Aerosols Analysis by LIBS for Monitoring of Air Pollution by Industrial Sources

Transcription

1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Aerosols Analysis by LIBS for Monitoring of Air Pollution by Industrial Sources G. Gallou, J. B. Sirven, C. Dutouquet, O. Le Bihan & E. Frejafon To cite this article: G. Gallou, J. B. Sirven, C. Dutouquet, O. Le Bihan & E. Frejafon (2011) Aerosols Analysis by LIBS for Monitoring of Air Pollution by Industrial Sources, Aerosol Science and Technology, 45:8, , DOI: / To link to this article: Published online: 12 Apr Submit your article to this journal Article views: 1031 View related articles Citing articles: 23 View citing articles Full Terms & Conditions of access and use can be found at

2 Aerosol Science and Technology, 45: , 2011 Copyright C American Association for Aerosol Research ISSN: print / online DOI: / Aerosols Analysis by LIBS for Monitoring of Air Pollution by Industrial Sources G. Gallou, 1,2 J. B. Sirven, 1 C. Dutouquet, 2 O. Le Bihan, 2 and E. Frejafon 2 1 Department of Physical Chemistry, Commissariat à l Energie Atomique, Nuclear Energy Direction, Gif-sur-Yvette, Paris, France 2 Institut National de l Environment Industrial Risques (INERIS), Verneuil en Halatte, France In the context of the air quality improvement, there is an increasing need to monitor gas and particle emissions originating from exhaust stacks (incinerators, foundries, etc.) for regulation enforcement purposes. Lots of pollutants are targeted; among them, heavy metals are mostly found in particulate forms. Hence, there is a need to promote the development of suitable on line analytical techniques. To that end, laser-induced breakdown spectroscopy (LIBS) appears to be a good technique. Indeed, it is quantitative, fast (<1 min), requires no sample preparation, and can be performed at remote distance. The instrumentation is compact and offers the possibility to be used for continuous and in-situ monitoring. Two different approaches have been tested by several authors to analyze aerosols by LIBS, by focusing the laser either on particles collected on a filter or directly into the aerosol. In this work, these two approaches, aiming at measuring the mass concentration of micrometer metallic particles in air, are investigated and compared. The experimental setup includes an aerosol source (an ultrasonic nebulizer producing a diluted aerosol of CuSO 4 particles); two sampling lines for particle sizes and, for reference concentration measurements, a line for direct LIBS analysis; and a fourth one devoted to filter sampling for subsequent LIBS measurements. Calibration curves were obtained with those two experimental approaches and the results are compared. In terms of sampling particles number, indirect analysis appears to be more efficient than direct analysis for our experimental conditions. Better detection limits were found with direct analysis when comparing the two approaches under similar sampling conditions (analysis time and sampling flow). 1. INTRODUCTION In the context of the air quality improvement, gas and particle emitted by incinerators and foundries are going to be more and more controlled. Lots of pollutants are referenced; among them, Received 9 July 2010; accepted 16 November Address correspondence to G. Gallou, Department of Physical Chemistry, Commissariat à l Energie Atomique, Nuclear Energy Direction, F-91191, Gif-sur-Yvette, Paris, France. g.gallou@iveasolution.com heavy metals are mostly found in particulate forms (Fernandez et al. 1992). Besides their chemical toxicity due to the elementary composition, heavy metals can generate serious toxic and carcinogenic effects for humans when inhaled in high concentrations (Fergusson 1991; Adriano 1992). Hence, the size of those particles appears to be a major parameter due to the penetration depth in respiratory system (Brauer et al. 2001; Churg et al. 2003). Current reference methods dedicated to combustion waste detection and composition determination such as atomic absorption spectroscopy (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), or mass spectrometry (ICP-MS) involve collecting sample and examining them over the course of often several days. This approach is limited by time-consuming digestion procedure. Moreover, those analytical techniques are not designed to allow on-line measurements, notably in terms of size and robustness. Hence, there is a need to promote the development of suitable on line analytical techniques with reduced analyzing time. Some punctual pollution peaks could be detected. In this context, laser-induced breakdown spectroscopy (LIBS) is developed for heavy metal particle measurements at different concentrations in air or industrial exhausts. This multielemental analysis technique consists in focusing an intense pulsed laser beam onto the sample. The resulting breakdown dissociates and partially ionizes all species within the plasma volume. During plasma cooling, it gives off a continuous spectrum called bremsstrahlung (emission of electrons decelerated in the vicinity of atoms and ions) superimposed to discrete atomic emission lines. As the temporal decrease of the bremsstrahlung is faster than the atoms emission, time-resolved detection of the signal provides the analytical information about the composition of the sample. Typically, spectral peaks observed early after plasma formation are mostly due to ionized species in the plasma. Later the lines observed are due to the deexcitation of neutral atoms. Even later, the emission peaks are primarily due to excited molecules generated from the optical 918

3 AEROSOL ANALYSIS BY LIBS 919 breakdown or atomic recombination. LIBS can yield quantitative measurements of constituent species, enabling a direct, simultaneous detection of several elements. Moreover, LIBS offers a rapid response time compared with the other techniques. Aerosol analysis by LIBS has been implemented by several authors for the analysis of particles. Two approaches are usually considered. The first one consists in focusing the laser beam directly into the aerosol. This is implemented for example for the analysis of combustion particulates (Ottesen et al. 1989; Zhang et al. 1995; Hahn et al. 1997; Buckley et al. 2000; Blevins et al. 2003). The other method consists in analyzing the filter enriched with particles (Panne et al. 2001; Kuhlen et al. 2008). Direct analysis permits notably to make fast and on line measurements while indirect analysis permits to analyze aerosol with low particle concentrations by working with important enrichment time. Very few studies compare those two approaches in terms of analytical performances. The comparison of the results obtained with the different studies referred above is not relevant because of the different nature of particles in terms of chemical composition, density, and concentration. An experimental study has been realized on nanometer NaF particles (Tran et al. 2001). The authors obtained a limit of detection (LOD) of 9 mg/m 3 for direct analysis and of 2.4 µg/cm 2 with indirect analyses, (corresponding to 75 µg/m 3 with enrichment time of 10 min and a sampling flow of 10 lpm). However, they did not use conditional analysis for data processing in case of direct analysis. This method permits to significantly enhance the analytical performance in terms of LOD for direct analysis in case of diluted aerosols (Hahn and Lunden 2000). Another study has been realized on nanometer metallic particles produced with a spark generator (Park et al. 2009). The main particularity consisted in focusing the aerosol flow with an aerodynamic lens focusing. The authors have notably compared the lower size detectable with direct and indirect LIBS system. They obtained, respectively, 100 nm and 60 nm. The aim of this article is to compare the performances of two LIBS approaches for aerosols analysis. In this view, LIBS is used to measure the mass concentration of micrometer copper (Cu) particles in air by both methods using the same aerosol generation system. The diluted aerosol generated is qualified in terms of particles number density using an Aerosol Particle Sizer (APS), but also in terms of particles mass concentration using a Tapered Element Oscillating Microbalance (TEOM). The two LIBS approaches are conducted simultaneously: indirect analyses are performed on particles collected on quartz filters and direct analysis of the same particles is performed by focusing the laser beam in the aerosol flowing through a cell dedicated to on-line LIBS analyses. Calibration curves are obtained, and the LOD is estimated. Results and system performances of the two approaches are compared and discussed. 2. PARTICLES GENERATION The experimental setup is composed of three main elements: the aerosol generating and characterizing system and two LIBS setups for direct and indirect analysis Experimental Setup Cu was chosen as a representative element for environmental monitoring purposes because its sulfate form is highly soluble in water. Therefore, we used a solution of CuSO 4 5H 2 O. Particles with a size bigger than 1 µm were generated by an FIG. 1. Experimental configuration for particles generation and characterization.

4 920 G. GALLOU ET AL. ultrasonic nebulizer (CHGA-75, Synaptec, France) working on the following principle: an aqueous solution was nebulized on the surface of a vibrating ceramic (frequency 80 khz) and the droplets were carried and dried in a gas flow. The droplet size ( 25 µm) is determined by the ceramic vibration frequency, whereas the dried particles size increases with the concentration of the solution. The dried particles are of various shapes and contain 25.5 wt% of Cu. Their number density depends on the solution flow rate on the ceramic. Using this particle generation system, an experimental setup was designed to measure simultaneously and in real time the particle size distribution of the dry particles and their mass concentration as shown schematically in Figure 1. At the output of the generator, a flow splitter was used to inject the aerosol into four analysis lines. The particle size distribution of the dried particles was measured in the first line L 1 with an Aerosol Particles Sizer (APS 3321, TSI, USA) working for particles with an aerodynamic diameter of µm. Figure 2 represents the number and mass distributions obtained with a CuSO 4 solution of 4 g/l. The mass distribution is polydisperse with a median diameter of 3.2 µm and a geometric standard deviation of 1.4 µm. The particles mass concentration is 100 µg/m 3 of CuSO 4 5H 2 O particles (i.e., 25 µg/m 3 of Cu). The global number density (integrated over all diameters) is 50 particles per cm 3. The particles smaller than 500 nm were not taken into account by APS measurements. However, the generated particles were larger than 1 µm (Bermer and Tierce 1996). Moreover, the nanometer particle effects on LIBS measurements, which are mass concentration measurements, of particles would be extremely small. The total aerosol mass concentration was measured with a Tapered Element Oscillating Microbalance (TEOM1400, Thermo Electron Corporation, USA) in the second line L 2. In this device, an inertial balance measures the change in the natural oscillating frequency of a tapered element when particles are collected on an exchangeable filter. The LIBS sample chamber devoted to direct analysis was connected to the third line L 3. It was a stainless steel 2.75 in six-way vacuum cross. The aerosol was injected vertically in the cell with a 5 lpm flow rate. The four horizontal flanges were fitted with optical quality quartz windows. The focal point of the lens was centered on the central vertical axis of the section. With the fourth line L 4, filters were enriched with particles duringdifferentsamplingtimes,witha5lpmaerosolflow.in this study, we worked with quartz filters (QM-H, Whatman, UK) with low heavy metals blank values (Berg and Royset 1993), a high loading capacity (Liu et al. 1983), and a high temperature resistance. These filters are commonly used for metal particles sampling. Their diameter was 47 mm, and the effective sampling diameter was 36 mm Characterization of the Aerosol Generation Setup Using the flow splitter at the generator output, the sampling was homogeneous in the different lines if the flow was approximately the same in all of them. The APS and the TEOM run with imposed flows (5 lpm and 3 lpm, respectively), which led us to set the L 3 and L 4 lines with flow rates of 5 lpm. L 2 was the only line dedicated to TEOM measurements. Particles of the same size distribution were flowed through the four tubes. However, particle size distribution may be altered when flowing through the four sampling lines. Indeed, the way these are bent, flow rates (others) may influence particle losses within the lines thereby modifying mass concentration at the outlet of the sampling lines. Thus, in order to verify that the aerosol mass concentration was identical in the four lines, the sampling efficiency of each line was calculated (Brockmann 2001). As particles generated with our device were bigger than 1 µm, only gravitational and inertial losses were taken into account. The maximum sampling efficiency was obtained for line L 2 (TEOM) essentially due to the weaker flow rate of 3 lpm imposed by the TEOM which significantly reduced the inertial losses, as shown in Figure 3. The correction factors depending on the particle size were calculated to estimate the particles mass concentrations in L 1,L 2, and L 3 from TEOM values. It is equal to the ratio between the two curves shown in Figure 3. FIG. 2. Particles size distributions of the dry particles (4 g/l CuSO 4 solution, 2 ml/mn nebulization flow): number distribution (line) and mass distribution (histogram). FIG. 3. Total sampling efficiency calculated for each sampling line for particle sizing from 1 to 10 µm.

5 To validate these calculations, a comparison was made between the TEOM data (L 2 ) and ICP-OES measurements of the Cu mass deposited on quartz filters (L 4 ). The TEOM mass measurements are related to the aerosol particle density (µg/m 3 )by the expression AEROSOL ANALYSIS BY LIBS 921 m Cu = C m t Q p 0.255, [1] where C m is the particles mass concentration of the aerosol, t is the experiment duration, Q p is the sampling line flow rate (3 lpm), and accounts for the 25.5 wt% of Cu per particle. Results are shown in Figure 4 for the ICP-OES measurements corrected using correction factors for the filter sampling line. The particles size distribution used to calculate the correction is given by the APS. The ICP-OES measurements show good agreement with TEOM measurements, which validates both the experimental sampling setup and the TEOM measurements to estimate the particle mass concentration in other sampling lines. 3. LIBS MEASUREMENTS 3.1. Direct Analysis In the stack of a combustion device following the air pollution control equipment, temperatures are such that most toxic metals have a negligible vapor pressure. Then, at stack temperatures, most metals are present in condensed phase in the form of particles in suspension in air (Fernandez et al. 1992). In this case, the LIBS direct analysis consists in focusing the laser beam directly into the aerosol. Each laser shot creates a plasma in air, and if a particle is present in the plasma volume, it is vaporized and yields an emission signal. For all experiments, 1000 laser shots were fired for each measurement, and we observed the most sensitive emission line of neutral Cu at nm. FIG. 4. Comparison between TEOM and ICP-OES measurements corrected by using correction factors for sampling line L 4 FIG. 5. LIBS experimental setup for direct analysis (top view) Experimental Setup The experimental setup used for direct analysis is shown in Figure 5. The laser-induced plasma was generated using a Q- switched Nd:YAG laser at 1064 nm (Brilliant, Quantel, France), witha5nspulsewidth,150mjpulseenergy, and a 20 Hz pulse repetition rate. The laser beam was first reflected by three mirrors (M1, M2, and M3), then expanded to three times its original diameter using a beam expander, reflected by a dichroic mirror (M4) transparent to wavelengths ranging from 300 nm to 600 nm and fully reflective for the laser wavelength, and finally focused in the sample chamber with a 75-mm focal length and 50-mmdiameter lens. The corresponding plasma sampling volume was approximately equal to 10 3 cm 3. The aerosol sampled in line L 3 (Figure 1) was injected in the sample chamber. The plasma emission was collected along the direction of the incident beam by using the focusing lens, injected into an optical fiber using a 25-mm focal-length lens and then guided to a spectrometer (Jobin-Yvon, ihr 320, 1200 grooves/mm grating, 0.3-m focal length, 400-µm slit width, λ/ λ 1,200, resolution FWHM) equipped with an intensified charge-coupled device (CCD) camera (Andor Technology istar, pixels). Signal integration was performed at each laser shot using a delay time of 30 µs with respect to the initial laser pulse (t = 0) and a total integration time of 80 µs Conditional Analysis In direct analysis, the LIBS spectrum is recorded at each laser shot. For diluted aerosols, most spectra exhibit no analyte signal but only the air plasma emission, therefore a large proportion of single-shot spectra do not contain any spectral information related to targeted species. Therefore, conditional analysis (Hahn and Lunden 2000) is used to extract hit spectra, corresponding to those for which a particle is vaporized by the LIBS plasma. This spectrum identification is performed by applying a threshold value to the nm Cu emission line. In the direct LIBS analysis, it is assumed that LIBS always detect a single particle

6 922 G. GALLOU ET AL. in the plasma volume. Considering the low particle number concentration for measured aerosols ( 20 #/cm 3 ), the single particle detection is confirmed for those following experimentations. An example of conditional analysis is presented in Figure 6 for the detection of Cu. The ensemble-averaged spectrum corresponds to 1000 laser pulses. The other spectrum is the average of the 59 spectra showing a Cu signal. We observe a significant increase in signal-to-noise ratio (SNR) when conditional data analysis is used. Finally, to trace a calibration curve, the measurement signal is obtained by multiplying the analyte average signal for hits by the particle sampling rate Assessment of the Upper Particle Size Limit of Complete Dissociation and Vaporization As long as the plasma energy is sufficient to vaporize a particle, the relation between the LIBS intensity and the particle mass is linear. But there exists an upper size limit beyond which the particle is not completely vaporized and the emission signal does not vary with the mass any more. Therefore, this upper size limit is a key element for quantitative analysis of aerosols using LIBS, and it is necessary to determine it to estimate the operating size range for direct analysis. It depends on the experimental setup (laser pulse energy, focusing optics, particle types, etc.). An American research team (Radziemski et al. 1983; Essien et al. 1988) explored direct LIBS analysis of beryllium aerosols using particles of size lower than 10 µm and noted that such a particle size is consistent with complete particle vaporization. Other study (Hahn and Carranza 2002) showed that the upper size limit for complete vaporization of silica particles corresponds to a diameter of 2.1 µm for a laser pulse of 320 mj with a 1064-nm Q-switched Nd:YAG laser. For glucose particles, Vors and Salmon (2006) evaluated this upper size limit to be approximately 5 µm for a laser pulse of 270 mj with a 532-nm Q-switched Nd:YAG laser. A comparison is difficult because the experimental conditions are different for each experiment, but we can see that the upper size limit is always close to some µm. To estimate the upper size limit of complete vaporization for micrometer Cu particles, we used the experimental setup shown on Figure 1 with a monodisperse Vibrating Orifice Aerosol Generator (VOAG 3450, TSI). This aerosol generator produces solid or liquid particles that have a uniform size, shape, density, and surface (Berglund and Liu 1973). The VOAG produced uniform particles by controlling the breakup of a liquid jet and the size of the dried particles depending on the solution concentration. When it decreases from 3.5 to 0.2 g/l, the median diameter of the size distribution decreases from 10.5 to 2.5 µm for CuSO 4 particles. As an example, for a 3.5 g/l solution of CuSO 4, the values of corresponding mean particle diameter and geometric standard deviation were, respectively equal to 10.5 and 1.1 µm. Using the VOAG, particles with different mean diameters were produced. For each size distribution, conditional analysis was performed to extract hits corresponding to the presence of Cu and to calculate the analytical signal. This operation was repeated six times per size distribution to estimate the signal dispersion. Then the total measurement time for each point was 5 min at 20 Hz acquisition rate. Figure 7 displays the peak-tobase (P/B) ratio as a function of the cube diameter of the particle proportional to its mass. A linear relation between the Cu P/B ratios and the cube diameter is observed for the smallest particle diameters (2.5, 4.5, and 5.4 µm). As expected, an abrupt deviation from this linear trend appears for particle diameters larger than an estimated limit of 7 µm. This value represents the upper size below which an aerosolized Cu particle is completely vaporized in the plasma produced by our experimental setup, which is then suitable to quantitatively analyze aerosols with a size distribution similar to that of Figure 2. Lower size limit was referred to in some articles. For diluted aerosol, in which one particle per plasma is vaporized, this inferior limit is due to lower mass detectable by LIBS with a direct analysis setup. For example, Carranza et al. (2001) FIG. 6. Average spectrum of Cu hits (n = 59) and corresponding ensembleaveraged spectrum (n = 1000) for a Cu mass concentration in air of 25 µg/m 3. Spectra have the same intensity scale and are vertically shifted for clarity. FIG. 7. Ensemble-averaged spectra corresponding to individually detected monodisperse Cu microparticles.

7 have obtained a lower mass detectable of the order of 1 fg corresponding to a particle size of 100 nm. In our present study, the lower particle size was 1 µm. Therefore, it was possible to detect all generated particles. AEROSOL ANALYSIS BY LIBS Calibration Curve Calibration was performed by analyzing dry aerosols of known concentrations, using the Sinaptec particle generator and the TEOM online reference measurements. The spectra acquisition and processing were the same as those described in the previous paragraph. Figure 8 displays the calibration curve obtained with an aerosol of CuSO 4 particles with Cu concentration range between 20 and 70 µg/m 3. A linear relationship with correlation coefficient equal to 0.98 was observed between the LIBS intensity and the particles mass concentration. The LOD for a given element is defined as the concentration producing a net intensity equal to three times the standard deviation of the background signal σ B. The LOD is determined from the relation (Cremers et al. 1995) LOD = 3σ B S, [2] FIG. 9. LIBS experimental setup for indirect analysis. where S is the slope of the calibration curve. For each measurement, σ B is estimated from the spectra rejected by the conditional analysis. For these spectra, the standard deviation is calculated from the background intensity values of the six measurements performed for each particle mass concentration. Thus, the LOD obtained for Cu by using the nm emission line is 15 µg/m Indirect Analysis The indirect analysis of aerosols by LIBS is similar to that of a solid. The laser beam is focused onto a filter enriched with particles Experimental Setup The experimental setup for indirect analysis is shown in Figure 9. The LIBS system employed a Q-switched frequencyquadrupled Nd:YAG laser (Brilliant B, Quantel, France) delivering 12 mj/pulse at 266 nm with a pulse duration of 5 ns and a repetition rate of 20 Hz. The pulse energy was adjusted by a variable attenuator (Optec, AT4020). The laser beam was reflected by a dichroic mirror (M4) transmitting wavelengths from 300 to 600 nm and reflecting the laser wavelength. The plasma was created by focusing the laser beam with a plano-convex lens (f = 25 mm) onto the surface of the quartz fiber filter (QM-H, Whatman) located on xy-translation stages. The diameter of the focused laser beam spot was approximately 250 µm. The plasma emission is collected along the direction of the incident beam using the focusing lens. A 15 mm plano-convex lens was employed to focus the transmitted plasma emission into an optical fiber of 910-µm core size and then guided to the entrance slit of the spectrograph (Acton, SP2358i, 600 grooves/mm grating, 0.3-m focal length, 50-µm slit width, λ/ λ 1200, resolution FWHM) equipped with an intensified CCD camera (Andor Technology istar, pixels). The detection delay was fixed at 100 ns, and the integration time was fixed at 1 µs, corresponding to the maximum SNR of the nm Cu line. System automation and analysis of spectra were performed with the industrial software (AnaLIBS, IVEA, France). FIG. 8. Calibration curve for Cu (LIBS direct analysis) Filters Enrichment and LIBS Analysis As described in Setion 2.1, the sampling line L 4 of the experimental setup for aerosol generation (Figure 1) enables to deposit a determined mass density of particles on a filter in order to make

8 924 G. GALLOU ET AL. calibration samples for the LIBS measurement. The Cu mass on the filter was calculated from the TEOM measurements (line L 2 ) using Equation (1) and taking into account the correction factor between both the lines. The filter sampling surface was 10.2 cm 2, and the flow rate in line L 4 was 5 lpm. The Cu concentration obtained on the samples for different enrichment times varied from 0.05 to 0.8 µg/cm 2 of Cu (corresponding to 0.2 and 3.1 µg/cm 2 of CuSO 4 5H 2 O). Figure 10 shows an scanning electron microscopy (SEM) picture of a quartz fibers filter enriched with CuSO 4 5H 2 O particles. We observe that most particles have a diameter lower than 5 µm, but some have a larger size, up to 10 µm. With those fiber filters, particles were located primarily on the surface but minor proportion can be embedded inside the filter matrix. Therefore, it was useful to estimate the particule proportion that was analyzed per laser shot. Figure 11 shows the LIBS intensity of the nm emission line of Cu measured for three successive laser shots on the same location, for two quartz filters enriched with a different Cu mass (0.07 and 0.35 µg/cm 2, corresponding to 0.3 and 1.4 µg/cm 2 of CuSO 4 5H 2 O particles). It was noted that both filters were not perforated with three laser pulses. The Cu line intensity proved to be higher at the first shot. Although the ablation depth depends on the mass loading (Panne et al. 2001), in both cases nearly all the aerosol mass was ablated by the first pulse. Then, we restricted the acquisition to a single shot per spot for all measurements. For this study, samples were scanned using a matrix with spacing between points of 1 mm. One spectrum per position was recorded. Therefore, four hundred eighty-four spectra per sample were recorded. For a given filter, in order to decrease the signal intensity fluctuations mainly due to the substrate density inhomogeneity and laser energy fluctuations, an average spectrum was calculated from the twenty-two spectra of the same matrix line: then twenty-two averaged spectra were obtained. FIG. 11. Depth profile of Cu LIBS intensity at nm obtained on quartz filters with different CuSO 4 5H 2 O particles mass loading. A final spectrum was computed by averaging those twenty-two spectra and the standard deviation was calculated Calibration Curve Figure 12 shows the calibration curve obtained with quartz filters enriched with CuSO 4 5H 2 O particles. In order to minimize fluctuations which were not correlated to Cu concentration variations, the Cu was normalized by Si (which is the main fiber component). This normalization makes it possible to correct ablated mass variations due to different operative conditions (mainly laser energy and sample surface condition) (Aragon et al. 1999). An inflexion is observed for Cu mass loadings higher than 0.5 µg/cm 2. Indeed the correlation coefficients for polynomial and linear regression are and 0.945, respectively. This behavior is due to self-absorption in the plasma. This phenomenon occurs when the radiation emitted at a given wavelength by the plasma core is absorbed by atoms located in the peripheral regions. This essentially depends on the spectral parameters of the line considered (energy levels and oscillator strength) and FIG. 10. MEB picture of a filter (QM-H, Whatman) enriched with CuSO 4 5H 2 O particles (C s = 3.1 µg/cm 2 ). FIG. 12. Calibration curve for Cu (indirect analysis). The points represent the Cu/Si ratio.

9 AEROSOL ANALYSIS BY LIBS 925 on the species concentration in the plasma. The consequence of self-absorption is that the detected signal intensity for highconcentration samples is lower than the intensity that was actually emitted, hence the curvature of the calibration curve. In our case the lower level of the nm Cu line is the fundamental level and it has a strong oscillator strength, which makes it very prone to self-absorption. But this line is the most sensitive one for Cu, which is favorable to obtain the best LOD. Self-absorption could be reduced by choosing another line with higher energy levels but at the expense of the measurement sensitivity. Considering the first points (linear part) of the calibration curve, for which one the correlation coefficients for polynomial and linear regression are and 0.975, respectively, we can nevertheless estimate the LOD using Equation (6). In our experimental conditions, the LOD for Cu is 25 ng/cm COMPARISON OF DIRECT AND INDIRECT ANALYSIS Different criteria can be taken into account to compare those two approaches: some can be quantified, as the sampling efficiency, the LOD, others are rather estimated, as the possibility to perform the measurement with more or less perturbation for the process. The measurements are more representative of the real aerosol when the sampling particle number is important. For a defined aerosol volume, the sampling efficiency is composed of two terms: the ratio of the particles number that are really analyzed with the total particles number and the ratio of the sampling surface with the total surface of the gas pipe. With direct LIBS analysis, the sampling efficiency η D could be calculated by the expression η D = n pv sp f L t. n p Q p t ( φsp φ T ) 2 = V spf L Q p. ( φsp φ T ) 2, [3] where n p is the particle density, V sp is the plasma sampling volume, f L is the laser frequency, t is the experiment duration, and Q P is the sampling line flow. The plasma sampling volume is different from the plasma total volume due to the inhomogeneity of temperature and free electron density in the plasma. Both depend on the experimental parameters (laser pulse energy, focusing optics, particle types, etc.). In our case, the plasma sampling volume is estimated to 10 mm 3. Hence, with f L of 20 Hz and Q P of 5 lpm, the direct sampling efficiency is estimated to 0.024%. For indirect analysis, the sampling efficiency η I could be estimated by the relation η I = η filter S ablation S filter, [4] where η filter is the filtration efficiency of the filter, S abaltion is the ablation surface, and S filter is the filter enrichment surface. With a filtration efficiency of the filter of 95%, an ablation surface (corresponding to four hundred eighty-four pulses with an ablation spot diameter of 200 µm) and a filter enrichment surface of 10.2 cm 2, the indirect sampling efficiency is estimated to 1%. Hence, in terms of sampling particles number, indirect analysis appears to be more efficient than direct analysis for our experimental conditions. However, it is necessary to make allowance of uncertainties on the experimental parameters, notably concerning the plasma sampling volume and the ablation surface. Indeed, the given sampling volume is estimated for particularly spectral line. Another line has notably different transition probability for a given temperature. Hence, for the Cu nm line with a transition probability lower than that of the nm line, the sampling volume is estimated to 10 1 mm 3. With indirect analysis, the ablation surface can significantly change because of laser pulse energy variations or filter surface inhomogeneity. For each example, the sampling efficiency appears to be more important for indirect analysis than for direct analysis. Hence, to have higher sensitivity, indirect analysis seems to be more competitive. In terms of LOD, those two approaches can be compared by converting the Cu LOD obtained in ng/cm 2 with indirect LIBS analysis into air concentration in µg/m 3 by using Equation (1). The measurement duration chosen for this calculation is equal to the time used to perform one direct analysis in accordance with the experimental protocol described in Section 3.1, i.e., 50 s for 1000 laser shots at 20 Hz. Considering this enrichment time, the sampling area S filter (10.2 cm 2 ), and the sampling flow rate D L4 (5 lpm), the equivalent volume LOD of indirect analysis is 60 µg/m 3. This value could be easily decreased by reducing the sampling area and by increasing the sampling flow rate. However with the experimental conditions chosen in this study, direct analysis yields better LOD than indirect analysis. Moreover, the implementation of direct analysis is easier since it does not require any consumable contrary to indirect analysis that uses filters and, in practice, would need a system to move those filters from the sampling device to the LIBS analysis setup. Hence, for short time on line measurement, direct analysis appears to be more efficient. However, for short enrichment time and then short particle number on filter, conditional analysis could be applied to increase the sensitivity of indirect analysis. This implies to make important analysis spot number and then to have either high surface enrichment surface or small spot surface (of few µm). 5. CONCLUSION LIBS was evaluated as a mean to quantitatively analyze micrometer Cu particles (particles size ranges between 1 and 7 µm) using two approaches. The first approach consisted in focusing the laser beam directly into the aerosol flow. With the

10 926 G. GALLOU ET AL. experimental setup tested in this work, the minimum detectable concentration of CuSO 4 particles in air was evaluated to be 15 µg/m 3 for a measurement time of 1 min, and the upper size limit for complete vaporization in the LIBS plasma was found to be approximately 7 µm. The second approach consisted in analyzing by LIBS quartz fibre filters enriched with the same CuSO 4 particles. In this case, we obtained a higher LOD of 60 µg/m 3 for a similar measurement time. The two approaches were also compared in terms of sampling particles number. Indirect analysis appears to be significantly more efficient than direct analysis. Measurements were made with Cu particles like representative element. Results may probably be transposable to other metals. Nevertheless, in order to confirm this point, more analysis with particles composed by other heavy metals (like, Pb, Cr, Ni, etc.) will be soon realized. 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