Anal. Chem. 1990, 62, 2071-2074
(10) Lieberman, S. H.; Inman, S. M.; Stromvall, E. J. Anal. Chlm. Acta 1989, 217, 249-262. (11) Sepanlak, M. J.; Tromberg, B. J.; Alari, J-P.; Bower, J. R.; Hoyt, A. M.; Tuan. V . 4 . In Chemlcal Sensors and Microlnsttumentation; Murray, R. W., et ai., Eds.; ACS Symposium Series 403; American Chem ical Society: Washington, DC; 1989, pp 318-330.
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(12) Berman, R. J.; Burgess, L. W. Chemlcal, Biochemical and €nvironmental flbef Sensors; Lieberman, Robert A., Wkdarczyk, Marek T., Eds.; SPIE: Bellingham, WA, 1990, pp 206-214; No. 1172.
RECEIVED for review May 23,1990. Accepted July 11,1990,
Aerosol Photoemission for Quantification of Polycyclic Aromatic Hydrocarbons in Simple Mixtures Adsorbed on Carbonaceous and Sodium Chloride Aerosols Reinhard Niessner,* Beate Hemmerich, and Peter Wilbring Institute of Hydrochemistry, Technical University of Munich, Marchioninistrasse 17, 0-8000 Munich 70, Federal Republic of Germany
The photoelectrk a e r d sensor was applied as a tool for the In situ and on-ilne detection of surface-enriched polycyclic aromatic hydrocarbons (PAHs). Carbon aerosol and sodlum chloride aerosol were coated stepwlse with up to four dlfferent PAHs or shnultaneously wlth three different PAHs (lnternally mlxed aerosol). The measured photoelectric signal of the internally mlxed aerosol was compared wRh the expected slgnal, whlch was calculated from the previous calibration of the sensor. An additlvlty of the lndlvidual contrlbutlons of the adsorbed PAHs on the sum signal was found. Experknents wlth photoelectrlcallyinactlve paraffin adsorbed on photoelectrically actlve aerosol particles have demonstrated that only the surface composltlon contrlbutes to the photoernisslon signal.
INTRODUCTION During the last years the interest in particulate emissions from combustion processes has increased. Large amounts of particles are emitted by various emission sources because of incomplete combustion (1,2). Many organic compounds are adsorbed on these particles, of which polycyclic aromatic hydrocarbons (PAHs) are of special interest, since potential health effects resulting from acute and chronic human exposure to PAHs are of special concern (3). After incorporation of particulate polycyclic aromatic hydrocarbons during breathing, the PAHs become biologically available and many of them are identified as directly acting carcinogens (4, 5 ) . Most present techniques for analysis of combustion processes consist of time-consuming filter sampling, extraction, and separation methods, which allow only off-line applications (1,2). In order to minimize PAH emissions, it is of considerable interest to use an analytical method with the ability of in situ and on-line detection of particulate PAHs. A technique that partially meets these requirements is the photoelectric charging of ultrafine PAH-coated particles with the photoelectric aerosol sensor (PAS), which was published by Niessner et al. (6-8). From laboratory studies (6, 8-10) it is known that the charging rate reflects the surface coverage of particles with photoemitting material and that the charging rate of the irradiated particles is a function of various parameters such as energy of UV photons, photoelectric yield of surface-adsorbed material, particle radius, etc. Studies with the photoelectric aerosol sensor (PAS) with artificially coated aerosols at one wavelength (185 nm) have 0003-2700/90/0362-2071$02.50/0
shown that the senor signal is linearly correlated to the amount of one single PAH adsorbed on the particle surface ( 8 , I l ) . However in real exhaust systems PAH-coated particles occur not only with one adsorbed PAH on a particle but in a very complicated mixture with a variety of combustion products adsorbed on one particle core. For the employment of the sensor in real combustion systems, it had to be examined whether the photoelectric signal reflects the sum of the single signals of the different types of adsorbed PAHs. Therefore monodisperse carbon and sodium chloride particles were artificially generated and coated by different techniques with various amounts of several PAHs. Additionally it has been examined whether adsorbed paraffims will influence the aerosol photoemission process. EXPERIMENTAL SECTION Reagents. The PAHs under investigation were perylene, anthanthrene, coronene, indeno[ 1,2,3-cd]pyrene, and benzo[a]pyrene. All substances used were of analytical grade and purchased from W. Schmidt (Ahrensburg, FRG) or Fluka AG (Neu-Ulm, FRG). Sodium chloride, toluene, methanol, and tetrahydrofuran (all of analytical grade) were purchased from Merck (Darmstadt, FRG). All work with the mutagenic substances must be carried out in a well-ventilated hood. Generation and Characterization of Monodisperse PAHCoated Particles. The experimental setup for generation and characterization of the coated particles has been described earlier (8, 11).
Internally mixed aerosols with several PAHs adsorbed on the particle surface were generated by conducting monodisperse primary aerosols sequentially through one, two, three or four coating equipments which were filled with the respective PAHs. By that way it was possible to cover the aerosols step by step with up to four PAHs. The coating sequence was chosen such that the PAH with the lowest vapor pressure adsorbed first and the PAH with the highest vapor pressure last. This kind of particle coating required high concentrations and a long-time stability of the aerosol generator because losses in particle number concentration were very high. Moreover it was very difficult to investigate a combination of PAHs at several vapor pressures. For this reason an alternative method was applied: a mixture of up to three PAHs (50 mg each) in a single coating vessel was prepared. Thus the aerosol needed only to pass one coating equipment. By that way up to three PAHs were adsorbed simultaneously on the single particles. Parallel to the sensor measurements the particle number concentration was controlled continuously with a condensation nucleus counter (TSI, Model 3020). The aerosol photoemission measurements were correlated with the amounts of adsorbed PAHs which were analyzed with an independent analytical technique. The particulate part of the 0 1990 American Chemical Society
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aerosol was collected by means of a back-up glass fiber filter (Whatman GF/C; sampling time = 0.5-2 h, Q = 7 L/min). After sampling the glass fiber filters were extracted by ultrasonic agitation ( t = 15 min) with 1mL of toluene and 10 pL of an internal standard (benzo[b]chrysene;15.7 ng/mL) was added. The extracts were centrifuged and 20 pL of the sample was analyzed by means of a high-performance liquid chromatography technique with fluorescence detection (Shimadzu, system controller (Model SCL-GA),two pumps (Model LC-GA),data recorder (Model DR-3), spectrofluorometer (Model RF-540); column, Macherey & Nagel ET 250/8/4 Nucleosil 5 PAH at a column temperature of 310 K; flow rate of the mobile phase, 1.5 mL/min; mobile phases (A) tetrahydrofuran, (B)methanol). To keep the sampling time below 2 h, the filter extracts with very low PAH concentrations had to be concentrated by evaporation in a nitrogen stream at room temperature. Photoelectric Aerosol Charging. The photoelectric aerosol sensor has been described earlier for detection of PAH-coated particles (6,8). The UV lamp is operated at a wavelength of 185 nm 6.7 eV (Hamamatsu Co., Type L 937-002). The laser system consisted of an excimer laser (ArF radiation at 193 nm, Lambda Physik, Gottingen, FRG, Model EMG 201 MSC). The pulsed UV laser beam was direct through the irradiation cell onto an energy meter (Laser Precision Corp., Utica, NY, Model RJ 7100 and RJP 735). The pulse energy was kept at 3 pJ throughout the experiments. The laser pulse width was approximately 20 ns, and therefore the power density (3 X lo4 J/cm2)/2 X lo-* s = 150 W/cm2. With a pulse frequency of 10 Hz, each particle was irradiated not more than once by the laser within the residence time in the intersection volume. The illuminated volume within the cell was also kept constant and the beam area as well by means of variable apertures. RESULTS A N D DISCUSSION For interpretation of the photoelectric signals, it was assumed that the signals of the different PAHs simultaneously adsorbed on a particle were additive. Therefore for each single PAH-coated aerosol a calibration function was set up by adsorbing only one PAH onto the primary aerosol a t various PAH coverages. Then the photoelectric signal was measured and the PAH content was determined. With these data we were able to calculate the photoelectric signal that would be produced by a certain amount of adsorbed PAH. Under these conditions an expected signal SE = CSi could be calculated from a single signal Si. If the measured signal S was a sum of the single signals, then the measured and the predicted photoelectric signal would be identical. In order to compare the results of the different experiments, all data were normalized to a number concentration of 100000 ~ m - ~ . Internally Mixed PAH-Coated NaCl Aerosols. First two PAHs (coronene (at TI) and perylene (at T z )with T z < T I )were adsorbed successively on sodium chloride particles. In this case the calculated signal SEcorresponded with the measured photoelectric signal S within the standard deviation. With stepwise adsorption of three PAHs on NaCl particles (coronene, anthanthrene, perylene) the differences between the measured and predicted signals increased. Contrary to that a good correlation between the measured and predicted photoelectric signals was found (Figure 1)when a mixture of three PAHs (see Table I) was adsorbed by a one-step procedure onto the trace catcher aerosol (here NaCl). Through this arrangement the reproducibility of the measurements could be clearly enhanced. Moreover the adjustment of different coating temperatures (that means different combinations of PAH vapor pressures) was much easier in this configuration. No correspondence between the measured and the expected signal was found when four PAHs were adsorbed stepwise. The resulting signal was as high as the single signal of the last adsorbed PAH (here BaP). During this coating process presumably three-dimensional crystals were formed on the particle surface, which resulted in a sandwich formation of
Photoelectrlc Slgnal [fAl
8000
1
6000
1 3 PAHs on NaCl
-
a rnsaaurrd slpnal cnIculatBd slpnal
4000
-
3000
-
2000
-
10000-
303
383
378
373
368
383
Adsorption Temperature [K]
Figure 1. Comparison between the calculated and measured photoelectric signals of a three-component PAH mixture (anthanthrene, perylene, benzo[a ] pyrene; simultaneously adsorbed onto NaCI) at six different vapor pressures of all components on NaCl carrier particles: A = 185 nm; No lo5 ~ m - d~, ;= 50 nm.
-
Table I. PAH Concentrations Determined in the Internal Mixture of Anthanthrene, Perylene, and Benzo[a Ipyrene on NaCl Aerosol as a Function of PAH Source Temperature source temp, K
393 383 378 373
368 363
anthanthrene 31
13 8 2
n.dSa n.d.
concn, ng/m3 perylene benzo[a]pyrene 667 179 162 85 68 69
4720 1717
1198 670 527 503
"n.d. = not detected. the PAH adsorption layers, so that the signal is determined by the upper PAH layer (here BaP). A particle growth was not observed with a tandem differential mobility analyzer arrangement (12). Internally Mixed PAH-Coated Carbon Aerosols. If a PAH adsorption onto the carbon aerosol occurs, then the photoelectric signal of this trace catcher aerosol can either be increased or diminished. Either the photoelectric signal of the carbon aerosol will be enhanced due to the available photoelectrically active surface of the adsorbed PAH or it will be decreased if a PAH is adsorbed which is less photoelectrically active than the carbon particles. Under these experimental conditions (No= lo5~ m -d,~ = ; 50 nm; constant photon flux of the UV lamp (185 nm) and aerosol flow rate of Q = 7 L/min through the irradiation cell) the basic photoelectric signal of the carbon aerosol was about 2000 fA. Traces of adsorbed PAH ( < l o ng of perylene/m3) produced only very low photoelectric signals so that a relatively small signal had to be measured on a relatively high background. For that reason highly reproducible measurements were necessary hence all PAHs were deposited simultaneously onto the trace catcher aerosol. The comparison between the predicted and the actually measured signals for a mixture of three simultaneously adsorbed PAHs (see Table 11) a t six different vapor pressure combinations is depicted in Figure 2. It is obvious that the expected and measured signals are only comparable at low and medium vapor pressures. If high amounts of gaseous PAH were available in the coating unit, the measured photoelectric signal was distinctly smaller than was expected from the calibration functions of the experiments with single PAHcoatings. In nearly all investigated series with internal PAH
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990 ~~
Photoelectric Signal [fAl
~
Table 11. PAH Concentrations Determined in the Internal Mixture of Anthanthrene, Perylene, and Benzo[a Ipyrene on Carbon Aerosol as a Function of PAH Source Temperature source temp, K
anthanthrene
383 378 373 368 363 358
concn,. n-,d m 3 perylene benzo[a]pyrene
34 19
602 343 192 113 54 32
9
4 1
0.5
3755 2287 1331 832 417 276
6000
= I::::::::I 150
100
60 n
/
5000 calculated
4000
303
120
2000
1000
Photoelectric Signal
378
373 388 383 Adiorption Temperature 1K1
358
373 388 363 Adeorption Temperatur 1K1
358
lfAl
looh 80
0
378
Figure 3. Comparison between the calculated and measured photoelectric signal for an internal PAH mixture of three PAHs (anthanthrene, perylene, benzo[ a ] pyrene; simultaneously adsorbed on carbon aerosol): = 193 nm; N o = lo5 ~ m - d~, ;= 50 nm.
3000
383
Anthanthrene Benm(a)pyrene
200
-
Photoelectrlc Slnnal lfAl
2073
Carbon dp
-
30 nm - * -Trlacontane on C
-fTrlacontane on C+er
*
Tetracoaane on NnCI
-S-
Tetraconane on C
++
Tetraconme on CIPer
80
Figure 2. Comparison between the calculated and measured photoelectric signals of a three-component PAH mixture (anthanthrene, perylene, benzo[a ] pyrene; simultaneously adsorbed onto carbon particles) at six different vapor pressures of all components on carbon carrier particles: h = 185 nm; N o lo6 ~ m - d~, ;= 50 nm.
-
mixtures and carbon as carrier aerosol at low vapours pressures the measured signal was higher than the expected one and vice versa at high vapor pressures. Hence one may infer that different adsorption places for PAHs exist, further it cannot be excluded that the PAH molecules were adsorbed on top of existing PAH-layers and not onto free adsorption sites of the carrier particle. Yet no particle growth or any other change in particle diameter could be noticed. In addition solid-state NMR studies with deuterated PAHs on the same carbon particle surfaces revealed that the PAH molecules prefer to stay side by side on the surface as long as monolayers exist (13). These findings were quite different from studies of the residence probability of PAHs on submicroscopic Aerosil-200 surfaces, where a three-dimensional PAH "island" formation was observed. According to this the PAH molecules prefer to stay near other PAHs on this kind of surface (13). Similar conditions may be expected for high vapor pressures and low number concentrations of any trace catcher aerosol (like here in the case of carbon aerosol with Tcoating < 363 K). In Figure 3 the comparison between the measured and the calculated photoelectric signals is shown for three PAHs adsorbed simultaneously on carbon aerosol and illuminated with laser light at a wavelength of 193 nm. The PAH concentrations were the same as given in Table 11. The measured and predicted values were in good agreement. Aerosol Photoemission of Paraffin-Coated Particles. As mentioned above pure PAH-coated particles are not the only species in real exhaust systems. There are also a variety of other combustion products like paraffins. During cooling down they are adsorbed on the particle surfaces similar to the adsorption process of the PAH molecules. Therefore it had to be examined whether adsorbed paraffins would influence
M
0 0
YI
10
20 30 40 50 Thickness of the paraffin layer
60
70
80
lnml
Figure 4. Photoelectric signal of triacontane-coated carbon particles, perylene- and triacontane-coated carbon particles, tetracosane-coated sodium chloride particles, tetracosane-coated carbon particles, and perylene- and tetracosane-coated carbon particles: N o = lo5 ~ m - ~ ; dp(carbon) = 30 nm; dNMc,,= 30 nm. the response of the aerosol photoemission process, e.g. by variation of the attenuation length of the photoelectrons (14). Carbon particles with a diameter of d, = 30 nm were coated with tetracosane (C,,H,) and triacontane (C,H,) analogous to the coating process of the PAHs. The photoelectric signal, which was measured as a function of the coating thickness, which was controlled with a diffusion battery (15),decreased with increasing paraffin coating. A 60 nm thick tetracosane coating (Figure 4)and 30 nm thick triacontane coating (Figure 4) erased the photoelectric signal of the primary particles completely. The same effect was observed when submonolayers of perylene were adsorbed onto the carbon particles and subsequently coated with paraffin. These experiments depict that the paraffin coating on the primary particles erased the photoelectric signal of the carbon particles itself and also the one of the adsorbed PAHs. Furthermore a nonphotoemitting sodium chloride aerosol was coated with tetracosane and, independent of the coating thickness, only a negligible photoelectric signal was noticed (Figure 4). The observed reduction of the photoelectric signal of photoemitting particle surfaces is probably due to the coating of the photoelectrically active parts of the carbon surface with tetracosane or triacontane. This will result in an increase of the work function of the paraffin-coated particles. This effect
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Anal. Chem. 1990, 62,2074-2079
can be obtained only with less-volatile paraffins, since highvolatile paraffins will vaporize from ultrafine particles within milliseconds due to the Kelvin effect (16). A solution to this problem may be the application of a heated (ca. 323 K) thermodenuder in front of the photoelectric aerosol sensor, which will remove not only high-volatile paraffins but also other interfering products (8). CONCLUSIONS The presented investigations have shown that an additivity of the photoelectric signals is given only for small PAH coverages on particles. In real exhaust situations similar PAH concentrations exist while the particle concentrations are substantially larger at least by a factor 1000; hence only low PAH surface coverages will result from real combustion processes except for residential wood combustion. Under these circumstances the photoelectric signal is likely to represent the sum of the single signals of all adsorbed PAHs. Analyzing cigarette smoke aerosols with the photoelectric aerosol sensor (A = 185 nm) and parallel wet-chemical techniques have confirmed the strong correlation between photoelectric signal and adsorbed PAH amounts (17). Also the applicability of this sensor as on-line and in situ detector for the measurement of PAHs in exhaust gases from waste combustion has been tested (18). Experiments with paraffin coatings have confirmed that aerosol photoemission only probes the surface composition of an aerosol system.
LITERATURE CITED Levsen, K. Fresenius' J. Anal. Chem. 1988,333,467-478. Partrklge, P. A.; Shah, F. J.; Cernansky, N. P.; Suffet, I . H. Envhon. Sci. Techno/. 1987,27,403-408. Davis, C. S.; Fellin, P.; Otson, R. J. Air Pollut. Control Assoc. 1987, 3 7 , 1397-1408. Szentpaly, L. V. J. Am. Chem. Soc. 1984, 706, 6021-6028. Tiwary, R. K.; Singh, T. P. N.; Gosh, S. K. Indian J. Environ. Rot. 1985,5 , 209-213. Niessner, R. J. Aerosol Sci. 1086, 17, 705-714. Niessner, R. Fresenius' J. And. Chem. 1988,329,406-409. Niessner, R.; Wilbring. P. Anal. Chem. lS89, 67, 708-714. Masuda, S.; Mizuono, A.; Tanaka, S. R o c . Symp. Aerosol Sci. Technol., 7st 1983, 35-37. Burtscher, H.; Schmidt-Ott, A. Sci. Total Environ. 1984, 365, 233-238. Niessner, R.; Robers, W.; Wilbring, P. Anal. Chem. 1989, 67, 320-325. Liu, B. Y. H.; Pui. D. Y. H.; Whitby, K. H.; Kittelson, D. E.; Kousaka, Y.; McKenzie, R. L. Atmos. Environ. 1978, 72, 99-104. Neue, G.; Niessner, R. J. C o l M Interface Sci., in press. b i n , C.; Whitesides, G. Anal. Chem. 1989,67, 1673-1679. Brown, K.; Gentry, J. Sci. TotalEnvkon. 1984. 36,225-232. Rader, D. J.; McMuny, P. H.; Smith, S. J. Aerosol Sci. 1987, 6 , 247-260. Niessner, R.;Walendzik, G. Fresenius' J. Anal. Chem. 1989, 333, 129-133. Zajc, A.; Uhlig, E.; Hackfort, H.; Niessner, R. J . Aerosol Sci. 1989, 20, 1465-1468.
RECEIVED for review February
26, 1990. Accepted June 2, 1990. We gratefully acknowledge financial support of Daimler-Benz AG (Stuttgart, FRG), Gossen GmbH (Erlangen, FRG), and the Deutsche Forschungsgemeinschaft.
Quantitative Analysis of Solids in Motion by Transient Infrared Emission Spectroscopy Using Hot-Gas Jet Excitation Roger W. Jones* and John F. McClelland
Center for Advanced Technology Development, Iowa State University, Ames, Iowa 50011
Quantitative compositlonal analysis of optically tMck SOW In motlon Is demonstrated by urbrg trandent Infrared emlsslon spectroocopy (TIRES). TIRES greatiy reduces the self-absorptkn that nonndly degrades comrenUona1emlsskn spectra so that they dosdy resembb Msckbody spectra. Quantltathre composltlonal analyses of poly[( methyl methacrylate)-co(butyl methacrylate)] and poly[ethylenaco-(vlnyl acetate)] with standard errors of predlctlon under 1% were achleved with only a few seconds of data acqulsition uslng prlnclpal compomtt regresgkn. Use of a hotgas jet in place of a laser In the TIRES technique allows study of materials that do not absorb strongly at common laser wavelengths whlle reducing cost and complexlty.
INTRODUCTION Quantitative infrared spectroscopy of solids is limited by the high optical density of most solid samples. Such samples produce highly saturated transmission spectra because of their opacity and produce blackbody-like emission spectra because of self-absorption (1). Conventionally, dilution and physical thinning of samples are used to reduce optical density, but they preclude real-time analysis, destroy the physical structure of samples, are labor intensive, and are not always possible. 0003-2700/90/0362-2074$02.50/0
Recently a new technique called transient infrared emission spectroscopy (TIRES) was introduced that can analyze optically thick samples in motion by reducing self-absorption ( 2 , 3 ) . In TIRES a thin surface layer of the sample is rapidly heated and thermal emission from this layer is collected before it thickens and cools by thermal diffusion. Previous studies of TIRES have demonstrated qualitative analyses on moving samples with continuous-wave (2)and pulsed (3)lasers as the heat sources. This article examines the quantitative abilities of TIRES on moving samples and introduces the use of a versatile, much more economical heat source consisting of a hot-gas jet. In hot-jet-based TIRES, a stream of sample material moves through the field of view of a spectrometer, and a jet of heated gas is aimed onto the sample surface within the field of view. The jet initially produces a very thin, heated layer at the sample surface from which the spectrometer observes thermal emission. Because the layer is optically thin, the amount of self-absorptionis modest. The deposited heat quickly diffuses into the sample, and the emitting layer thickens and cools, but the thickening layer is also carried laterally out of the spectrometer field of view by the sample motion. The emitting layer leaves the observation zone before thickening results in an excessive increase in self-absorption. As a result, the spectrometer detects only the low-self-absorption emission from the thin layer initially produced. 0 1990 American Chemical Society
