Progress toward the Quantitative Analysis of PAHs ... - ACS Publications

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Progress toward the Quantitative Analysis of PAHs Adsorbed on Soot by Laser Desorption/Laser Ionization/Time-of-Flight Mass Spectrometry Alessandro Faccinetto,*,† Cristian Focsa,‡ Pascale Desgroux,† and Michael Ziskind‡ †

Laboratoire de Physico-Chimie des Processus de Combustion et de l’Atmosphère (PC2A), UMR CNRS 8522, Université de Lille Sciences et Technologies, 59655 Villeneuve d’Ascq Cedex, France ‡ Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Université de Lille Sciences et Technologies, 59655 Villeneuve d’Ascq Cedex, France ABSTRACT: Ex situ analyses of substances extracted from flames provide useful albeit mostly qualitative information on the formation process of soot and on the impact of exhausts on the environment. An experimental setup based on the coupling of laser desorption, laser ionization and time-of-flight mass spectrometry (LD/LI/ToF-MS) is presented in past works as an alternative means to more traditional techniques like gas chromatography (GC) to characterize the polycyclic aromatic hydrocarbons (PAHs) content of soot. In this paper, we go one step further in the understanding of the laser desorption/laser ionization dynamics and propose a combined experimental/simulation approach: we estimate the limit of detection of LD/LI/ToF-MS as low as [0.2, 2.8] fmol per laser pulse and we make quantitative predictions on the concentration of PAHs desorbed from soot. In particular, external calibration with model samples where PAHs are adsorbed on black carbon at known concentrations allows us to link the concentration of PAHs desorbed and detected by photoionization ToF-MS to the concentration of PAHs adsorbed on soot. The comparison of data obtained from the analysis of flame sampled soot with standard commercial GC−MS run in parallel validates the approach and defines limits and potentialities of both techniques.

1. INTRODUCTION Combustion processes are responsible for most of the pollutants contained in anthropogenic emissions. Carbon, nitrogen and sulfur oxides, volatile organic compounds and polycyclic aromatic hydrocarbons (PAHs) are all found adsorbed on the surface of the carbon-based particulate matter (soot) also formed during combustion. To develop efficient measures for reducing exhaust emissions, it is necessary to investigate the mechanisms of formation and the surface chemical composition of such particulates. More specifically, in the last three decades a variety of original ex situ techniques to characterize the role of PAHs in combustion and environmental sciences has been developed.1−5 The identification and quantification of PAHs adsorbed on soot at several steps of the soot formation process are often perceived as critically important information to track the reaction routes leading to the formation of aromatic molecules, of soot nanoparticles and eventually of the aggregates found in the exhausts.6,7 Despite this variety of experimental techniques, the quantification and often even the identification of large PAHs adsorbed on soot remain difficult tasks. One of the key problems is to understand the PAHs partitioning between the phase where they are naturally found (flames or adsorbed on particles) and the phase where the analytical procedure is meant to be performed (gas or solution). Such partitioning is related to several thermodynamic variables, like the PAHs vapor pressure and the structure and chemistry of the adsorbing surface. As a rule of thumb, the most volatile PAHs (up to three © XXXX American Chemical Society

aromatic rings) are mainly found in the gas phase, whereas the larger ones (four rings upward) are found structurally integrated into the nascent soot nanoparticles or adsorbed on the carbon-rich soot matrix.8,9 The different extraction efficiency of various PAHs and their chemical degradation during the time lapse between the sampling and the analysis are therefore important factors that affect outcome and reliability of any ex situ approach. Traditional and widely followed analytical approaches like gas chromatography (GC) are well suited for detecting relatively light molecules (mass typically up to 400 u depending on the used protocol, analyte, chromatography column and so on), as they provide isomer identification and quantifications, but generally fail when heavier masses are investigated.10,11 Indeed, the presence of an insoluble graphitic matrix forces separation and preconcentration of the analytes, therefore limiting the analysis to the soluble fraction only. In combustion and for samples of atmospheric interest, several extraction protocols are available8 that are often defined by heuristic approaches rather than a detailed understanding of the PAHs partitioning. On the opposite side, the desorption techniques, thermaland laser-based, produce the transition of the adsorbed PAHs Received: June 2, 2015 Revised: July 22, 2015 Accepted: July 28, 2015

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DOI: 10.1021/acs.est.5b02703 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

doubled Quantel Brilliant Nd:YAG laser, λdes = 532 nm, pulse width Δτdes = 4.6(2) ns, operating at the repetition frequency νrep = 10 Hz. The beam transverse intensity profile is Gaussian having e−2 diameter at the sample surface ddes = 1.83(1) mm. The beam-sample energy transfer induces the smooth desorption of neutral species, which form a gas plume expanding in the vacuum normally to the sample surface and between the repeller and extraction plates of the mass spectrometer. These species are photoionized by a second laser beam orthogonal to both the desorption beam and the ion acceleration axes. The ionization beam is generated by a Qswitched frequency quadrupled Continuum Powerlite 8010 Nd:YAG laser, λion = 266 nm, Δτion = 6(1) ns, νrep = 10 Hz. A top-hat transverse intensity profile having diameter dion = 1.27(1) mm is obtained by relay imaging the center of the beam between the repeller and the extraction plate by means of a diaphragm and an f = 500 mm fused silica plano-convex lens. A quasi-collimated beam covering the whole length of the ToFMS acceleration plates is so generated. The desorption/ ionization time delay is set to 50 μs (DG-535 Stanford Research System delay generator) to maximize the signal by selecting the velocity class corresponding to the maximum of the desorbed molecules velocity distribution (v ≈ 600 ms−1). Ions are mass-analyzed in a custom Jordan ToF Products Inc. 1.72 m long reflectron ToF-MS (m/Δm ≈ 1000). The ion detector signals are recorded using a 2 GHz digital oscilloscope LeCroy Waverunner 6200A. In LD/LI/ToF-MS, the laser peak irradiance strongly affects the mass spectrum.16,29 The working range of desorption irradiance Ides ∈ [5, 25] MW cm−2 and ionization irradiance Iion ≈ 2 MW cm−2 are chosen in order to maximize the ion signals and at the same time to minimize postionization dissociation. Especially, the top-hat transverse intensity profile of the ionization beam allows good control over dissociation reactions and fragment-free mass spectra of PAHs with excellent sensitivity.18,29 2.2. GC−MS. GC−MS analyses are performed on a commercial Agilent 5975C series GC/MSD equipped with a column HP 5 SM (0.25 μm, 30 m, 0.250 mm) using helium as carrier gas. In preparation for GC−MS analyses, soot samples are sonicated in benzene to extract the PAHs, then the solution is injected in the GC−MS in 0.111 mL volumes. 2.3. Sample Preparation. In this work, we present the analyses of four different series of samples. Series S01 and S16 consist of model soot obtained from black carbon as adsorbing substrate and pure PAHs at known concentration. They are used to determine the PAHs analytical response, the effect of the surface concentration and the minimum surface coverage that results in a detectable signal. Series SPRE and SDIF are obtained by sampling real flame soot and represent the operational test of the LD/LI/ToF-MS. Pureblack 100 Carbon from Columbian Chemicals Company (average particle diameter 80 nm, specific surface 80−150 m2 g−1) is chosen as blank and adsorbing substrate in S01 and S16 samples preparation: its low hydrogen-to-carbon ratio (H:C ≈ 0.1) assures it is free of PAHs that might interfere with LD/LI/ ToF-MS analyses. Furthermore, its physical−chemical properties are close to those of isotropic graphite used in the calculations of the limit of detection (see below). The stability of Pureblack Carbon to laser ablation is tested up to 42 MW cm−2. The appearance in the mass spectra of signal peaks around 25 MW cm−2, identified as carbon-containing fragment ions, indicates that soft desorption turns to ablation and defines

to the gas phase by uniform heating of the whole sample (thermal desorption, TD12,13) or alternatively by local heating of the sample surface (laser desorption, LD14−16). With respect to solvent extraction, desorption-based techniques reach higher mass limit and require little sample preparation. Samples are usually deposited on supports suitable for the subsequent analysis: polymer membrane,17 porous glass and activated carbon filters,18 electron microscope grids19 and metallic plates.20,21 Online measurements can also be achieved.14,22,23 Desorption techniques are often affected by reproducibility issues inherently linked to the sample surface heterogeneity, and generally fail to provide isomer identification and quantitative information. The complementary information required for identifying the analytes in the extracted solution (chromatography) or in the desorbed gas phase (desorption) is often obtained by mass spectrometry14,18,20,23 or alternatively by infrared spectroscopy.24 Particularly, time-of-flight mass spectrometry (ToF-MS) has become more and more the technique of choice on account of the virtually unlimited mass range and the simultaneous detection of multiple species. ToF-MS is especially suited to investigate individual short-time events like the plume of desorbed gases formed during pulsed laser ablation. Additional specificity can be gained by laser ionization (LI). When the ions are produced by photoionization, the energy transfer can be limited to specific analytes, and parasite phenomena like photodissociation avoided.25 Ionization techniques like resonance enhanced multiphoton ionization (REMPI) enable an even more sophisticated and detailed investigation.26−28 In this work we focus on the quantitative detection of PAHs adsorbed on soot by laser desorption, laser ionization, time-offlight mass spectrometry (LD/LI/ToF-MS) using a combined experimental/simulation approach. The characterization of the analytical response of LD/LI/ToF-MS is the subject of several papers published by our group. Desorption and ionization steps have been thoroughly investigated, leading to a quite complete view of the parameters affecting this technique’s efficiency in detecting PAHs with high sensitivity.15,29,30 A semiquantitative approach has been proposed to relate the detected signal to the surface concentration of PAHs adsorbed on soot.18 Analyses of surface composition have been performed on soot sampled from premixed low-pressure methane18 and atmospheric ethylene31 flames, a jet-diffusion diesel flame,32 and the exhausts of a ship diesel engine.33 However, the complete characterization of LD/LI/ToF-MS remains challenging. Thorough parametric studies have been already performed by various groups, including the influence on the signal of the desorption and ionization wavelength and irradiance.17,34 In this work, we propose a new approach toward the complete characterization of LD/LI/ToF-MS that partly relies on theoretical simulations. A validation comparison with results from standard commercial GC−MS technique defines some limits and potentialities of our approach.

2. EXPERIMENTAL SECTION 2.1. LD/LI/ToF-MS. The analysis procedure and the experimental setup are detailed elsewhere.18 Briefly, the samples are located in the ion source of a custom time-offlight mass spectrometry (ToF-MS) instrument where they are cooled down to 150 K to avoid sublimation of the volatile compounds at the spectrometer working pressure (10−8 mbar). Here they are irradiated at normal incidence by a slightly focused desorption beam generated by a Q-switched frequency B


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Table 1. List of the 16 PAHs Contained in the TraceCERT PAH Standard (Samples S16, See Section 2.3 for Details) and of the PAHs Identified and/or Quantified by GC−MS (Samples SDIF) Sorted by Nominal MassA

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For each PAH the table shows: IUPAC name, nominal mass, structural formula, retention time tr in the analysis conditions (see text), molar absorption coefficient ε at 266 nm wavelength measured in cyclohexane solution,36 concentration on sample S16 cS16 and concentration on sample SDIF cSDIF when pertinent.

treated black carbon are finally pressed on a custom-made sample holder to a solid sample. The concentration of the adsorbed PAHs is calculated from the difference between the initial and the residual amount of matter in the exhaust solution after filtration. The initial amount is known from the concentration of the mother solution, while residual nonadsorbed PAHs are quantified by UV−vis absorption spectroscopy performed on the exhaust solution. Volatility losses of PAHs during sample preparation are considered as negligible: the vapor pressures of naphthalene (the most volatile of the PAHs we used) and dichloromethane are 4.8·10−3 and 41.1 kPa at 290 K, respectively.35 A simple model at constant molar volume to calculate the repartition between the solution and the gas phase shows that the concentration of naphthalene in the gas phase is around 0.01% than in solution. Samples S01 and S16 have target concentration of PAHs cS01 = 10−4 mol

the highest achievable irradiance for nondestructive analytical purposes (carbon matrix ablation threshold Iabl = 25 MW cm−2). At higher irradiance the samples are damaged, and ablated around 42 MW cm−2. Series S01 and S16 are obtained from PAH standards purchased from Sigma-Aldrich. A mother solution is prepared by diluting in dichloromethane one PAH at a time for samples S01, or a TraceCERT standard (16 PAHs, 1 mg mL−1 in dichloromethane:benzene 1:1) for samples S16. The mass range spanned by these PAHs is 128−278 u, see Table 1. The mother solutions are further diluted in dichloromethane to reach the desired concentration then mixed with finely ground Pureblack Carbon (15 min magnetic stirring). The black carbon that now contains the adsorbed PAH(s) is recovered by vacuum filtration on porous borosilicate glass filters (pore size 10−16 μm, P4 European standard). About 150 mg of the C


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Figure 1. Comparison of the desorption yields of three low-mass pure PAHs (left column) and the same PAHs adsorbed on Pureblack 100 carbon (right column) at the concentrations 1.50(2)·10−4 mol kg−1 (naphthalene), 1.08(1)·10−4 mol kg−1 (anthracene) and 9.52(9)·10−5 mol kg−1 (pyrene). Iion = 1.9 MW cm−2.

kg−1 and cS16 = 10−5−10−3 mol kg−1, respectively, see Table 1. In this concentration range, the adsorption efficiency of benzene, present in the original standard used for sample S16, is too small to allow competition with PAHs. Samples SPRE contain soot extracted from a premixed atmospheric ethylene/air flame stabilized on a standard commercial McKenna burner equipped with a 60 mm diameter sintered bronze porous plug. Our interest in premixed flames is due to the coexistence of soot and PAHs along virtually the entire reaction coordinate.20,37 The flame equivalence ratio and the standard total gas flow rate are ϕ = 2.10 and Qt = 1.67 × 10−4 m3 s−1. The flame is stabilized by a circular stainless steel plate having the same diameter of the burner and located 21 mm above the burner surface (height above the burner, HAB) and by a 3.33 × 10−4 m3 s−1 nitrogen shielding flow running on the burner outer ring. A stainless steel extractive probe is inserted throughout a stabilization plate axial hole to reach the flame at 21 mm HAB. Soot is pumped with the combustion gases, undiluted and deposited on the surface of borosilicate filters. Samples SDIF contain soot extracted from a diffusion atmospheric diesel jet turbulent flame. Contrary to the premixed flame described above, this flame features little overlap between the regions of PAH and soot formation as revealed by in situ optical diagnostic performed in parallel.32 Such little overlap allows the sampling of the PAH-rich phase with little interference by the nascent soot and vice versa. The soot volume fraction ( f v) and the PAH profiles are measured along the entire height of the flame using two-color laser-

induced incandescence (LII) and laser-induced fluorescence (LIF) techniques.32 The sampling height corresponds to the f v maximum. The flame is stabilized on a modified McKenna burner equipped with a direct injection high-efficiency nebulizer (DIHEN-170-AA) for the atomization of liquid fuels. The nebulizer is inserted in the center of the burner bronze plug (60 mm diameter) throughout a 6.35 mm diameter tube. A complete vaporization of the liquid spray is obtained above 15 mm HAB. After this height, the flame behaves similarly to a turbulent diffusion gas flame. The nitrogen nebulization gas flow rate is fixed at 5.33 × 10−6 m3 s−1, and the fuel is introduced in the nebulizer capillary with a constant mass flow rate 12.8 mg s−1. The fuel jet exiting the injector is ignited by a lean premixed methane/air flat flame stabilized on the porous plug of the burner. Soot and gases are extracted from the flame by a quartz probe radially introduced in the flame at 92 mm HAB, then deposited on the surface of borosilicate filters. The thin end of the probe (0.9 mm diameter aperture) is positioned in the flame with ±0.1 mm accuracy. The probed flame depth is estimated as twice the diameter of the probe aperture.38

3. RESULTS AND DISCUSSION 3.1. Methodological Considerations. The behavior of the desorption yield vs Ides is described as a competition between the desorption efficiency that releases the neutral molecules adsorbed on the carbon matrix, and photodissociation phenomena that remove precursor ions from the plume.29,34 In one of our previous works, we detailed the D


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Figure 2. Samples SPRE, LD/LI/ToF-MS single pulse analyses at m/z = 152 (left column), 178 (middle column) and 202 (right column) below the desorption threshold (top row), between the desorption threshold and the ablation threshold of the carbon matrix (middle row) and above the ablation threshold of the carbon matrix (bottom row). Data points are attributed to the condensation mode (red-colored), to the desorption mode (green-colored), and to the ablation mode (black-colored). Iion = 2.2 MW cm−2. All data points are shown.

every adsorbed PAH, i.e., there is no need to establish a different Ides working point for each PAH. Differences in the response of PAHs adsorbed on black carbon and bulk PAHs are attributed to different surface coverage. Whether the surface coverage is submonolayer (samples S01), then PAH−matrix interactions dominate and the desorption occurs via the excitation of the carbon matrix rather than the PAHs themselves. Because of the broad spectral absorbance of carbon, no wavelength selectivity can be expected in the sub-monolayer regime.39 Whether the surface coverage is much larger than one monolayer,15 then PAH− PAH interactions dominate and the PAHs are desorbed from successive layers rather than from the carbon matrix, in a situation similar to the desorption of bulk PAHs. As detailed in section 2.3, the concentration of PAHs in real and model soot samples spans the range 10−6−10−3 mol kg−1, always well below one monolayer. The decay of the signal vs the number of laser pulses on the same location at the sample surface (single pulse analysis, section 3.2) provides further information on the nature of the PAH-carbon matrix interactions. Single pulse analyses are performed on samples SPRE. As shown in Figure 2, we identified three different behaviors as a function of Ides. (1) Ides < Ides,th (Figure 2, top row): some PAHs are detected nevertheless. A single decay mode can be observed, having its maximum on the very first laser pulse and quickly disappearing in the following 1−2 laser pulses. (2) Ides,th ≤ Ides < Iabl (Figure 2, middle row): a bimodal behavior occurs. The first mode roughly corresponds to the decay observed in case (1). This second mode follows a log-normal probability distribution function, is shifted to a higher number of pulses with respect to the first mode and persists up to a few tens of laser pulses. (3) Ides ≥ Iabl (Figure 2, bottom row): a new mode appears after the

desorption yields of six pure PAHs in bulk form: naphthalene, acenaphthene, anthracene, phenanthrene, fluoranthene and pyrene.15 The desorption yields of three bulk PAHs are shown in Figure 1 and compared to the same PAHs adsorbed on black carbon. When desorbing bulk PAHs, the desorption yield rises at a specific Ides,th (named desorption threshold), comes to a maximum, then falls (Figure 1, left column). At low Ides values, the number of desorbed molecules increases with Ides, leading to the initial sharp rise. At higher Ides values, the excess of population in the excited rovibronic levels progressively leads to a competition between desorption and photodissociation: the desorption yield reaches a maximum then falls when photodissociation becomes dominant. The position of Ides,th depends on the analyte’s spectroscopic and molecular properties, like the absorption cross section and the enthalpy of sublimation, and it is then characteristic of each PAH. This difference in the response to the desorption process, which is preserved in the case of mixtures of pure PAHs in bulk, in principle provides the selectivity required to discriminate different isomeric PAHs.15 The behavior of PAHs adsorbed on black carbon (samples S01) is remarkably different (Figure 1, right column). First, the desorption yield rises again above Iabl, indicating that the destruction of the matrix releases additional PAHs initially in the matrix core as well as fragment ions and carbon clusters. This observation is confirmed by the extinction of the PAH signal after a few laser pulses when Ides < Iabl (the exact value depending on the surface coverage as detailed below), whereas it persists for thousands of pulses when Ides ≥ Iabl. Furthermore, the trend of the desorption yield is virtually the same for all tested PAHs. From a practical point of view, the selectivity of the response to Ides demonstrated for bulk PAHs is lost. Supposing a similar behavior for natural soot, this simplifies the analysis because the same Ides optimizes the desorption yields of E


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Environmental Science & Technology previous ones and remains visible up to thousands of laser pulses. The very existence of these modes suggests that different PAH−matrix interactions occur at the sample surface.40 The series described in case 1 and corresponding to PAHs desorbed during the first laser pulses, which reasonably feature weaker bonds to the carbon matrix, are possibly the result of the condensation of the gas phase during sampling from a flame region where gaseous PAHs and nascent soot coexist. Herein, it is important to remember that samples SPRE are obtained from a premixed flame where PAHs and soot coexist all along the reaction coordinate.37 The interference of gas-phase PAHs condensing on soot particles as a result of the sampling procedure is a well-known issue during ex situ analyses of soot surface composition. A strong dilution (1:103−105) with a cold, inert gas while sampling is a widely adopted procedure that slows down the reaction kinetics by lowering the sampled gas temperature and that reduces condensation by lowering the partial pressure of all gas-phase species.14,23 An alternative approach is proposed that compares the desorption mass spectra of samples collected without dilution on porous glass filters (to collect mainly soot particles) and activated carbon filters (to sample both the particles and the gas phase) in parallel. The composition of the gas phase is then inferred from the difference between the mass spectra.18 According to our experiments on model soot, the series described in cases 2 and 3 are attributed to the desorption of PAHs adsorbed on soot surface and to the release of deeper PAHs after the destruction of the carbon matrix, respectively. This assumption is supported by the appearance of fragment ions in the mass spectra at high Ides values consistently to the dissociation mechanisms previously described. In our former investigations on real and model soot,18 we irradiated the samples at Ides = 5.1 MW cm−2 and the overall decay lasted a few laser pulses only. The mass spectra there presented were the average of the first five mass spectra obtained from the same decay. These observations point out the need to consider the whole signal decay to obtain pertinent information on the adsorbed phase only. A simplified approach may consist in cleaning every sample by irradiating at low Ides values or in selecting the mass spectra after a given number of laser pulses. 3.2. Determination of the Limit of Detection. In this section, we estimate the limit of detection (LOD) of LD/LI/ ToF-MS on model soot as the lowest concentration of a reference PAH adsorbed on soot that generates a signal that can be distinguished from a blank value. During single pulse analyses, each laser pulse delivered to the sample surface removes some matter from the amount initially adsorbed in the volume heated by the laser pulse. Therefore, when repeatedly irradiating the sample at the same location, the peak intensity decays as a function of the number of laser pulses. The integrated area of the decay curve is proportional to the total amount of substance contained in the heated volume Vth (volume of the thermal region). The decay function of pyrene vs the number of laser pulses at different Ides obtained from sample S01 is shown in Figure 3. The additional decays at Ides > 12.8 MW cm−2 in Figure 3 are attributed to the desorption occurring from a larger spot area (laser beam wings). When Ides < Iabl, the decay curve integrated signal is proportional to the total amount of desorbed pyrene. The minimum amount of desorbed matter nLOD that still produces a

Figure 3. Sample S01, decay of the signal intensity vs the number of laser pulses obtained at different Ides values. Iion = 2.1 MW cm−2. The curve shown is the average of three decays collected from three different spots at the sample surface.

detectable signal corresponds to the end of the leftmost decay in Figure 3: nLOD = ntot

SLOD Stot

(1)

where SLOD is the signal recorded at the LOD (the fourth laser pulse produces SNR ≈ 3), and Stot is the total integrated signal for Ides < 12.8 MW cm−2. Although SLOD and Stot are calculated from the decay curves, a challenge lies in the determination of the total desorbed amount of matter ntot. In this work, we use two different approaches to estimate ntot under the hypotheses of surface desorption and volume desorption that give the extremes of the interval in which ntot is expected to lie. Both approaches are detailed below. 3.2.1. Surface Desorption. The limit inferior n−tot of the desorbed amount of PAHs is estimated by operating under the assumption that only the PAHs adsorbed on the sample surface and directly exposed to the laser beam are desorbed into the gas phase, i.e., by neglecting any form of heat transfer inside the sample: − ntot =

A irr cS01 ASSA

(2)

where Airr (m2) is the area irradiated at the sample surface (calculated from the laser beam diameter ddes), ASSA (m2 kg−1) is the specific surface area of black carbon and cS01 (mol kg−1) is the concentration of the reference sample (pyrene in this case). Solving eq 1 with the condition 2 gives n−tot ≈ 3 fmol that provides the lower estimation of the LOD n−LOD ≈ 0.2 fmol per laser pulse. 3.2.2. Volume Desorption. A more refined approximation is used for estimating the limit superior of the desorbed amount of PAHs n+tot by assuming that thermal desorption occurs from a sample volume which reaches a high enough temperature under laser irradiation. Implicitly, we assume that the molecules desorbed from underneath the surface are readily released into the ion source thanks to the large sample porosity. In this case: + ntot = VthρS cS01

(3)

−3

where Vth (m ) is the volume of the thermally affected region, ρs (kg m−3) is soot bulk density and cS01 (mol kg−1) is the PAH concentration of the reference sample. The problem is now moved to the estimation of Vth that can be obtained from a F


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Environmental Science & Technology qH(r , z , t )= (1 − R )I0f (r )g (t )e−αz

sample temperature map. A numerical approach is implemented to calculate the space-time evolution of the temperature in the irradiated zone. As further detailed below, in such a model we considered the desorption process as occurring from the soot layer generated by compacting the soot aerosol sampled from the flame down to the filter surface. The deposited soot is approximated as a continuous porous medium characterized by apparent density ρapp, thermal conductivity kapp and specific heat capacity cp. As the laser light propagates into the sample volume, it is progressively absorbed by the material and, assuming that the temperature does not rise enough to initiate a phase explosion, is ultimately dissipated by thermal relaxation. The section of Vth is given by the desorption laser pulse. Indeed, although a Gaussian beam diameter does not depend on the pulse energy, the higher the pulse energy and the larger the spot on which the irradiance reaches a desorption-effective value (Ides > Ides,th). However, to simplify the heat transfer model, the e−2 diameter measured by the beam profiler is used in all of the following calculations. The depth of Vth is given by the desorption beam penetration in the sample. We define the thermal desorption depth dth, which is required to calculate Vth, as the distance from the sample surface and the isosurface of a given temperature as detailed below. The optical penetration depth dop at a given wavelength can be calculated using Beer−Lambert law once the complex refractive index is known. The absorption coefficient α (m−1) is related to the complex refractive index m = mR + imI by the relation:

α=

4πmI λ0

⎧1 if r ≤ ddes/2 f ( r )= ⎨ ⎩ 0 if r > ddes/2 ⎪

⎪

⎧1 if r ≤ Δτdes g (t )= ⎨ ⎩ 0 if r > Δτdes ⎪

⎪

where I0 = 5.1 MW cm−2 is the pulse peak irradiance set during routine analyses, α (m−1) is the absorption coefficient at 532 nm wavelength calculated from eq 4 and R is the sample surface reflectivity (R ≈ 0.04).35 Moreover, the high sublimation temperature of the carbon matrix removes the need for any further term describing the carbon phase transitions. In practice, this hypothesis is verified by following the presence of carbon clusters in the mass spectra of a blank sample as a function of Ides. The diagnostic ions attributed to the ablation of the carbon matrix only appear at much higher Ides values than the one set in routine analyses and used for the following heat transfer calculations. The boundary conditions required to solve eq 5 are given by ⎧ ⎪T (z → ∞ , t ) = Ti ⎨ ⎪ ⎩ T (z , 0) = Ti

∂T (z , t ) ∂ 2T (z , t ) = kapp + qH(z , t ) ∂t ∂z 2

(7)

where Ti = 150 K is the temperature of the sample before laser heating and on the sample surface sufficiently far from the irradiated volume. The assumption T(z → ∞, t) = Ti comes from the liquid nitrogen thermostat that keeps the temperature of the sample holder constant (section 2.1). The effect of thermal emission from the irradiated spot and convective heat transfer in the vacuum of the mass spectrometer ion source is tested as well, and eventually proved to be negligible with respect to heat conduction along the sample thickness. To solve eq 5, the computational fluid dynamics module integrated to the commercially available software Solidworks 2012 is used. The model grid was meshed with the default cubic cells, with higher refinement and improved spatial resolution (20 nm) at the surface sample and thermal penetration region. Modeling the heat transfer between a laser beam and soot particles is no trivial task that requires detailed knowledge of the aggregate size and of the thermophysical properties of soot in a wide temperature range. In our experiments, soot is compacted from the aerosol state down to a thin porous layer upon the filter surface. Since the properties of such layer are not easily measured, we used the data of bulk graphite and semiempirical calculations to account for the layer porosity. Also, the contribution to the thermal properties of the adsorbed phase is neglected due to the very low surface coverage. As briefly mentioned above, the required properties of the porous layer are its density ρapp, specific heat capacity cp and thermal conductivity kapp. ρapp enters the heat transfer calculations directly through eq 5 and indirectly in the porous layer thermal conductivity in eq 9 as detailed below. The value ρapp = 0.4(1) kg m−3 is obtained directly from density measurements on the porous layer and its consistency postverified from the parametrization T vs ρapp knowing that the desorption occurs below the sublimation temperature of carbon. The data of isotropic graphite are assumed to be a good approximation for the porous layer.25 On the other hand, kapp strongly depends on

(4)

where mR and mI are the complex refractive index’s real and imaginary parts respectively, and λ0 (m) is the irradiation wavelength in vacuum. Data on the refractive indexes of carbon-based materials are readily available in the literature from coal up to disperse soot aerosols.41 For soot, the values mR = 1.57 and mI = 0.56 are often used in the visible spectral region.42 Although such values are measured for soot aerosols, mI falls well inside the variability range of more compacted carbon-based materials, and thus it is considered as representative of the porous layer and used in all following simulations. The resulting value of dop at λ0 = 532 nm is of the order of 80 nm, much smaller than the beam diameter at the sample surface. Therefore, it can be assumed that the thermal effect on the direction transverse to the beam is negligible when compared to the longitudinal effect (thermal confinement regime). A one-dimensional heat equation can then be considered to simplify the thermal analysis:43 ρapp c p

(6)

(5)

where T (K) is the local temperature of the sample as a function of the time t (s) and of the depth z (m) from the sample surface on the beam axis. ρapp (kg m−3), cp (J K−1 kg−1) and kapp (W K−1 m−1) are the apparent density, specific heat capacity and thermal conductivity of the porous layer as detailed below. The term qH (z, t) describes the heat source provided by the laser beam. For sake of simplicity, the Gaussian spatial and temporal profiles of our desorption laser are replaced in calculations by flat profiles: G


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laser pulse g(t). In Figure 4b, the temperature isosurfaces as a function of time are shown. The surface temperature reaches its maximum at the end of the laser pulse. As the time passes, the region of maximum temperature expands and progressively cools down due to the thermal conduction in the sample. In Figure 4b, dth(T) is the maximum distance from the sample surface (z = 0) reached by the isosurface of temperature T. In order to estimate the thermally affected volume Vth (cylinder defined by the depth of the thermal region dth and the laser beam section at the sample surface Airr) which will efficiently promote PAH desorption, one has to define a threshold temperature for this process. PAH thermal desorption experiments performed in conjunction with aerosol mass spectrometry or GC−MS usually display temperatures around 300 °C for effective desorption of this class of compounds.12,13 On the basis of this, we chose a temperature of 600 K, leading to dth ≈ 0.60 μm and Vth ≈ 1.6 × 10−12 m3. Notice that Vth is sufficiently large to be considered as homogeneous with respect to the size of soot nanoparticles. When going back to eq 3, we obtain n+tot ≈ 60 fmol that provides the upper estimation of the LOD n+LOD ≈ 2.8 fmol. 3.2.3. LOD Summary. To summarize, the limit of detection nLOD ∈ [0.2, 2.8] fmol per laser pulse is obtained from single pulse analyses on model soot samples having the concentration 10−4 mol kg−1 that approximately corresponds to 10−4 monolayers for PAHs adsorbed on black carbon. The exponential decay is also a strong evidence for the submonolayer coverage of the surface47 that indeed turns into a limit of detection for LD/LI/ToF-MS in the range of 10−6 monolayers per laser pulse. This excellent sensitivity to PAHs arises from their large REMPI cross sections at 266 nm wavelength.28 As a concluding remark, we point out that the LOD depends on the (carbon) matrix surface state and can be further pushed by improving the sample surface flatness and smoothness. This was proven by Haefliger and Zenobi who obtained nLOD as low as 1 amol in desorption experiments of PAHs from a glass matrix.17 The same desorption experiment from rougher PVC filters returned nLOD = 0.5 fmol, which is consistent with our results and is claimed to be representative of most PAHs adsorbed on a carbon matrix.17 3.3. Quantification of PAHs Adsorbed on Soot. The goal of the analyses described in this section is to correlate the concentration of PAHs adsorbed on soot with the concentration of PAHs in the desorption plume and detected by photoionization/mass spectrometry. To this purpose, samples S16 and SDIF are analyzed by LD/LI/ToF-MS and by GC− MS in parallel. 3.3.1. Concentration of PAHs in the Plume. When analyzing complex samples containing several PAHs at trace concentration it is important to remember that the same Ides optimizes the desorption yields of every adsorbed PAH (section 3.1): the detection step is ionization-driven. The signal peak intensity depends on the concentration of the analyte in the desorbed plume, which is related to both the sample concentration and the analyte photophysics. Therefore, the ratio of two signal peaks is not representative of the ratio of two concentrations unless only one isomeric structure is present at each m/z.34 However, if the photoionization cross sections of the analytes are known, a linear relation between LD/LI/ToF-MS signal and sample concentration exists that does not depend on the nature of the adsorbed PAHs.

the number of contact points between particles/aggregates that can drastically reduce the efficiency of phonon propagation. The thermal conductivity of a porous medium can be described by the following equation:44 ⎡ k (T ) ⎤ ⎥ kapp(T ) = kgraph(T )⎢(1 − ζ )3/2 + ζ 1/4 air kgraph(T ) ⎥⎦ ⎢⎣

(8)

where kapp is the calculated thermal conductivity of the porous layer, kgraph is the thermal conductivity of bulk isotropic graphite45 and kair is the conductivity of the air filling the pores.35 For comparison, whether a graphite foil instead of a porous medium is used as target material, the much higher thermal conductivity of the foil compared to the one estimated from eq 8 assures that the thermal energy is not confined to the particles. Therefore, a much higher Ides is required to produce surface ablation of the foil.46 Once kair(T) and kgraph(T) are known, eq 8 can be used to estimate kapp(T). The last required parameter, the layer porosity ζ, is given by

ζ=1−

ρapp ρsoot

(9)

where ρapp and ρsoot are the apparent density of the porous layer and the bulk density of soot, respectively. The results of the simulations of heat propagation in the sample are shown in Figure 4. In Figure 4a, the time-dependent maximum temperature is compared to the time shape of the

Figure 4. Simulation of the maximum temperature in the sample vs time (blue dashed line) and comparison with the time shape of the desorption laser pulse (black solid line). (b) Simulation of the heat propagation in the sample. The colored lines represent the deepest dth value reached by the corresponding temperature as a function of time. H


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i.e., it is not possible to know the contribution of every isomer to the total signal when more than one isomer at the same m/z occurs. Isomers having a small absorption cross section result in weak LD/LI/ToF-MS signals even when their concentration in the sample is large. A possible approach to directly compare LD/LI/ToF-MS and GC−MS is therefore to multiply each isomer concentrations cSDIF,j measured by GC−MS to the absorption cross section of the corresponding isomer (photoionization-corrected concentration), then sum over all the contributions at the same m/z. This quantity ΣjεjcSDIF,j is directly comparable to S(m) = kLDΣjεjcSDIF,j measured by LD/ LI/ToF-MS and calculated as detailed in section 3.3.1. The comparison of the photoionization-corrected concentrations obtained from GC−MS and LD/LI/ToF-MS is shown in Figure 6. Two groups of data points are immediately

Samples S16 contain 16 PAHs spanning the mass range 128−278 u (see Table 1). Some of the detected PAHs are isomeric structures, thus their contributions to the total signal at specific m/z cannot be isolated only using information provided by the mass spectrum. Whether no dissociation phenomena occur during the mass analysis, the peak intensity as a function of the mass S(m) is given by18 S(m) = kLD ∑ εjcS16, j j

(10)

where the index j runs over all the isomers having the same nominal mass, εj are the molar absorption coefficient at the ionization wavelength,36 and cS16,j are the PAH concentrations on carbon of sample S16. We used the molar absorption coefficients at the excitation wavelength (266 nm) to multiply each concentration following the idea that the photoionization is governed by the resonant absorption of the first photon.28 Therefore, the ionization cross section is well approximated by the absorption cross section at the excitation wavelength. Such an approach is effective because most low-mass PAHs feature extended resonances in the wavelength range 248−280 nm.28 The constant kLD depends on the overall system configuration, and can be easily measured from the linear fit of the ToF-MS signals vs the sum of the corrected concentrations of all isomers occurring at the same m/z, as shown in Figure 5. The lack of

Figure 6. Sample SDIF, ΣjεjcSDIF,j measured by LD/LI/ToF-MS vs GC−MS are directly compared. LD/LI/ToF-MS and GC−MS give consistent concentration values (points overlap in the top figure/all lie in the bisector line in the bottom figure) whenever the absorption cross sections of all the isomers at specific m/z are known (128, 152, 166 and 178 u). The error bars represent the data standard deviations. The nominal mass of each PAH is shown next to each point.

recognizable. The first group contains the four data points corresponding to the nominal masses 128, 152, 166 and 178 u. These data points lie on the bisector (black solid line in Figure 6), thus GC−MS and LD/LI/ToF-MS return consistent results. The second group contains the data points corresponding to the nominal masses 202, 226, 252, 276 and 300 u that lie above the bisector, suggesting either that GC−MS is underestimating the actual PAH concentrations, or LD/LI/ToF-MS is overestimating them. In the first group, only a few isomers at each mass exist and all their concentrations are quantified by GC−MS, i.e., all possible isomers are considered in the mass balance. In the second group, some isomers are not identified or quantified by GC−MS, and therefore they are not considered in the balance. The much smaller photoionization-corrected concentration obtained from GC−MS in this case (Figure 6) suggests that the unidentified isomers might be adsorbed on soot at not negligible concentration, thus playing an important role in soot surface chemistry as well. In our opinion, this is a relevant result that highlights at the same time potentialities and limits of both techniques. LD/LI/ ToF-MS can perform measurements on an extended m/z range, and detect PAHs up to 800−900 u. On the other hand, the lack of selectivity toward isomeric structures prevents quantitative analyses with the only notable exception of m/z = 128, where naphthalene is the only existent isomer. Conversely,

Figure 5. Sample S16, peak signal vs ΣjεjcS16,j for 16 different PAHs and linear fit. The concentrations are calculated as the sum of the concentrations of all PAHs having the same nominal mass. The error bars and the grayed out region represent the standard deviation of the data points and the linear fit, respectively. The nominal mass of each PAH is shown next to each point.

selectivity of the desorption process assures that in our current setup the value kLD = 2.0(1) V kg m−2 is the same for all the investigated PAHs, and therefore it can be essentially used as a calibration constant. 3.3.2. Quantification by LD/LI/ToF-MS−GC−MS Comparison. Samples SDIF are analyzed by LD/LI/ToF MS and GC− MS in parallel. Nineteen different PAHs are identified and quantified by GC−MS (see Table 1). All PAHs detected by GC−MS are associated with the most intense signal peaks in the LD/LI/ToF-MS mass spectra. Seven other signal peaks attributed to PAHs in the range 178 < m/z < 300 are detected but not identified. A correlation between the PAHs concentration and the corresponding LD/LI/ToF-MS signals is established by widening the approach presented in section 3.3.1. Mass spectrometry alone cannot distinguish conformational isomers, I


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the French Civil Aviation Authority (DGAC) through the MERMOSE project (http://sites.onera.fr/MERMOSE/).

GC−MS can distinguish isomeric structures, but is limited by the extraction procedure during sample preparation, and especially by the incomplete knowledge of soot surface properties that so far prevented the development of a theoretical model to reliably calculate the extraction efficiencies. In conclusion, from the comparison of LD and GC−MS, the quantification of the amount of PAHs adsorbed on soot is only possible when certain conditions are met. In LD/LI/ToF-MS with UV excitation the contribution of the desorption to the peak intensity is outweighed by the ionization. Therefore, the identification of all isomers contributing to the signal at a certain m/z and the knowledge of their photoionization cross sections at the excitation wavelength are mandatory for analytical purposes. Whenever only one isomer is present at a given m/z, the quantification is straightforward, like in the case of naphthalene. The total number of possible isomers increases rapidly as the molecular weight increases, thus the photoionization spectrum-based isomeric identification can hardly be sufficient to distinguish isomers for heavy molecules. From a practical point of view, the limitation of our ionization sources (maximum photon energy around 5.5 eV) has not allowed so far access to single photon ionization (SPI) mechanisms. In particular, the combination of desorption-based techniques with VUV single photon ionization can make things easier for quantification, as a single photon is involved then a linear dependence between signal and ionization irradiance is expected (involving only one cross section), in contrast to more complicated (fractional) power laws associated with multiphoton ionization. Moreover, tunable VUV (synchrotronbased) near-threshold SPI in photoelectron-photoion coincidence (PEPICO) configurations can provide a powerful capability to distinguish isomers (through the difference in their photoionization cross section spectral profiles), and can be used for combustion diagnostics in the future.48−50

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AUTHOR INFORMATION

Corresponding Author

*A. Faccinetto. E-mail: [email protected]. Phone: +33 (0)3 20 43 49 85. Fax: +33 (0)3 20 43 69 77. Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors thank Dr. E. Therssen and Dr. I. Ortega for the useful discussions, the Centre d’Etudes et de Recherches Lasers et Applications (CERLA) for the laser desorption facility and the logistic required to run the experiments, and Dr. F. Casier and the Centre Commun de Mesures (CCM) of Dunkerque for the GC−MS analyses. The two laboratories participate to the Institut de Recherche en Environnement Industriel (IRENI), which is financed by the Région Nord Pas-de-Calais, the Communauté Urbaine de Dunkerque, the Ministère de l’Enseignement Supérieur et de la Recherche, the CNRS and the European Funds for Regional Economic Development (FEDER). The CaPPA project (Chemical and Physical Properties of the Atmosphere) is funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement d’Avenir) under contract ANR-11LABX-0005-01 and by the Regional Council Nord-Pas de Calais and the European Funds for Regional Economic Development (FEDER). This work is further supported by J


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