Sensitive in situ detectionof chlorinated hydrocarbons in gas mixtures

Charles S. McEnally, Robert F. Sawyer, Catherine P. Koshland, and Donald Lucas

We detect chlorinated hydrocarbons (CHC's) in gas mixtures by dissociating the CHC's with a 193-nmlaser and measuring the subsequent concentration of the CCI fragmentation by means of laser-inducedfluorescence. Sub-ppm detection, where ppm indicates parts in 106, is achieved for C2H5 Cl with a10-mm3 measurement volume and integration over 50 laser shots. Every other CHC tested is alsodetectable, with the same or better detection limits. The CCl forms promptly during the fragmentationlaser pulse through unimolecular dissociation of the parent CHC's. The technique should be a usefuldiagnostic for CHC incineration systems.

Key words: Continuous monitoring, chlorinated hydrocarbons, laser fragmentation-laser-inducedfluorescence.

Introduction

New concentration-measurement techniques are ur-gently needed in the hazardous waste incinerationfield. Currently the only established method formeasuring trace gases in incinerator emissions is gaschromatography-mass spectrometry analysis of ex-tracted and concentrated samples, which requireshours to weeks to produce results.' Therefore directon-line monitoring of incinerator emissions is notpossible. Instead, during a trial burn phase theincinerator operator develops correlations betweenemissions and easily measured process variables suchas furnace temperature and outlet 02 concentration,which are then the only quantities monitored duringactual operation. This indirect method is unsatisfac-tory from a scientific standpoint, and it is an impor-tant reason for public opposition to incineration.Furthermore, the lack of diagnostic capability hin-ders research on the fundamental processes thatoccur during hazardous waste incineration. Thispaper concerns the development of a measurementtechnique suitable for the continuous monitoring of

D. Lucas is with the Energy and Environment Division, LawrenceBerkeley Laboratory, Berkeley, California 94720. The other au-thors are with the University of California at Berkeley, Berkeley,California 94720; C. S. MeEnally and R. F. Sawyer are with theDepartment of Mechanical Engineering, and C. P. Koshland is withthe Department of Biomedical and Environmental Health Sci-ences.

Received 4 December 1992; revised 16 November 1992.0003-6935/94/183977-08$06.00/0.C) 1994 Optical Society of America.

chlorinated hydrocarbons (CHC's), which account for20% of the incinerable wastes generated in the U.S.and are often very toxic.2

Continuous monitoring requires a technique thatcan measure concentrations down to 10 ppb (whereppb indicates parts in 109) in near-real time (a fewseconds or less) and that can distinguish CHC's fromhydrocarbon combustion products and from HC1,which is the desired end product for the Cl atomscontributed by the CHC's. In addition, the tech-nique should not be species selective: the speciesconstituting the greatest emission is seldom known inadvance because emissions can be composed of inter-mediates as well as the compounds present in theinitial waste stream. For example, in a study ofwastes cofired in industrial boilers, emissions ofintermediates were an order of magnitude greaterthan those of unburned fuel compounds.3 Becauseincinerators are engineered to provide thermal condi-tions that destroy all organics, large emissions of anycompound reflect off-design operating conditions andrequire corrective action. The technique of reso-nantly enhanced multiphoton ionization can achievethe necessary detection limits with a rapid timeresponse, but it is inherently species selective.4 5

The diagnostic method we are developing is laserfragmentation-laser-induced fluorescence (LF-LIF),wherein the CHC's are photodissociated into smallerfragments, including CCI that is subsequently mea-sured with LIF. The LF-LIF method shares severalof the characteristics of LIF: high sensitivity, fastresponse, in situ operation, and high spatial resolu-tion.6 Scientists have used it in several atmospheric

20 June 1994 / Vol. 33, No. 18 / APPLIED OPTICS 3977

sensing and laboratory applications to detect nonfluo-rescent species.7-9 Our strategy uses fragmentationnot only to render the CHC's fluorescent but also toprovide broadband sensitivity: because CCl is apossible dissociation product of all CHC's, the tech-nique can in principle detect every CHC. Con-versely, HCl or Cl2 should not interfere with themeasurements because CCl is not a unimoleculardissociation product of either. This strategy con-trasts with recent experiments by Jeffries et al., 0 whoalso applied LF-LIF to CHC measurement by detect-ing the atomic Cl fragment. In these experiments,HCl generated a strong interference because it photo-dissociates to Cl + H.

In an earlier publication we demonstrated that ourLF-LIF strategy could detect a particular CHC,C2H5Cl."1 Here we apply the method to CHC-containing mixtures to determine its sensitivity, theCCI formation mechanism, and the range of detect-able CHC's.

Experimental Procedures

The apparatus that performs the LF-LIF measure-ments is shown in Fig. 1. Briefly, CHC-containingmixtures flow through a photolysis cell or out of asection of tubing open to the room air, where theyinteract with the laser beams. Any subsequent emis-sion is collected and measured. The photolysis cellallows us to vary the total pressure and to control theexact composition of the gas mixture. Unfortu-nately, black deposits rapidly accumulate on thewindows and significantly attenuate the laser beams,especially when chloroethylenes are present. Conse-quently we also use the open-tube configurationshown in Fig. 1, which requires no windows. Ineither case most of the experiments occur at atmo-spheric pressure.

Two lasers are required: one to photolyze theCHC's (the fragmentation laser), and one to excite theproduct CCl molecules (the probe laser). We use aLambda-Physik EMG 102MSC pulsed excimer laseroperating at 193 nm as the fragmentation laser. All

P;1~ 1% \/ /\monochromator

digital ' fluorescence

oscilloscope PMT

dichroic mirror metal mirrorbeam stop: I

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I/

/ < 1 \ ~~~~laser laserK aser beams\

Fig. 1. Diagram of the experimental apparatus; PMT, photomul-tiplier tube.

compounds with C-Cl bonds absorb at this wave-length.12 Photons at 193 nm contain 620 kJ/mol ofenergy, which exceeds most bond energies, so absorp-tion usually causes dissociation. If the energy fromonly one photon were available, only one or two bondsof the parent compound would be broken, so mostfragmentation pathways, including those leading toCCl, would not be energetically feasible. However,excimer lasers produce enough photons in a pulsethat multiphoton processes become possible. Eitherthe initial fragments absorb additional photons laterin the fragmentation pulse and dissociate further, orthe precursor CHC absorbs additional photons beforedissociating.'3 Consequently, almost every smallfragment possible has been observed in CHC photoly-sis at 193 nm: Cl2 , CH and CCl,'4 CCl2 ,'5 HCl,' 6 C,' 7

C2 and HCl+,"1 and Cl.'0 Because only multiphotonprocesses create CCl, we focus the fragmentationbeam with a UV-grade quartz lens with a focal lengthof 250 mm. Detection occurs at the focal point,where the cross section is a 2 x 0.5 mm rectangle andthe energy density is 104 J/m 3. The focused beamdoes not break down air through multiphoton ioniza-tion, which could alter the subsequent chemistry.

An eximer-pumped Lambda-Physik FL3002 dyelaser serves as the probe laser. Using Coumarin 153dye and a barium borate frequency-doubling crystal,it produces approximately 1 mJ of tunable wave-length light between 270 and 280 nm. Wavelengthsare read directly from the dye laser display withoutfurther calibration; the manufacturer states that thisscale is accurate to within 0.05 nm. Several vibra-tional lines of the CCl X 21I - A 2 electronic transi-tion lie within this range.'8 For the experimentsdescribed here (with a few exceptions that are dis-cussed later), the Q, head of the X 2

1 / 2(v" = 0) _A 2A(v' = 0) transition is excited, and all fluorescencein the v' = 0 ->v" = 0 emission band is detected.

A dichroic mirror reflects the fragmentation beaminto the measurement region; the probe beam passesthrough the rear edge of this mirror so that bothbeams follow the same path through the measure-ment region. The probe beam has a circular crosssection several millimeters in diameter that com-pletely overlaps the fragmentation beam. A func-tion generator-pulse generator combination triggersthe lasers with an adjustable time interval, desig-nated At, between them, which is reproducible towithin 0.05 s. A photodiode in the vicinity of thelens and mirror sees reflections from both lasers; itsoutput provides a synchronization signal for thedetection system and gives a measure of At that isaccurate to within the reproducibility.

A 20-mm-diameter, 75-mm-focal-length lens col-lects fluorescence emitted at a 90° angle to the beampaths. The intersection of the fragmentation beamwith the solid angle viewed by the detection lensdefines the actual measurement volume: a roughly5-mm-long prism with a 1 mm x 1 mm cross section.A second lens directs the fluorescence through theentrance slit of a 0.3-m McPherson scanning mono-

3978 APPLIED OPTICS / Vol. 33, No. 18 / 20 June 1994

chromator coupled to a Hamamatsu R928 photomul-tiplier tube, whose output is digitized and recorded bya LeCroy 9410 digital oscilloscope. Experiments witha mercury lamp show that the wavelength of themonochromator is accurate to within 0.2 nm and thatthe bandpass is 5 nm. We average the emission from50 successive laser shots to remove changes in thesignal caused by variations in the energy output ofthe lasers and aliasing from the digitization process;successive averages agree to within 10%.

The target mixtures are composed of air, N2, C02,and Ar (Airco, 99.99+% purity); CH3 Cl, C2 H5 Cl, andSF6 (Matheson, 99.5%, 99.7%, and 99.8% purity,respectively); and a variety of liquid CHC's (Aldrich,99+% purity). Mixtures of a CHC and N2 are pre-pared manometrically in stainless-steel sample cylin-ders and combined with a N2 coflow before passingthrough the measurement region. For the open-tube configuration we find flow rates that preventsignificant mixing of air into the measurement vol-ume by increasing the flow rate of a constant CHCconcentration mixture until the CCl signal is constant.For both configurations the flow rates through thesample region ensure that each laser shot photolyzesa fresh mixture. The CHC concentrations in themeasurement region are estimated to be accurate towithin 20%.

Results and Discussion

Laser Fragmentation-Laser-Induced FluorescenceMeasurement of Chlorinated Hydrocarbon Concentrations

Figure 2 shows the optical signal at 278 nm duringone set of fragmentation and probe laser pulses with100 ppm of C2H5Cl (where ppm indicates parts in106), 0.1% 02, and N2 flowing out of the open tube.The dashed trace at the bottom of the figure indicatesthe firing point and width of each laser pulse. Thefragmentation pulse is accompanied by 02B 3 Z-(v' = 4) X 31g-(V'4 = 0) emission excited by

the 193-nm light,' 9 A 2 A(v' = 0) -* X 211(v" = 0) fluo-rescence from CCl formed directly in the A stateduring photolysis, and red-shifted laser scattering.The probe pulse is accompanied by a much largerburst of CCl fluorescence, whose amplitude is theLF-LIF signal, which we will designate by Sf.Because Sf is related to the CCl concentration, whichis in turn a function of the initial concentration ofCHC's, Sf is a measure of the CHC concentration, bymeans of conversion factors that can be determinedin calibration experiments.

The dependence of Sf on the probe wavelength,which is plotted in Fig. 3 for 430 ppm of C2H5Cl andN2 in the photolysis cell, proves that CCl is theemitting species. The data points are separated by0.05 nm (6 cm-') and were averaged over five lasershots. The figure contains four bands, each labeledby its peak wavelength, which closely match four CClX 2fl -> A 2A transitions. These are identified in theenergy-level diagram in Fig. 4, which is based onmolecular constants from M6len et al.' 8 The bandsat 277.84 and 278.98 nm are due to v" = 0 -' = 0absorption, and those at 271.47 and 272.48 are due tov" = 0 -v = 1 absorption. Each vibrational band issplit into two components by the large spin-orbitsplitting of the X 2fl state (135 cm-'), as shown byFig. 4. The A 2A state is also split, but the splitting(14 cm-') is buried within the rotational fine struc-ture of the bands.

CCI Laser-Induced Fluorescence Strategies andDetection Limits

We now describe three LIF strategies for detectingCCl, each of which we have successfully implemented,and we discuss their detection limits. A LIF strategyconsists of specific choices for the fluorescence wave-length, excitation wavelength, and detection band-width. Several spectroscopic characteristics of CClaffect these choices. The A 2A state is the onlyexcited state at sufficiently low energy to be accessibleto dye lasers through one-photon transitions,20 so any

'a.L5

0E

CCI fluorescence

02, CCI fluorescence S

pump laser pulse At probe laser pulse

… I…W., …I- ---- __------1 -- -- -- -

o 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Time (psec)

Fig. 2. Temporal behavior of the optical signal at 278 nm duringone set of fragmentation and probe laser pulses. The photolysiscell contains 100 ppm of C2H 5CI, 0.2% 02, and N2 at 1 atm totalpressure.

II

E

265 267 269 271 273 275 277Excitation Wavelength lnm)

277.85

278.75

271.40 272.25 pt'\ e,

Fig. 3. Fluorescence at 278 nm as a function of the excitationwavelength. The photolysis cell contains 430 ppm of C2H 5Cl andN2 at 1 atm total pressure.

20 June 1994 / Vol. 33, No. 18 / APPLIED OPTICS 3979

279

v=1

l vO

A

t272.48

277.84 i

A

1278.98

2A A

A II II II II II II a

(a)

v"=0 - v'=0, 278nmv' = 0 - v" = 0, 278nm

v=1

2v=O X 3/2

Fig. 4. Energy-level diagram showing the first two vibrationallevels of theX 2f andA 2A electronic states. The spacing of levelsis approximately to scale, except for that between the X and Amanifolds. The wavelengths of four transitions are indicated innanometers.

v"=0 - v'=1, 271nmv'=1 - v"=1, 278nm

(b)

strategy must be based on X <-* A transitions. As wediscussed above, the X 2fI state is split into twospin-orbit components. Because the 2111/2 compo-nent has the lower energy, it always contains moreCCl molecules than the 213/2 component (by a factorof 2 at 300 K; less at higher temperatures). Thus allof the strategies excite molecules out of this compo-nent. The X and A states have similar molecularconstants, so the J = 0 transitions overlap; byexciting the resultant Q heads, we can probe mol-ecules from many rotational levels simultaneouslyand obtain stronger fluorescence. Finally, Fig. 3shows that a detection bandpass of 5 nm will encom-pass both spin-orbit components and all of the rota-tional lines within a particular vibrational emissionband.

The detection limits for each strategy are defined asthe concentration of C2H5Cl that gives a signal-to-noise ratio of one. Previous research has shown thatsystematic errors in concentrations caused by absorp-tion onto walls and the like can become important forsub-ppm CHC mixtures.5 Therefore we have mea-sured the signal-to-noise ratio at concentrations of1-10 ppm. In all measurements a background sig-nal caused by Rayleigh scattering is present; thisbackground is subtracted from the signal and noisebefore their ratio is calculated. The backgroundmust be laser scattering because it always occurs as anarrow band at the probe wavelength and displays nostructure as this wavelength is changed. The back-ground decreased by 85% when helium was flowingout of the tube, indicating that most of the signal wasRayleigh scattering, with some scattering from sur-faces. The background is measured by blocking thefragmentation light, because this is the approachmost likely to be used in a real application. Experi-ments show that similar background levels are mea-sured by tuning the excitation or detection wave-length away from the CCl bands, or with pure N2 inthe measurement region.

llv'=0 - v'=0, 278nmv'=0 - v = 1, 285nm

(c)

Fig. 5. Schematic diagrams of three different LIF strategies fordetecting CCl: (a) strategy A, (b) strategy B, (c) strategy C.

The three strategies we have studied differ in thevibrational bands excited and monitored, as shownschematically in Fig. 5. Strategy A excites the v" =0 -v' = 0 transition and monitors emission from thesame transition. Transitions with Av = 0 are muchstronger than those with Av • 0; therefore strategy Auses both the strongest absorption and the strongestemission transitions, and it generates the maximumfluorescence. However, the fluorescence is at thesame wavelength as the probe light, so it cannot bedistinguished from Rayleigh scattered probe light.We minimize this background by aligning our detec-tion optics in the plane of polarization of the probelight, where the Rayleigh scattering intensity is at aminimum.2' With this configuration the signal wemeasure at 1 ppm of C2H5Cl is one half Rayleighscattering and one half CCl fluorescence. 50-shotaverages of the Rayleigh scattering vary by less than10%, so the detection limit for C2H5Cl is 100 ppb orless. Because of the variation in the pulse energiesof the lasers, we must average 50 successive shots toobtain results that are reproducible to within 10%.Our lasers are limited to 10 Hz, so detection requires5 s. However, state-of-the-art excimer lasers canoperate at repetition rates of 350 Hz or more, someasurements in less than s are possible.

3980 APPLIED OPTICS / Vol. 33, No. 18 / 20 June 1994

Energy

271.47

i

I

2X T

1/2 v=O

l

v-.

Strategies B and C suppress the interference fromRayleigh scattering by using different transitions forfluorescence and excitation; however, this requiresusing less intense Av 0 transitions. Strategy Bexcites v" = 0 v = 1 and monitors fluorescence at278 nm. Emission can occur from the v' 1 =1 transition, and if molecules in the A state collision-ally relax to v' = 0, from the v' = 0 -v" = 0transition. All Av = 0 transitions are closely spaced,so the detection system collects both. Figure 3shows that the fluorescence at 278 nm is 20 timesstronger for excitation at 278 nm than for excitationat 271 nm; therefore strategy B collects 20 times lesslight than strategy A. The difference is caused bythe unfavorable transition probabilities for Av • 0transitions. In addition, predissociation reduces thecollision-free lifetime of the A 2A state from 110 ns inv' = 0 to 17 and 35 ns for the two spin-orbitcomponents of v' = 1.22 The collisional lifetimes atatmospheric pressure are even shorter, but becausethe molecules that predissociate cannot be excitedlater in the probe pulse, the total fluorescence signalcould still be reduced.

Our monochromator discriminates between emis-sion at 278 and 271 nm by 4000; thus Rayleighscattering still presents a background to strategy B.At 7 ppm of C2H5Cl the signal is 400 times strongerthan the background, which varies by 20%, so thedetection limit is 5 ppb. The background variation isgreater than for stratregy A because the absolutesignal levels are much smaller; therefore shot noise ismore important.

Although most of the fluorescence occurs in theAv = 0 bands, there is some in other bands. StrategyC monitors the fluorescence in the v' = 0 -v" = 1band near 285 nm following excitation through theintense v" = 0 -v' = 0 band. We find that the signalfrom strategy C is less than half of that from strategyB. These signals possess the same advantages anddisadvantages otherwise, so strategy C is inferior.

In summary, we have shown that detection limitsunder 10 ppb are possible for our apparatus. Op-tions for improving the detection limits exist. NewerArF lasers produce 500% greater pulse energies thanour laser. Experiments in which the fragmentationbeam was attenuated with mesh screens show thatthe CCl signal depends strongly on the 193-nmenergy density, so increasing the energy density atthe measurement region translates into lower detec-tion limits. Even if the increased energy densitycaused air breakdown or saturation, one could stilluse the additional light to increase the size of themeasurement region. Enlarging the solid angle ofthe detection system would increase the amount offluorescence collected. The detection limit for strat-egy A could also be improved by increasing thepolarization of the dye laser beam.

Mechanisms of CCI Formation and Destruction

On one hand, if the CCl that we observe results fromunimolecular dissociation of the CHC's, then interfer-

.2

3a _

0 -. -. ' unimol~~~~~~~~~~ecular Islope=1)2 5 10 20 50 100 200 500

C2HC Concentration (ppm) [log scale]

Fig.6. CCl fluorescence as a function of the C2 H5Cl concentrationfor C2H5Cl-N2 mixtures flowing out of the tube. The CCl isexcited at 277.8 nm.

ing CCl cannot be created from HCl and sources of C;on the other hand, if the CCl is formed by bimolecularreactions among primary dissociation products, thenHCl could generate an interference. Therefore wesought to determine which of these mechanisms wasoccurring. Figure 6 shows the dependence of Sf onthe C2H5Cl concentration for C2H5 Cl-N2 mixturesflowing out of the open tube, plotted on a log-logscale. The first and last data points are separated inconcentration by 2 orders of magnitude. The datashows a first-order dependence on the C2H5Cl concen-tration, implying that the process is unimolecular.

To obtain further information on the formationprocess we investigated the dependence of the CClfluorescence on the time interval between the frag-mentation and probe lasers, At, which is defined inFig. 2. Figure 7 plots Sf against At for three concen-trations of C2H5Cl in N2 flowing out of the open tube.At 140 ppm (100 mTorr) of C2H5Cl, Sf rises for several

I

E

r,N

0 10 20 30Laser Interval (psec)

Fig. 7. CCl fluorescence as a function of the time interval betweenthe fragmentation and probe lasers, for three concentrations ofC2H5Cl mixed with N2 at 1 atm total pressure and flowing out ofthe tube. The CCl is excited through the v" = 0 - v' = 0

transition at 277.8 nm.

20 June 1994 / Vol. 33, No. 18 / APPLIED OPTICS 3981

microseconds, then decreases, falling to half of themaximum value after 15 pus. At lower concentra-tions the same qualitative behavior occurs but overlonger time scales. The shortest reproducible At is0.1 ps; some signal is always present at this interval,but the trend appears to be toward Sf = 0 at At = 0.

The initial increase in Sf could result from changesin the bulk gas density, diffusion or convection of CClinto the measurement volume, formation of CCI bymeans of reactions, or relaxation of vibrationallyexcited CCl to v = 0. Measurements reveal that theprobe laser Rayleigh scattering signal is independentof At, so the bulk gas density in the measurementregion is not changing. Diffusion and convection ofCCl in and out of the measurement volume is ruledout by the C2H5Cl concentration dependence. Chemi-cal formation of CCl would contradict the earlierconclusion that the CCl was formed in a unimolecularprocess. Furthermore, experiments in the cell withvarying N2 pressure showed that the CCI concentra-tion was unaffected by the N2 concentration, so onlyfragment plus fragment reactions could be formingCCl. The characteristic time for fragment plus frag-ment collisions is 1 Rus (for a fragment concentrationof 140 ppm), which is of the same order of magnitudeas the observed rise time. Consequently, reactionsamong fragments would have to proceed at gaskinetic rates for us to explain the observed CCI;however, all of the rate constants that have beenmeasured for CCl are at least 1 order of magnitudeless than gas kinetic.23-25

The best explanation for the initial rise is that theCCl molecules are formed in vibrationally excitedlevels of the ground state and Sf increases as theyrelax through collisions to u = 0, which is the onlylevel whose population is measured. Excess energyin multiphoton photolysis commonly reappears asvibrational excitation of the products.26 Tyermanobserved vibrationally excited CCl from photolysis ofCF2CC12 with a flash lamp.23 Moss et al. 2 7 foundmany vibrationally excited fragments in 193-nm pho-tolysis of chloroethylenes. Under our conditions,vibrational relaxation of CCl would be expected toproceed slowly. The efficiency of vibrational energytransfer in molecular collisions decreases strongly asthe vibrational frequencies of the collision partnersdiffer. The ground-state vibrational frequency ofCCl is 861 cm-1,'8 compared with 2360 cm-' for N2,28

and the density of other fragments that might havefrequencies similar to CCl is small.

To demonstrate that vibrational relaxation causesthe rise in S we added a more efficient collisionpartner, SF6. As noted by Tyerman,29 although noneof the SF6 fundamentals (347, 525, 616, 642, 774, and948 cm') 30 are near that of CCl, the combinationband of the two lowest fundamentals is within 15cm-'. We also added CO2 (vibrational frequencies of667, 1333, and 2349 cm-')30 and Ar (no vibrationalmodes) in separate experiments. None of these threespecies are photolyzed by the 193-nm light, so we canassume that they are still present in the measure-

0 5 10 15 20 25 30Laser Interval lpsec)

Fig. 8. CCI Fluorescence as a function of the time intervalbetween the fragmentation and probe lasers for 23 ppm of C2HrCland N2 at 1 atm total pressure, with 1% SF6 added in onecase. The CCl is excited at 277.8 nm.

ment region after the fragmentation pulse. Nochange in the dependence of Sf on At was observedwhen CO2 or Ar were added. The results for SF6addition are shown in Fig. 8. The initial rise isreduced from 15 pus to less than 2 [Ls.

The removal of CCl after the maximum concentra-tion is achieved is almost certainly due to chemicalreactions. When 25% of the N2 was replaced by Ar,no change in the falloff was observed, so thesereactions must be between CCl and other fragments.The falloff portions of the curves in Fig. 7 fit exponen-tial decay functions. The inverses of the time con-stants for each concentration are plotted against theC2H5Cl partial pressure in Fig. 9. Assuming that thefragment concentration is equal to the original C2H5CIconcentration, we see that a straight-line fit gives aCCl plus fragments rate constant equal to 2 x 10-"1cm3 molecule-' s. This rate is comparable with

0.08

0.06

- 0.04

0.02

00 20 40 60 80

C2HCI Parial Pressure (mTorr)100 120

Fig. 9. Inverses of the time constants characterizing the falloffportions of the data in Fig. 7, plotted against the C2H5Cl partialpressure for each case. A rate constant of 7.3 x 105 Torr' s- canbe extracted from the slope of the line.

3982 APPLIED OPTICS / Vol. 33, No. 18 / 20 June 1994

the CCl plus NO and CCl plus C2C14 rates and issubstantially faster than other CCl rates.23-25

CCI Yields of Different Chlorinated Hydrocarbons

C2H5Cl has been the target species in most of ourexperiments because it is the least toxic CHC. Wehave also tested CH3 Cl, 1,2-C2H4 C12, 1,1,1-C2 H3 C13,cis-1,2-C2H2 C12, C2HCl3, and C2C14. For the chloro-ethanes and chloroethylenes, the CCl fluorescenceyield increases with the Cl:H ratio. The chloroeth-ylenes yield roughly an order of magnitude more CClfluorescence than the chloroethanes with the samenumber of Cl atoms. The chloromethane tested,CH3Cl, generated roughly the same CCl fluorescenceas C2 H5Cl.

Conclusions: Implications for Incineration

Our earlier publication discussed the application ofthe LF-LIF technique to actual incineration sys-tems." The present results provide additional rea-sons for us to believe the technique can be imple-mented successfully in incineration.

Two useful CMl LIF detection strategies were iden-tified with detection limits of 100 ppb and 5 ppb forC2H5Cl in our apparatus, 10-mm3 spatial resolution,and 5-s measurement time. These characteristicsare sufficient for laboratory investigations of incinera-tion, including measurements in the actual flamezone. Emissions monitoring requires detection lim-its of 10 ppb or less, which has been achieved hereunder simplified conditions. Additional options forlowering the detection limits have been identified.

The CCl is formed by prompt, unimolecular decom-position of the parent CHC's after they absorb the193-nm light. HCl and Cl2 cannot create interfer-ences through this mechanism. Therefore the tech-nique is capable of distinguishing Cl in unburnedCHC's from Cl in the desired incineration end prod-ucts, HCl and Cl2, which is the fundamental require-ment for a CHC emissions monitor.

The CCl molecules are formed in excited vibra-tional levels of the ground electronic state and mustrelax to v = 0 before the strategies demonstrated herecan measure them. Hydrocarbon combustion prod-ucts and HCl should relax the vibrationally excitedCCl inefficiently. Thus the time interval betweenthe fragmentation and probe lasers must be chosencarefully so that vibrational relaxation can occur.Because the time scales for vibrational relaxation areshorter than those for chemical removal of CCl, thedelay before the probe pulse should not decrease thesensitivity.

The CCl-based LF-LIF technique successfully de-tected every species in our experimental program,including chloromethanes, chloroethanes, and chloro-ethylenes, so the technique offers broadband sensitiv-ity to CHC's. Furthermore, all of these speciesproduced the same or more CCl than C2H5Cl, so thedetection limits discussed above are worst-case limits;for most species the sensitivity will be greater.

The National Institutes of Environmental HealthSciences funded this research through the SuperfundBasic Research Program. We thank A. K. Oppen-heim and N. J. Brown for loaning us equipment.

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