708
Anal. Chem. 1989, 6 1 , 708-714
(9) Janghorbani, M.: Young, Vernon R. selenium in 8rologv and Medicine, Third International Symposium: Van Nostrand Reinhold Co.: New York, 1987; pp 450-471. (10) Janghorbani, Morteza; Ting, Bill T. G.: Young, Vernon R. Am. J. Ciin. Nutr. 1981, 3 4 , 2816-2830. (11) Reamer, Donald C.; Veillon, Claude J. Nutr. 1983, 713, 786-792. (12) Houk, Robert S. Mass Spectrosc. Rev. 1988, 7 , 425. (13) Powell, M. J.: Boomer, D.W.; McVicars, R. J. Anal. Chem. 1988, 58, 2664-2867. (14) Ting, Bill T. G.; Janghorbani, Morteza J. Anal. At. Spectrom. 1988, 3 , 325.
(15) Horlick. Gary: Tan, S. H.; Vaughan. M. A.: Rose, C. A. Specfrmhim. Acta, Part 6 1985, 406, 1555-1572. (16) Ebdon, Les; Sparkes, S. T. M/crochem. J. 1987, 36. 198-206. (17) Longerich, Henry P.: Fryer, 6. J.; Strong, D. F. Specfrochlm. Acta, Part 8 1987, 428, 39-48. (18) Russ, G. P., Bazan. J. M. Spectrochim. Acta, Part 8 1987, 428, 49-62.
RECEIVED for review July- 19,. 1988. Accepted December 19, lg8& This work was in part,-by Rol-CA 38943; DK-26678, and DAMD17-87-C-7235.
Ultrafine Particles as Trace Catchers for Polycyclic Aromatic Hydrocarbons: The Photoelectric Aerosol Sensor as a Tool for in Situ Sorption and Desorption Studies Reinhard Niessner* and Peter Wilbring Department of Chemistry, Inorganic and Analytical Chemistry, University of Dortmund, P.O. Box 5005 00, 0-4600 Dortmund 50, FRG
The adsorption, condensatlon, and desorption behavior of polycycllc aromatlc hydrocarbons (PAHs) on the surface of ultraflne partlcles has been studied. Four different klnds of monodisperse prlmary partlcles-carbon, sodium chlorkle, aluminum oxide, and Aerosll 200-were subjected to PAH adsorptbn under well-defined conditions. The photoelectric a e r d sensor was used as a sendtlve technique for “In Snu” and “on-llne” detection of the degree of surface coverage of the particles by PAHs. The results were controlled by fluorometrlc determination of the PAH amount from aerosol filter samples taken In parallel. The affinity of the PAHs to the partlcle material decreased from carbon to sodium chlorlde to aluminum oxlde and had Its mlnlmum for Aerosil 200. I n the second part of the study the thermal desorptlon of the adsorbed PAHs was investlgated by appHcatlon of a thermodenuder technique. From all partlcle materlals tested the PAHs wlth the largest ring system desorbed at the hlghest temperature. By the study of a single PAH compound on the dlfferent carrler aerosols, It was demonstrated that the temperature necessary for desorptbn decreased in the sequence Aerosll200 > alwnlnum oxlde > NaCl = carbon. I n case of aluminum oxide and Aerosil200 a strong chemisorption of the PAHs Is supposed to be the reason for the differences in the observed desorption temperatures.
INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are one of the most important and ubiquitous environmental pollutants. They are formed by the pyrolysis of carbonaceous materials at high temperatures or during oxidant-deficient combustion processes ( I , 2). Many of them are identified as carcinogens or mutagens (3-5). Within the last years the particulate emissions from diesel engines are recognized as one source of particle-bound PAHs, which contribute to the atmospheric
pollution in specific regions (6-9). During cooling down of the gas/aerosol mixture streaming from the center of combustion to the outlet of the exhaust pipe, the gaseous PAHs are enriched by adsorption and/or condensation on the surface of particles, especially on carbon-containing particles (10). Only PAHs with a low vapor pressure, e.g. PAHs with four or more condensed rings are enriched on particle surfaces at ambient temperatures (11,12). On the other hand the four-, five-, and six-ring PAHs are expected to be the most carcinogenic or mutagenic substances (13,14). They are therefore relevant for human health, because during breathing, the PAH-contaminated particles are transported into the human lung and are deposited there. Insoluble particles like aluminosilicates which are accumulated in the bronchial or alveolar regions possess a long residence time within the lung ranging from several days up to years, as shown by Bailey and Hodgson (15). A t present it is not clear whether such a long clearance time will influence the carcinogenic activity of possibly adsorbed PAHs. Sun et al. (16) compared the deposition, retention, and biological fate of inhaled benzo[a]pyrene (BaP) as a pure aerosol and BaP adsorbed onto ultrafhe particles. Particle adsorption significantly increased the retention of the BaP in the respiratory tract. Experiments with pure BaP aerosol and BaP adsorbed onto diesel particles showed an approximately 200-fold long-term lung retention compared to pure BaP (17). The primary aerosols generated were carbon, sodium chloride, Aerosil200 (spherical SiOz particles), and aluminum oxide. The most important material for these studies was the carbon aerosol because carbon particles are always formed by combustion processes which produce PAHs. Especially the diesel exhaust contains carbon particles (18). Sodium chloride is teated as a model aerosol for nonspherical and ionic particles. Aerosil200 and aluminum oxide aerosols are tentatively used as model aerosols for inert fly ash particles. The work presented here contains the results of PAH-adsorption and -desorption studies of well-defined PAH-coated
0003-2700/89/0361-0708$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
Photoelectric aerosol sensor .........
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test aerosols. The results obtained by application of the photoelectric aerosol sensor are compared to reference methods as fluorometry or synchronous fluorometry. It will be shown that under certain conditions the photoelectric aerosol sensor produces an electric signal that is proportional to the surface coverage of PAH-coated particles or the surface-enriched PAH mass, respectively. It was the aim of this study to compare sorption properties of different particle core materials toward different PAHs as well as to elucidate a way of separation of the PAHs adsorbed on ultrafine particles by a thermal stripping procedure. In both cases the PAS served as an in situ and on-line measurement technique (19, 20).
EXPERIMENTAL SECTION Reagents. The PAHs investigated were selected from their appearance in diesel exhaust aerosol (16,17).All substances used were of analytical grade and purchased from W. Schmidt (Ahrensburg, FRG) or Fluka AG (Neu-Ulm, FRG). Aerosil200 was delivered by Degussa (Frankfurt, FRG). Sodium chloride, aluminum chloride, and n-heptane (all of analytical grade) were purchased from Merck (Darmstadt, FRG). Generation and Coating of Monodisperse Particles. In order to obtain monodisperse particles and to detect them, an electrostatic classifier (Thermo Systems Instruments (TSI), Inc., St. Paul, MN, Model 3071) and an aerosol electrometer (TSI, Model 3068) were used. To produce the “trace catcher” particles, different methods were applied. The carbon aerosol was obtained from a sparking generator (Palas GmbH, Karlsruhe, FRG, Model GFG 1OOO) (19,21). The electrodes of the generator consisted of Spectrapure carbon. The particle size distribution and number concentration were varied by adjusting the spark frequency and the argon flow rate (Q= 120 L/h) across the electrodes. Sodium chloride aerosol was produced by an evaporation/condensation technique resulting in high number concentrations (>los cm”) of ultrafhe particles (22). Aerosil200 aerosol was obtained by atomizing an aqueous suspension (0.5 mg/mL, Q = 180 L/h) of the commercially available substance and subsequent drying of the water droplets by a diffusion dryer. Aluminum oxide aerosol was produced by
nebulizing an aqueous aluminum chloride solution (0.5 mg/mL, Q = 180 L/h), drying of the spray aerosol by a diffusion dryer, and subsequent decomposition of the particles at elevated temperatures (2’ = 998 K). Through this process the aluminum chloride aerosol was converted to a aluminum oxide aerosol. The liberated HC1 gas was removed by a sodium hydroxide coated denuder (23). By this technique amorphous and partially nonspherical aluminum oxide particles, contaminated with traces of residual chloride, were produced. From the different polydisperse aerosols monodisperse fractions were filtered out by electrostatic classification (part I in Figure 1). The required particle size and the monodispersity of the particles were verified by diffusion battery measurements (19, 24). The number concentrations of the monodisperse aerosols were controlled by an aerosol electrometer (25). The monodisperse aerosols were coated with PAHs by a condensation technique, earlier described by Niessner (26). The experimental setup for generation and characterization of the coated particles is shown in Figure 1 (parts 1-111). The PAH vapor necessary for coating the aerosol particles was taken up from the heated PAH by a small stream of nitrogen passing over its surface without whirling up the crystalline PAH. The PAH vapor was mixed with the aerosol in a ring-gap mixing nozzle. In the following temperature-controlled section vapor supersaturation and heterogeneous adsorbate formation or condensation on the particles took place. To obtain different PAH supersaturations only the oil bath temperature had to be varied. Measurement of the Particle Size. An important parameter for the interpretation of the findings was the thickness of the coating layer. Particle growth, due to adsorption or condensation of PAH vapor, was measured by a ten-stage screen diffusion battery (24)which had been calibrated with monodisperse carbon aerosol (part I11 in Figure 1). The determination of the increase in particle size due to the coating was accomplished by measuring the penetration of the monodisperse ultrafine particles through the diffusion battery before and after the coating procedure (19, 27). The increase in size or the layer thickness, respectively, could be determined very precisely, because particles below 20 nm in diameter exhibit a strong diffusion coefficient variation already for a small shift in particle size, resulting in an excellent size
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resolution. Therefore it was possible to detect an increase of 0.5 nm in diameter for monodisperse particles with a diameter of 20 nm. This size resolution allowed the measurement of one adsorbed monolayer of, e.g., anthracene on the particle surface, resulting in an increase in particle diameter of 0.8 nm (28). The Photoelectric Aerosol Sensor. A valuable method to detect the PAH-coated particles is the photoelectric aerosol sensor (part V in Figure 1). The sensor system consists of a @lo. source (TSI Model 3077),an electrostatic precipitator,an irradiation cell, and an aerosol electrometer (TSI Model 3068). The irradiation cell consists of an elliptically grooved aluminum block with an inserted UV lamp (Hamamatsu Co., Type L 937-002; 6.7 eV) aligned in one focal axis and an optical transparent quartz tube aligned in the other focal axis of the ellipse. In order to measure the amount of photoelectric charge as a substance relevant signal, precharged particles have to be neutralized. To achieve this, the aerosol was led through a neutralizer containing a @Krsource (3.7 X lo8 Bq). As the particle diameter of the particles used was smaller than 50 nm, most of the particles were uncharged (29). Remaining charged particles were removed in the following electrostatic precipitator. In the subsequent UV irradiation cell, the PAH-coated particles were charged due t o the emission of photoelectrons from the adsorbed PAH after irradiation with UV light at 6.7 eV. The charged particles were continuously detected and recorded by application of an aerosol electrometer. As a result of the short irradiation time (ti, = 65 ms) and the low light intensity, not more than one photoelectron per particle could be emitted during UV irradiation. On comparison of the number of charged particles N+ after the irradiation cell with the total incoming aerosol number concentration N , the photoelectric activity ( N + / N )has been calculated. The charging rate of particles under illumination is a function of the following parameters (30, 31): cW+/dt = ~ ~ F Prrp2, A HYPAH(~Y), N , IIJV, tirrl where N+ is the number concentration of the charged particles, N is the number concentration of all particles, hv is the energy of the UV photons, Ypmis the material and wavelength-dependent photoelectric yield, Zw is the photon flux, F p m is the fraction of coverage of photoemitting PAH on one particle surface, rrp2is the irradiated particle cross section, t is time, ti, is irradiation time, and rp is particle radius. Under the applied experimental conditions the following parameters were kept constant Zw, hv, tat N , and rrp2 and therefore the observed fraction of charged particles is directly proportional to the PAH-coated area of the particles (FPm rrp2).The photoelectric activity was measured as a function of the PAH vapor pressure and the particle size. The photoelectric aerosol sensor detected only the particulate phase of the PAH aerosol. Gaseous PAH molecules did not emit photoelectrons under the applied conditions. Thermal Desorption of the Particle Surface Enriched PAHs. The experimental setup for these experiments is presented in part IV of Figure 1. The polydisperse aerosols generated by different techniques were electrostatically classified. For the investigations only particles with an electrical mobility diameter of dp = 19 nm were used. The monodisperse particles were coated with one single PAH compound under well-defined conditions. The particle growth was controlled again by diffusion battery measurements. The PAH-covered particles were passed through a thermodenuder (Q = 180 L/h), consisting of a heated quartz tube and a subsequently mounted, unheated charcoal denuder. Temperature measurement in the denuder arrangement was accomplished by a thermocouple. The walls of the unheated denuder were covered by a charcoal filtersheet (Schleicher & Schull, No. 3375, FRG) acting as a sink for the PAH molecules. The thermally stable particles passed the denuder with only negligible loss due to diffusion deposition on the tube walls. Under the influence of the gradually increased temperature within the quartz tube, the PAH adsorbates were stripped off the PAHcoated particles. Therefore the PAH surface coverage was diminished and hence the photoelectric activity of the aerosol. Fluorometric Determination of the PAH Content of the Aerosols. The results of the photoemission measurements were correlated with the results of a fluorometric determination of the PAH amount on the particles. The particulate part of the aerosol was collected by means of a backup glass fiber filter (Whatman
[fAl' dpCorbmE19nm
I
I
i 23 Snn! c
Benzo(e)pyrene
200
400
600
Vapour pressure Figure 2.
800
'
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Iatml
Photoelectric signal vs vapor pressure for some PAHs on
carbon. GF/C). In order to remove the gas-phase PAH, which might have been adsorbed on the surface of the fiiter, the aerosol was passed through a charcoal-denuder in front of the filter holder. After sampling, the glass fiber filters were extracted with n-heptane under ultrasonic agitation (t = 30 min). The extracts were centrifuged and analyzed by fluorometry (32) or synchronous fluorometry (33). The measurements were carried out by a spectrofluorometer (Model RF-540) by Shimadzu (Shimadzu Co., Kyoto, Japan). The extraction yield was checked for trace amounts of PAH and various amounts of polydisperse carbon particles. Thus, a nearly quantitative extraction is expected.
RESULTS AND DISCUSSION Adsorption Studies. A monodisperse aerosol was coated with a single PAH at constant number concentration (N = 50000-60000 cm-9 and known particle size (dp = 19 nm).The photoelectric signal produced by the coated aerosol was measured. Some difficulties emerged in generating of coated particles with the three- and four-ring PAHs. The vapor pressure of these PAHs is so high that formations of submonolayers, which could be detected by aerosol photoemission, were often not stable within the observation time (t < 1 s). Only the four-ring PAHs chrysene and benz[a]anthracene were detectable on particles as a submonolayer. Similar PAHs such as anthracene, fluoranthene, pyrene, and triphenylene could be obtained only as condensates on the particles. The photoelectric activities of these multilayer particles were very small. In contrast to these findings most of the five-, six-, and seven-ring PAHs were easily detected as submonolayers on the different primary aerosol particles. In Figure 2 the photoelectric activities of coated carbon particles are presented as a function of the PAH vapor pressure. The vapor pressures were calculated fom published data (34-37). The results demonstrate that the photoelectric yields of the various PAHs are different. The photoelectric yield depends on the electronic and geometric structure of these molecules, i.e. the delocalized r-electron system and the planarity of the molecule. Additionally the interactions between the particle surface and the PAH molecule become evident. That means that for a large planar PAH molecule with a large r-electron system a high photoelectric yield has to be expected theoretically and has been observed in the experiment, too. These correlations demonstrate the fact that a linear relationship between the vapor pressure and the photoelectric signal was observed a t low vapor pressures. This linear correlation exists only as long as the diameter of the coated particles remained unchanged. Since the photoelectric signal ( N + / N )is also a function of irradiated particle surface, the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989 PE- Signal
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Sodium chloride Aluminium oxide
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Corbon
Benzo(b1fluoronthene
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101
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Figure 4. Photoelectric signal as function of the fluorometrically determined amount of PAH.
10 .10 -"
a x-x interaction between x-electron density of the PAH molecule and the graphitic domains of a carbon particle are responsible for the sorptive interaction. [ f A 1 / b Aerosols ' PA H In Figure 3a this typical behavior is illustrated for benzoSodium chloride Benzolbifluoronthene [ blfluoranthene on different "trace catcher" aerosols. It can Aluminium oxide be explained by different factors: The first possibility is that 60 70 AerosilRZOO the PAH coverages on the particle surfaces are really different for different materials. This means that under the same 50 Carbon conditions different amounts of PAHs were adsorbed on the I o various trace catcher particles. It is not surprising that the greatest photoelectric signal has been observed when a carbon particle was used as a sink. The similarity between the extended aromatic system of the polymeric parts of the carbon particles and the structure of the PAH molecule may explain this behavior (see also ref 40). Further, it seems to be possible /that the shape of the adsorbed-originally planar-PAH ",&$LkcL-. -, molecule is distorted by interaction of the ?r-electron cloud 50 100 150 200 of the PAH molecule and a cation (41) or the aromatic x Concentration of adsorbed PAH lnglm31 system of graphite. This modification of the molecular Figure 3. Photoelectrlc signal vs vapor pressure and vs concentration structure could result in a decrease of the photoelectric of adsorbed PAH for benzo[b]fluoranthene on different carrier aeroquantum yield. sols. Figure 3b shows the photoelectric signal as a function of observed deviation from linearity (see coronene or BaP in the adsorbed benzo[blfluoranthene mass (derived from PAH Figure 2) is due to the increased particle diameter. This fact extraction) on the different particles surfaces. Identical PAH has been checked by independent size measurements through amounts on the different particle materials gave different application of the diffusion battery technique. The size of photoelectric signals. Especially for Aerosil 200 very low photoelectric signals were observed. This may be explained the grown particles has been indicated in parenthesis in Figure 2. The linear correlation between photoelectric signal and by the smooth silica particle surface, only allowing the advapor pressure can be interpreted as the formation of subsorption of a few PAH molecules and hence resulting in a monolayers which is known to occur always at the beginning smaller PAH coverage of the particle itself. We believe that further adsorption of PAH molecules will only take place on of an adsorption process. Most of the known adsorption top of already sticking PAH molecules resulting in a growth isotherms show a linearity of surface coverage at low vapor of particle size but still at a low PAH coverage. pressures if the surface coverage is plotted against vapor pressure (38). From measurements of monodisperse pure If the surface of the particle is very porous, the PAH can PAH particles (FPAH be accumulated in pores and holes inside the particle. In this = 1)it is known (19) that the photoecase the photoelectric aerosol sensor shall not detect the PAH; lectric signal directly reflects the PAH surface, in this case the irradiated particle cross section. therefore a decrease of the photoelectric signal has to be expected. This effect obviously plays a role in systems like Different vapor pressures were necessary to obtain the same photoelectric activity if the same PAH was enriched on prithe aluminum oxide particles. mary particles consisting of different materials. The necessary The results of the photoelectric aerosol sensor were tested PAH vapor pressure increased in the order carbon < NaCl independently with well-known analytical methods. Figure < A1203< Aerosil200. Similar findings have been reported 4 shows the photoelectric signal as a function of the particulate by Griest and Tomkins (39) who have analyzed coal comPAH mass concentrations (derived from the PAH extraction). The linear correlation between the PAH amount and the bustion stack ash. The strong affinity of PAHs to particles may be influenced photoelectric signal was only observed as long as the particles by physical surface properties, too. Since we have not been were covered with submonolayers. able to sample the monodispersed ultrafine aerosols in order In Figures 2-4 it is demonstrated that identical amounts to accomplish a surface area measurement, we have measured of different PAHs yield different photoelectric signals. The the specific surface area (nitrogen adsorption) for the polyphotoelectric yield correlates well with the molecular structure of the PAH molecule. For instance benzo[b]fluoranthene disperse primary aerosol in the case of carbon and Aerosil200. The results were 395 m2/g for carbon and 205 m2/g for Aerosil (B(b)F) and benzo[k]fluoranthene (B(k)F) have the same 200. The value for carbon indicates that a certain inner molecular weight and a similar molecular structure. The porosity exists. Griest and Caton (40)have hypothesized that condensed aromatic three-ring system of the B(b)F molecule PE-Signal
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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Table I. Desorption Temperatures Tw of PAHs Adsorbed on Carbon Aerosols (dD= 19 nm)
i
c 5
3
Group A chrysene pyrene triphenylene benz[a]anthracene benzo[blfluoranthene benzoLjlfluoranthene benzo[k]fluoranthene Group B benzo[a]pyrene benzo[e]pyrene perylene 6-nitrobenzo[a]pyrene
4-ring PAHs
0,
nonplanar 5-ring PAHs
50
100
150
200
planar 5-ring PAHs
I'Cl
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temp (Td, "C
substance
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desorption
Benzolg,h,iiperylene Anthanthrene Coroneie
65 65
60 60 66 73
62 105 92 90 138
125
PAHs with same MG
Group C anthanthrene benzo[ghi]perylene dibenz[a,h]anthracene
7-rine PAHs
Group D coronene
195
6-ring PAHs
120
126
60
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80
Temperature
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Benzlaianthracene Benzo(k1fluoranthene Benzo1b)fluoranthene Benzoijifluopanthene . I n d e i o i l 2 3 - c dipyrene . 6-Nitro-Senzoioipyrene -
60 . \
Temperature Figure 5. Photoelectric signal from carbon particles coated with different PAHs as function of the desorption temperature.
seems to be responsible for the comparatively high photoelectric yield, while the two separated condensed aromatic two-ring systems of the B(k)F molecule cause a very low photoelectric signal. Thermal Desorption of the Adsorbed PAHs. In the thermal desorption studies submonolayer-coated monodisperse particles were used again. The Figure 5 presents the desorption curves for PAHs on carbon particles. During the experiments particle size and number concentration were always kept constant. The desorption studies include only those PAHs that were well detectable by the photoelectric aerosol sensor under submonolayer conditions. The PAHs of this group are the seven-, six-, and five-ringPAHs and some of the four-ring PAHs. As can be seen from Figure 5 the desorption curves of some PAHs are very similar.
1 - ring ?AH5 ionplanar
ii
u
uu
6 - ring PAHs
di 7
-
ring PAH
I
20 100 150 200 I'Cl Temperature Flgure 6. Comparison of the PAH desorption temperatures from monodisperse carbon particles (see Table I). 50
Figure 6 and Table I summarize the results that have been shown in Figure 5. PAHs with similar desorption temperature for 90% thermal desorption (determined by the photoelectric signal) can be arranged as follows: Group A consists of the four-ring PAHs and the nonplanar five-ring PAHs investigated here. The vapor pressures of the four-ring PAHs are so high, that thermal desorption properties of these PAHs appear very similar. The investigated nonplanar five-ring PAHs were the benzofluoranthenes. These PAHs have the lowest interaction with particle surfaces, because of their curved ring system. Consequently their desorption temperatures are rather low. Group B, where distinct higher desorption temperatures were found includes the five-ring PAHs with planar ring systems. Here optimal interactions are possible between the PAH and the planar graphitic parts of the particle surface. In group C the desorption temperatures for the six-ring PAHs anthanthrene and benzo[ghi]perylene were essentially higher. An exception is dibenz[gh]anthracene, a five-ring PAH, which belongs to group C. The obvious reason is that dibenzkhlanthracene has the same molecular weight as the six-ring PAHs. Thus the molecular weight next to the molecular structure plays an important role in the desorption temperature. Another example for this factor is 6-nitro-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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I
Carbon NaCl
100
Chrysene Perylene Benzoieipyrene Benzolalpyrene 0 Coronene Benzolg,h,ilperyiene x
100
80
80
60
60
40
40
20
20
50
100
150
200
I
Temperature Flgure 7. Photoelectricsignal from benzo[a Ipyrene coated particles as a function of the vapor pressure.
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0
80 . 60 .
40 20 .
Temperature
50
100
150
200
Flgure 9. Photoelectric signal from Aerosol 200 particles coated with different PAHs as a function of the desorption temperature.
I'CI
b
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I
rn
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Benzblonthracene Benzoiklfluoranthew Ben zo(bif Iuo ranthene Benzoijifluorontnene Indenoll 2 3-c,dlpyrene-
-
-
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t
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100
1
.\.-
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150
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Temperature Flgure 8. Photoelectric signal from aluminum oxide particles coated with different PAHs as a function of the desorption temperature.
benzo[a]pyrene. The desorption temperatures were higher than those of the six-ring PAHs. This may be explained by the high molecular weight of the nitro group in the molecule. Furthermore this group is in an equatorial position to the molecule and the ?r electron of the nitro group is completely in resonance with the aromatic ?r system. Therefore it does not hinder a complete adsorption onto the surface. The highest desorption temperatures were measured for the planar seven-ring PAH coronene (group D). Comparing the desorption temperatures of one PAH on different primary particles the following ranking was observed Aerosil200 > aluminum oxide > carbon > sodium chloride.
In Figure 7 this is demonstrated for benzo[a]pyrene. Relative high desorption temperatures were also observed for PAHs adsorbed on aluminum oxide particles. This is demonstrated in parts a and b of Figure 8. Surprisingly, with increasing temperatures we observe in the beginning an increasing photoelectric signal. This is particularly the case when perylene, coronene, and indeno[l,2,3-cd]pyrene are thermally desorbed. This fact may be explained by the morphology of the aluminum oxide particles. First of all they have an amorphous structure, and second, from the production process the particles are not smooth and spherically shaped. With increasing temperatures the PAH diffuses from holes or rents onto the surface of the particle, thus increasing the coverage and the photoelectric signal, too. The height of the desorption temperatures demonstrates that chemisorption of the PAHs happens on the aluminum oxide surface, thus obstructing an desorption step at lower temperatures. The same chemisorption process was observed in desorption investigations for the Aerosil 200 surface. High PAH supersaturations were n e c e s s q to produce a detectable amount of PAHs adsorbed on this surface. This was detected by aerosol photoemission and also by fluorometric studies. In contrast to these results the highest desorption temperatures, presented in Figure 9, have been observed in the m e of Aerosil 200. The silica particles are spherical and nonporous, so only a strong chemisorption between SiOH groups and PAH molecules will explain these results. Similar findings were recently reported by Saucey et al. (42). The relatively low desorption temperatures for PAHs adsorbed on sodium chloride as primary aerosol reflect the minor interaction between a PAH molecule and an ionic cristalline surface. The desorption curves are shown in the Figure 10.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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[ O/O
a
1
dpNac, = 15 nm x
Perjlene Chrjsene Benzo(e)pyrene
100
80
Registry No. Chrysene, 218-01-9; pyrene, 129-00-0; triphenylene, 217-59-4; benz[a]anthracene, 56-55-3; benzo[b]fluoranthene, 205-99-2; benzoVlfluoranthene, 205-82-3; benzo[klfluoranthene, 207-08-9; benzo[a]pyrene, 50-32-8; benzo[e]pyrene, 192-97-2; perylene, 198-55-0; 6-nitrobenzo[a]ppene, 63041-90-7;anthanthrene, 191-26-4;benzo[ghi]perylene, 191-24-2; dibenz[a,h]anthracene, 53-70-3; coronene, 191-07-1; sodium chloride, 7647-14-5; aluminum oxide, 1344-28-1; Aerosil 200, 7631-86-9;carbon, 7440-44-0;indeno[ 1,2,3-c,d]pyrene,193-39-5.
60
LITERATURE CITED
41)
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20
Temperature
PE- Sianal Benzlalanthracene * B e n z o l blfluoranthene Q BenZO(j)flUOranthene Indenol?,2.3-c.d)pyrene
0
-
100 80
60
LO 20
[
I,'
.
100 150 200 ['Cl Temperature Figure 10. Photoelectric signal from sodium chloride particles coated with different PAHs as a function of the desorption temperature. 50
CONCLUSIONS We report for the first time on the use of monodisperse ultrafine aerosols as "trace catchers" for PAH molecules. Especially carbon aerosols act as an effective sink. As an "in situ" and "on-line" monitor we have introduced the photoelectric aerosol sensor. The sensor allows a continuous "in situ" monitoring in the nanograms per cubic meter range of four and more ring PAHs, in the case of a submonolayer formation. Very important is the finding that same amounts of PAH being adsorbed as submonolayers onto different carrier particles offer a different photoelectric yield. Also we observed a different photoelectric yield for different PAHs each adsorbed onto the same carrier particle system. The consequence of this finding is that always the best photoemitting PAH will govern the signal formation. Experiments with mixed PAHs adsorbed onto one particle are in preparation. In connection with a thermodenuder, studies of interactions between the particle surfaces and the adsorbed PAHs were performed. The results indicate the possibility of thermal differentiation between strong and weak adsorbed PAHs. Especially aluminum oxide and silica interact in a yet unknown way with PAHs. From studies on PAH adsorption on laponite particles three possible binding functions are known: silanol groups, siloxane groups, and cationic counterions on the surface (42). Further research is clearly necessary to understand these adsorbate systems.
RECEIVED for review July 14, 1988. Accepted December 14, 1988. We gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (Bonn, FRG), DaimlerBenz AG (Stuttgart, FRG), and Gossen GmbH (Erlangen, FRG).
