. What limitations do these difficulties pose to the application of your technique to on-line monitoring of hazardous waste incinerators? Author's Reply. As discussed in our paper, several likely interferant molecules present no problems in the detection of benzene, toluene, chlorobenzene, and tetrachloroethylene. The methods used here would also permit studies of detection thresholds and selectivities for on-line measurements of the effluent of typical hazardous waste incinerators. Indeed, our plans for future work include on-line detection of a wide range of both aliphatic (focused 2 + 1 REMPI) and aromatic (unfocused 1 + 1 REMPI) compounds.

Edward R. Ritter, Villanova University, USA. You have presented results which show a dramatic decrease in the sensitivity of your R E M P I / T O F M S technique to aromatics with increasing degree of chlorination. The sensitivity to benzene is 10 times greater than chlorobenzene and nearly 100 times greater than dichlorobenzenes, ff this is due to rapid internal conversion of the absorbed photon energy to vibrational energy (with increasing degree of chlorination), (1) what sensitivity might you expect your technique to have toward more highly chlorinated aromatics? (2) what impact might this have on the feasibility of monitoring emissions from an incinerator which is being fed a mixture of highly chlorinated aromatics (such as hexa and hepta chlorobiphenyls)? In the effluent from such a system, concentrations of the surrogate mono and dichlorinated biphenyls and arenes may be insignificant by comparison to their more chlorinated analogs. Author's Reply. The decrease in sensitivity with increasing chlorination of benzene and its deriva-

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tives, e.g., toluene, phenol and aniline, may indeed be caused by an increased density of vibronic states associated with coupled substituents that increases the rate of radiationless E ~ V transfer (internal conversion) to produce a strong quenching similar to that seen in brominated and iodinated benzenes [Tembreull, R., Sin, P. L., Li, P., Pang, H. M., and Lubman, D. M., Anal. Chem. 57, 1186 (1985)]. Similar difficulties may be expected for polycyclic chlorinated aromatics, although REMPI studies of these compounds have yet to be performed. Highly chlorinated aromatics are thus not expected to be appropriate surrogates for monitoring with this technique. Moreover, the R E M P I / T O F M S method, as implemented in our studies, is only appropriate for volatile surrogates that may be sampled in real time in the gas phase. Highly chlorinated polycyclic compounds of low volatility would require the use of filtering and sample concentration methods employed in conventional G C / M S sampling and analysis. Measurements of the relative concentrations of the chlorinated congeners of polycyclic aromatics as a function of their degree of chlorination, under actual incinerator operating conditions, are needed to assess the utility of mono- and dichlorinated aromatics as surrogates.

Donald Lucas, Lawrence Berkeley Laboratory, USA. None of your "soups" contain HC1 and/or C12, the desired products of chlorinated hydrocarbon incineration. Do you expect any problems with your method from these species?

Author's Reply. HC1 and C12 are unlikely to cause interference since they cannot produce ions in the mass channels used to detect the surrogates studied (m/z = 47 for the chloroethylenes and m/z >- 78 for the aromatic species). In addition, there are no discrete absorption bands for these species in the spectral region considered here. Nevertheless, inclusion of these species in future soups may be useful, if only to ensure chemical compatibility of the monitoring system with these compounds.

Eric A. Rohlfing, Sandia National Laboratories, USA. Have you observed any interferences from photochemical processes in your 2 + 1 REMPI diagnostics in the soup samples? One might be concerned with multiphoton chemistry/ionization producing ions in the fragment channel that you monitor.

Author's Reply. Previous tests of detection of tet-

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DIAGNOSTIC MFTIIODS

rachloroethylene (Ref. 7) in a soup of similar composition to those used here showed virtually no interference problems. While the high laser intensities necessary to stimulate the 2 + 1 resonant ionization did result in significant fragmentation and ionization of soup species at the 344 nm C2C1, detection wavelength in various mass channels, no signal was apparent in the mass 47 channel used for monitoring of C2CI4 and most of the other chloroethylenes. Thus it appears that aliphatic CHCs (at least those which may be ionized to CCV) may be selectively detected in spite of the high laser intensities used. Planned extensions of these studies to other thernmlly stable aliphatie compounds, e./~., nitriles, chloroalkanes, chlorocarhonyls, etc., ,nay, however, reveal problems with photochemical origins.

J. Wolfrum, University of lleidelberg, Germany. Could you estinaate the chances to detect dioxines and furanes on line in incinerations by your approach?

Author's Reply. Proposed detection limits by the EEC (European Economic Community) for dioxin/

furan emissions (sub part-per-trillion) are probably beyond the sensitivity of this approach without sample pre-coneentration methods. Nevertheless, studies of dioxins and furans using REMPI may be vahlable for studies of mechanisms for their formation and the establishment of correlations of dioxin and furan concentrations with the concentrations of various surrogates. The REMPI detection of monoand dichlorinated dioxins and filrans is presently under investigation in our laboratory.

John W. Daily, University of Colorado at Boulder, USA. Your method of determining detectability seemed a bit odd. Why not use dilution until a S/ N of unity is observed?

Author's Reply. It was not practical to prepare samples of less than about 20 ppb with the present sample handling system. Tests at smaller concentrations were subject to wall adsorption/desorption effects that yielded pnorly reproducible nonlinear relationships between signal and nominal concentrations. We therefore worked at concentrations high enough to minimize this source of error in the detection limit determinations.