Volume 52, Number 10, 1998 APPLIED SPECTROSCOPY 13210003-7028 / 98 / 5210-1321$2.00 / 0

q 1998 Society for Applied Spectroscopy

Time-Resolved Laser-Induced Breakdown Spectroscopy:Application for Qualitative and Quantitative Detection ofFluorine, Chlorine, Sulfur, and Carbon in Air

L. DUDRAGNE,* Ph. ADAM, and J. AMOUROUXCentre d’ Etudes du Bouchet, Direction des Centres d’Expertises et d’Essais, B.P. N 8 3 91710 Vert-Le-Petit, France (L.D., Ph.A.);

and Ecole Nationale SupeÂrieure de Chimie de Paris, Laboratoire de GeÂnie des ProceÂdeÂs Plasma, 11 rue P. et M. Curie, 75231

Paris Cedex 05, France

Detection limits for the main heteroatoms in pollutants and chem-

ical agents have been determined in atmospheric conditions with

the use of the time-resolved laser-induced breakdown spectroscopy(TRELIBS) method. This method presents many advantages for de-

tection in hazardous or corrosive gas mixtures where sampling sys-

tems are not usable. Moreover, low concentrations of ¯ uorine, chlo-rine, sulfur, and carbon can be measured with short analysis times.

Currently, concentration limits are close to 10± 50 ppm (w/w) for F,

Cl, and C atoms, while presently only 1500 ppm (w/w) limits arereached for S. These measurements are obtained with an analysis

time of under 20 s.

Index Headings: Time-resolved laser-induced breakdown spectros-

copy; LIBS; Atomic emission spectroscopy; Fluorine; Chlorine; Sul-fur.

INTRODUCTION

Chemical weapons such as mustard gas and neurotox-ics have been banned in most countries, and the chemicalweapons control (CWC) treaty will have been rati ® ed, asof the end of 1997, by more than 60 States. Nevertheless,these weapons are still stored in many arsenals. Conse-quently, their control and destruction are today at theheart of many international projects.

Our goal is to propose new tools to detect in a veryshort time (1±10 s), low concentrations (close to 1±10ppm) of these kinds of toxic molecules both for militaryand civilian needs.

The main objective of this study is to obtain the de-tection limit thresholds for atomic elements such as ¯ uo-rine, sulfur, phosphorus, and chlorine in air, because theseelements are present in hazardous gases or in the organiccompounds which are used as chemical weapons such asnerve agents and blister agents.

In our study, chemical compounds such as CFC or SF 6,diluted at atmospheric pressure in air, are selected to rec-ognize the elements. With this method, we can qualifythe experimental setup of time-resolved laser-inducedbreakdown spectroscopy (TRELIBS) without the precau-tions needed when working with toxic molecules.

In the early 80s Radziemski and co-workers1±3 dem-onstrated that time-resolved laser-induced breakdownspectroscopy can be used as a method for real-time in-vestigation in corrosive and toxic environments. Over thelast ® fteen years, research teams have reported manyanalysis results showing that TRELIBS is a method of

Received 10 March 1998; accepted 19 June 1998.* Author to whom correspondence should be sent.

qualitative and quantitative detection for almost all theatomic species. The interest in this method is mainly dueto the many advantages it has to solve problems of insitu and quasi-real-time analysis:

1. There is no need for a sampling system.2. Only optical components are used, thereby avoiding

any contamination.3. The analysis method can be automated.4. This method can be extended to both polluted sur-

faces and liquids.

Today most of these research groups are interested inthe ability of TRELIBS to analyze solid surfaces (steels,iron ores, alloys, polymers, coal samples, etc.) and gasesand liquid samples (uranium solution, saline aerosols,etc.).

Recent results on detection by plasma atomic emissionspectroscopy have led to low detection limits for ¯ uorine,sulfur, phosphorus, and chlorine atoms (Table I). Usually,detection limits by TRELIBS are higher than those ofMIP-AES (microwave-induced plasma atomic emissionspectroscopy) or ICP-AES (inductively coupled plasmaatomic emission spectroscopy). We observe that detectionlimits are lower when the plasmas are generated in he-lium or argon. Cremers and Radziemski,4 using the sameionic emission line for the phosphorus atom used byCheng et al.5 but operating in an atmospheric medium,reported a detection limit more than two orders of mag-nitude higher. Atmospheric conditions, involving thepresence of nitrogen and oxygen atoms, seem to limit theability to detect by using plasma emission spectroscopy.Nevertheless, LIBS appears to be one of the few tech-niques able to detect ¯ uorine and chlorine atoms.

We have observed that sulfur and phosphorus detectionhas commonly used atomic emission lines in the vacuumultraviolet: S(I) 180.73 nm, S(I) 182.03 nm, P(I) 177.5nm, and P(I) 213.618 nm. These lines provide better de-tection limits than those in the near-infrared.

To our knowledge, studies in the near-infrared regionfor S, Cl, F, and P detection by TRELIBS, with shortanalysis times, have not been previously reported in thepublished literature. In previous papers,6,7 a spatial andtime study of radiative emissions of the plasma provideda better understanding of the main steps in the evolutionof a plasma induced by a pulsed laser. We have studieddependencies of spectroscopic analysis on laser pulse en-ergy, laser wavelength, and number of accumulated spec-tra. The method has been validated for qualitative detec-tion of ¯ uorine, carbon, chlorine and sulfur atoms, using

1322 Volume 52, Number 10, 1998

TABLE I. Analysis procedures by plasma atomic emission spectroscopy for sulfur, phosphorus, chlorine, and ¯ uorine elements.

Atoms Methods Molecule supportGas

diluting Lines (nm)Acquisition

durationa

Detectionlimit

Sulfur LIBS Steel S(I) 180.73 20 s 70 ppmb

ICP-AES (NH4)2SO4 aq He S(I) 182.03 N/A 1.7 ppmc

(C6H5)2S2 Ar S(I) 921.29 N/A 100 ppmd

CuSO4 Ar S(I) 180.73 20 s 24 ppbe

MIP-AES H2S He S(I) 921.29 N/A 30 ppbf

H2S He S(I) 180.73 N/A 0.4 ppbf

Phosphorus LIBS PH3 He P(II) 604.3 N/A 3 ppmg

DIMP Air P(II) 604.3 N/A 690 ppmh

ICP-AES (NH41 ,H2PO4

2 ) aq He P(I) 213.618 N/A 80 ppbc

NaH2PO4 aq Ar P(I) 177.5 20 s 8 ppbe

MIP-AES KH2PO4 aq He P(I) 213.618 N/A 4.5 ppbi

Chlorine LIBS NaCl aq Air Cl(I) 837.594 45 min 8 ppmh

CCl4 Air Cl(I) 837.594 N/A 1.5 ppmj

MIP-AES NaCl aq He Cl(II) 479.45 N/A 120 ppbi

Cl (II) 481.0 N/A 350 ppbi

Fluorine LIBS CCl2F2 Air F(I) 685.604 45 min 38 ppmh

ICP-AES C2ClF5 Ar F(I) 683.426 ´́ ´ QualitativeF(I) 685.604 ´́ ´ detectionk

a N/A 5 not available.b See Ref. 9.c G. F. Kirkbright, A. F. Ward, and T. S. West, Anal. Chim. Acta 62, 241 (1972).d M. W. Blade and P. Hauser, Anal. Chim. Acta 157, 163 (1984).e T. Hayakawa, F. Kikui, and S. Ikeda, Spectrochim. Acta 37B, 1069 (1982).f J. S. Alvarado and J. W. Carnahan, Anal. Chem. 65, 3295 (1993).g See Ref. 5.h See Ref. 4.i Qinhan Jin, Hanqi Zhang, Dongmei Ye, and Jinsheng Zhang, Microchem. J. 47, 278 (1993).j C. Haisch, R. Niessner, O. I. Matveev, U. Panne, and N. Omenetto, Fresenius’ J. Anal. Chem. 356, 21 (1996).k J. M. Keane and R. C. Fry, Anal. Chem. 58, 790 (1986).

FIG. 1. Experimental setup of TRELIBS.

atomic emission lines, respectively, at 685.604 nm [F(I):3s4P 5 /2±3p4D 0

7 /2 ], 833.515 nm [C(I): 3s1P 02±3p 1S 1],

837.594 nm [Cl(I): 4s4P5/2±4p4D07/2], and 921.29 nm

[S(I): 4s5S0(2)±4p5P(1)] . Our purpose is now to determine

detection limits of these elements by TRELIBS in at-mospheric conditions, with different gases diluted in air.

EXPERIMENTAL

Apparatus and Procedure. The laboratory-construct-ed experimental apparatus (Fig. 1) allows time, spectral,

and space resolution of radiative emissions of the plasma.A Nd:YAG laser at 1064 nm with a 50 Hz repetition rate,a 25±160 mJ output energy per pulse, focused with a lens(63.5 mm focal lens) allows one to reach power densitiesof 4.1011 to 2.6.1012 W/cm2. The laser beam is focused ina stainless steel cell (316L), ® lled with gas mixture atatmospheric pressure. The gas mixture made with air [N 2

80%; O2 20% ( 6 1%)] and pure gases or premixed gasesin nitrogen (1% or 0.1%) is generated with a mass ¯ owmeter set. The radiation from the plasma is collected andtransmitted to a 1 m focal length spectrometer by meansof an optical ® ber probe containing 24 optical ® bers sideby side and labeled. With this setup, we can achieve spa-tial decomposition of plasma radiative emissions. Oneend of the optical ® ber without lens coupling is locatedat 3 mm from the plasma plume. The other end, equippedwith an SMA plug, is connected to the spectrometer. Theentrance slit is set to 80 m m width and 2 mm height.Emissions from the plasma are spectrally resolved by aspectrometer with a 1200 lines/mm grating and moni-tored with an OMA 4. A serial mounted gated intensi® erwith the OMA reaches a 5 ns time resolution with 1 nsjitters. A small part of the laser beam (5%) is sampledon a calorimeter (with beam sampler) for calibration pur-poses. The average pulse energy on 50 pulses is measuredby a crystalline pyroelectric calorimeter.

More details on equipment and conditions are listed inTable II.

RESULTS AND DISCUSSION

Optimization of Detection Parameters. To obtain thelowest detection limits, we have optimized different pa-rameters, such as delay between the end of the laser pulse

APPLIED SPECTROSCOPY 1323

TABLE II. Experimental apparatus and settings.

Laser Quantel Brilliant

Wavelength 1064 nmPulse width 6 nsEnergy 25±160 mJRepetition rate 50 Hz

Detection system

Optical ® ber probe 24 Fibers HCG MO365 T08 (SEDI)Numerical aperture 0.22Length 3 m

Spectrometer Jobin-Yvon type Czerny±TurnerFocal length 1 mGrating 1200 lines/mmSlit width/height 80 m m/2 mm

Gated image intensi® er EG&G: Gated Image Intensi® er 1435ADelay 2.5 m s (F), 2 m s (S), 4.2 m s (Cl), and 4 m s (C)Width 20 m sGain 5

Detector EG&G±OMA vision CCD-2CCD Thomson CSF 512 3 512 pixelsCCD size/pixel size 9.7 3 9.7 mm/19 3 19 m mCooling 2 60 8 C

Signal processing OMA 4000 (PAR)Data acquisition mode On trigTrigger External TTL (0±5 V) from laser Q-switch outAccumulated spectra 1000

Crystalline pyroelectric calorimeter Coherent LM-PlOi

Inlet gas ¯ ow metersFlow meters Tylan FC-280: 0±10, 0±300 cm3/min for CFC and 0±2 L/min for airFlow controller Tylan RO-28Gas ¯ ow in cell 300 cm3/min

TABLE III. Atomic and ionic sulfur emission lines monitored in apure SF6 plasma.

Wavelength(nm)

Elementalspecies

Wavelength(nm)

Elementalspecies

469.41 S(I) 674.88 S(I)481.55 S(II) 675.72 S(I)499.35 S(I) 416.27 S(II)a

542.86 S(II) 858.56 S(I)543.28 S(II) 868.05 S(I)545.38 S(II) 869.47 S(I)547.36 S(II) 921.29 S(I)550.97 S(II) 922.81 S(I)556.49 S(II) 923.75 S(I)560.61 S(II) 469.4 S(I)a

564.0 S(II) 492.41 S(II)a

564.7 S(II)

a Indicates lines in the second-order spectrum.FIG. 2. Evolution of the signal-to-noise ratio vs. logarithm of the num-ber of accumulated spectra.

and the opening of the optical detector, detector gatewidth, and number of accumulated spectra.

As is well known, the focusing of a powerful laserpulse induces, near the focal region of the lens, a high-temperature plasma (15 000±20 000 K). For a few nano-seconds, the plasma grows in time and space. At the endof the laser pulse, when the radiative power decreases,the plasma decays and emits the absorbed energy throughradiative deexcitations, electronic recombinations, andbremsstrahlung emissions. The atomic species radiate forseveral microseconds after the end of the pulse. The earlylines are mainly due to single ionized atoms. However,

in general, these ionic emission lines are very dif® cult toobserve: either they do not exist because of the high ion-ization potential of nonmetallic elements or, when theyexist, they are masked by the high continuum back-ground. After 1 or 2 m s, the high continuum backgrounddecreases noticeably, and more intense and persistentatomic lines appear. Therefore, by using time-resolvedplasma emissions, we are able to determine optimal timedelay and optimal gate width, in order to maximize thesignal-to-noise ratio (SNR).

Optimal time delay between the end of the laser pulse(corresponding to the initiation of the plasma) and theaperture of the detector depends strongly on the speciesto detect. To determine these, we studied the SNR vs.delay for each element. The optimal SNR for the F(I)line, at a concentration of 200 ppm (w/w) of ¯ uorine in

1324 Volume 52, Number 10, 1998

FIG. 3. Sulfur line intensity vs. sulfur concentration in ppm (w/w) forSF6/air mixtures. Delay 5 2 m s; width 5 20 m s; laser pulse energy 546 mJ; number of accumulated spectra 5 1000.

FIG. 5. Chlorine line intensity vs. chlorine concentration (ppm w/w)for CF2Cl2/air mixture. Delay 5 4.2 m s; width 5 20 m s; laser pulseenergy 5 130 mJ; number of accumulated spectra 5 1000.

FIG. 6. Zoom on chlorine emission spectra for low concentrations ofCl: (± ± ±) 10 ppm; ( ) 25 ppm; (Ð lÐ ) 50 ppm.

FIG. 4. Comparison between line intensity of ¯ uorine in SF6 and CF4

vs. concentration.

air, is obtained for a time delay of 2.5 m s. For chlorineand carbon detection [200 ppm (w/w)], we ® nd a highertime delay of 4.2 and 4 m s, respectively, while the bestfor sulfur is a 2 m s time delay.

Concerning the gate width, we observe that the SNRbecomes constant for an integration time higher than 1m s. Because of the lines long lifetime [S(I) at 921.29 nmand Cl(I) at 837.594 nm are persistent for more than 15m s], we can set a relatively long gate width of 20 m s.

To improve the SNR, one accumulates spectra for eachanalysis. This technique reduces the detection limit; how-ever, it increases the analysis time. We studied the in¯ u-ence of an accumulation of spectra on the SNR. In Fig.2, we observe that the SNR for ¯ uorine and chlorine linesvs. logarithm of the number of accumulated spectra ® tsa polynomial law. The optimal SNR is obtained for athousand accumulated spectra, which correspond to ananalysis time of 20 s for a 50 Hz pulsed laser.

Detection Limits. In all spectra we determined the sig-nal as the peak height. The noise was taken as the stan-dard deviation of the background measured over 30 pix-els on each side of the line, at the baseline level. Detec-

tion limits were determined by using the formula CL 52 s / a , where s is the root mean square (rms) noise and athe slope of the calibration curve.8 In our paper, s is theaverage of the standard deviation of noise for the ® vesamples with the lowest concentrations. For a given cal-ibration curve, the regression coef® cient is calculated onall the corresponding plotted dots.

Sulfur Detection. At ® rst, using pure SF6 shows sev-eral sulfur lines in the 450±1000 nm spectral region (Ta-ble III). Another scan using SF6 at 1% in air reveals onlya few sulfur lines; centered at 469.5 nm the triplet 469.41/469.54/469.62 nm from S(I) lines overlapped, as well asthe three S(I) lines at 921.29/922.81/923.75 nm.

We decided to determine the detection limit of sulfurby measuring the emission at 921.29 nm. For the detec-tion limit measurement, sulfur was provided by SF6 gaspremixed with nitrogen at 1% (v/v). The lower concen-trations were obtained by dilution with reconstituted air.

The detection limit for sulfur (Fig. 3) by accumulating1000 spectra is close to 1500 ppm (w/w). This detectionlimit is 20 times higher than the one obtained by Gon-zalez et al. with the use of the sulfur line at 180.73 nm.9

APPLIED SPECTROSCOPY 1325

FIG. 7. Carbon line intensity vs. carbon concentration in ppm (w/w).Delay 5 4 m s; width 5 20 m s; laser pulse energy 5 100 mJ; numberof accumulated spectra 5 1000.

TABLE IV. Detection limits for ¯ uorine, chlorine, carbon, and sul-fur atoms within a 20 s analysis time.

Element F Cl C S

Detection limit (ppm w/w) 20 90 36 1500Precision on measurements (% RSD) 7 12 8 7

FIG. 9. Concentration curves for alkanes molecules containing differ-ent number of carbon atoms.

FIG. 8. Fluorine line intensity vs. ¯ uorine concentration in ppm (w/w)for CF4/air mixtures. Delay 5 2.5 m s; width 5 20 m s; laser pulse energy

5 100 mJ; number of accumulated spectra 5 1000.

As SF6 is a gas often used as an electrical insulator, andas it is well known to be extremely stable and to recom-bine very quickly, we wondered whether it was totallydissociated into the plasma. To answer this question,knowing that CF4 is completely dissociated (see Fig. 11),we compared the ¯ uorine calibration curves for both CF4

and SF6 molecules.In Fig. 4 we observe that ¯ uorine line intensity vs.

concentration curves ® t linear laws. The ratio betweenboth slopes is almost equal to 3/2 (1.66) and in goodagreement with the ratio of ¯ uorine atom number in thesetwo molecules.

Now we have an explanation for the high detectionlimit: in air, sulfur may offer a great reactivity with atomsstemming from the air decomposition. Moreover, theatomic emission line at 921.29 nm has a low relativeintensity.

Chlorine Detection. To determine the chlorine detec-tion limit, we have used CF 2Cl2 as a test molecule. Weobserve that emission intensity of the chlorine line at837.594 nm vs. concentration ® ts a linear law over a

range of 1000 ppm (Fig. 5). The detection limit is 90ppm (w/w) in air.

In Fig. 6, we notice that the chlorine line is still iden-ti ® ed for concentrations of 50, 25, and 10 ppm (w/w).Nevertheless, for these concentrations the SNR is lessthan 2, because of artifacts or unidenti® ed lines on eitherside of the 837.594 nm chlorine line.

Subtracting an emission spectrum of air from all thesespectra may lead to a better SNR and therefore to a lowerdetection limit.

Carbon Detection. For carbon detection, the atomicemission line used is C(I) at 833.515 nm. This line is oneof the strongest in the near-infrared, and it is close to thechlorine line at 837.594 nm. When we use CFC mole-cules, we can simultaneously detect both elements.

In Fig. 7 we plotted the calibration curve for carbondetection leading to a detection limit of 36 ppm (w/w),using C4H10 and CF2Cl2 to provide C atoms.

Fluorine Detection. The ¯ uorine concentration limitwas acquired by using CF 4 as a source of F atoms. Theatomic emission line selected for acquisition of spectra isthe F(I) line at 685.604 nm. In Fig. 8, we observe that¯ uorine line intensities vs. concentration ® ts a linear lawwith a regression coef® cient very close to 1. The con-centration limit is 20 ppm (w/w).

In Table IV, we summarize the detection limits cur-rently available for ¯ uorine, chlorine, carbon, and sulfuratoms within a 20 s analysis time. The precision for 20

1326 Volume 52, Number 10, 1998

TABLE V. Comparison of stoichiometric ratio and slope ratio be-tween molecules containing different number of C atoms.

MoleculesStoichiometricratio in carbon

Measured sloperatio

nC4H10/C3H8 4/3 1.39nC4H10/C2H6 2 2.14C3H8/C2H6 3/2 1.54

FIG. 11. Concentration curves for CFC molecules containing differentnumber of ¯ uorine atoms.

TABLE VI. Comparison of stoichiometric ratio and slope ratio be-tween molecules containing different number of Cl atoms.

MoleculesStoichiometric

ratio in chlorineMeasured slope

ratio

CCl3F/CCl2F2 3/2 1.30CCl3F/CHClF2 3 2.95CCl3F/CClF3 3 2.79CCl2F2/CHClF2 2 2.27CCl2F2/CClF3 2 2.14

FIG. 10. Concentration curves for CFC molecules containing differentnumber of chlorine atoms.

consecutive measurements of sulfur (3000 ppm) and ¯ uo-rine (200 ppm) signals is close to 7% RSD (relative stan-dard deviation); for carbon (40 ppm), it is about 8%;while for chlorine (200 ppm), it is around 12%. Thisaccuracy takes into account the uncertainties with respectto concentration gas generation 3±9% (¯ ow meters havea precision of 1% full scale), and the 3% energy stabilityof the laser over the 20 s required for these measure-ments.

Atomic Stoichiometry for Fluorine, Chlorine, andCarbon in CFC and Alkanes Molecules. Laser-inducedplasma at atmospheric pressure presents initial tempera-tures up to 15 000 K. For this reason, the gaseous mol-ecules are completely dissociated, including very stablemolecules such as SF 6 or CF4.

As an example, Fig. 9 shows the linear behavior ofline intensities vs. concentration. Then, we de® ned a nor-malized curve obtained by dividing each intensity for allconcentrations by the number of C atoms in the corre-sponding parent molecule, and redrew the linear regres-sion.

We can see that the slopes are proportional to the num-ber of carbon atoms in the molecule (Table V). The sim-plest explanation is that, in the plasma, molecules aretotally dissociated. Later on in this paper, we will see that

this conclusion can be extended to other atomic compo-nents of interest.

Two possibilities of analysis are then possible: in the® rst case, the molecular formula is known, so by mea-suring atomic emission intensities we can deduce con-centrations in the analyzed sample. In this case, we usethe normalized calibration curve for this element (for ex-ample, see Fig. 10). In the second case, the molecularshape is unknown. So, performing an elementary analysisfor each atom, we are able to reach the stoichiometricratio between ¯ uorine, chlorine, and carbon. Then, wecan propose stoichiometric coef® cients and comparethem with a data bank to predict the nature of the mol-ecule. Indeed, let’s consider a molecule M with formulaFaClb, where a and b are the stoichiometric coef® cients.The unknown concentration of M is called [C], and wecan write [F] 5 a[C] and [Cl] 5 b[C].

The related measured intensities can be expressed as:IF 5 a F [́F] and ICl 5 a Cl [́Cl] where a is the slope of thecalibration curves normalized to one atom (see Fig. 11).We have

I /I 5 a / a ´[F]/[Cl]F Cl F Cl

so

APPLIED SPECTROSCOPY 1327

TABLE VII. Comparison of stoichiometric ratio and slope ratiobetween molecules containing different number of F atoms.

MoleculesStoichiometricratio in ¯ uorine

Measured sloperatio

CF4/CClF3 4/3 1.31CF4/CCl2F2 2 2.13CF4/CHClF2 2 2.01CF4/CCl3F 4 4.41CClF3/CCl2F2 3/2 1.62CClF3/CHClF2 3/2 1.53CClF3/CCl3F 3 3.36CHClF2/CCl2F2 1 1.06CHClF2/CCl3F 2 2.19CCl2F2/CCl3F 2 2.07

I /I ´1/X 5 a /b where X 5 a / a 5 constant.F Cl F Cl

Results for Figs. 9, 10, and 11 show that such analysesare currently achievable for ¯ uorine, chlorine, and carbonatoms. These results are from experiments performed un-der the same conditions:

c Gases used for dilutions in air: CFCl3 (R11), CF2Cl2

(R12), CF3Cl (R13), CF4 (R14), CHF 2Cl (R22),nC4H10, C 3H8, and C2H6 at 1000 ppm (v/v) in N2.

c Conditions of plasma initiation and spectrum acqui-sition:

±Delay 5 2 m s±Width 5 20 m s±Laser pulse energy 5 100 mJ±Number of accumulated spectra 5 1000.

The ratios between the experimental slopes obtainedwith CFC molecules containing different numbers of For Cl atoms are compared with stoichiometric ratios inTable VI and Table VII. They are clearly in good agree-ment. Moreover, it appears that the method is sensitiveto the number of atoms in the initial molecule and not toits molecular formula.

CONCLUSION

Our work has been focused on the ability of the TRE-LIBS method to analyze complex molecules without anysampling procedure. Our results point out that F, Cl, S,and C can be identi® ed and quanti® ed by TRELIBS, witha short analysis time, by using their atomic emission linesin the visible and near-infrared spectral ranges. One ofthe main results is that we are currently able to determinethe partial molecular formula of molecules such as CFCs.This result demonstrates that very stable molecules (SF 6,CF4) are completely destroyed in the high-temperatureplasma induced by a laser. However, the minimum de-tectable concentrations are the second challenge of thistechnique. The experimental results have shown detectionlimits for ¯ uorine and chlorine close to 20 ppm withoutsignal treatment, while for the sulfur detection the limitthreshold is presently only 1500 ppm. We do have toincrease, by a factor of 10, the sensitivity of the system,in order to respond to the detection requirements for toxicagents.

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949 (1991).6. H. Lancelin, L. Dudragne, P. Adam, and J. Amouroux, ``Time Re-

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8. C. Th. J. Alkemade, J. Snelleman, G. D. Boultiller, B. D. Pollard, J.D. Winefordner, T. L. Chester, and N. Omenetto, Spectrochim. Acta33B, 383 (1978).

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