Anal. Chem. 2001, 73, 3131-3139
Sampling and Raman Confocal Microspectroscopic Analysis of Airborne Particulate Matter Using Poly(dimethylsiloxane) Solid-Phase Microextraction Fibers Marek Odziemkowski,† Jacek A. Koziel,‡,§ Donald E. Irish,‡ and Janusz Pawliszyn*,‡
Department of Earth Sciences, University of Waterloo, ON N2L 3G1, Canada, and Department of Chemistry, University of Waterloo, ON N2L 3G1, Canada
Commercial poly(dimethylsiloxane) (PDMS) 7-µm solidphase microextraction (SPME) fibers were used for sampling and Raman spectroscopic analysis of a tailpipe diesel exhaust, candle smoke, cigarette smoke, and asbestos dust. Samples were collected via direct exposure of the SPME fiber to contaminated air. The mass loading for SPME fibers was varied by changing the sampling time. Results indicate that PDMS-coated fibers provide a simple, fast, reusable, and cost-effective air sampling tool for airborne particulates. The PDMS coating was stable; Raman bands of the PDMS coating were observed exactly at the same wavenumber positions before and after air sampling. Raman spectroscopic analysis resulted in identification of several characteristic bands allowing chemical speciation of particulates. The advantage of the SPME fiber is the open bed geometry allowing for application of various spectroscopic methods of particulate analysis. This paper describes the first-ever combined application of SPME technology with Raman confocal microspectroscopy for sampling and analysis of airborne particulates. Advantages of the combination of solid-phase microextraction and Raman microspectroscopy for airborne particulate analysis are discussed. Challenges associated with combined SPME sampling and Raman analysis of single particles are also described. The concern for air quality in the industrial world necessitates constant progress in the development of more sophisticated and also economical sampling and analysis methods. The evidence of adverse health effects associated with airborne particulate matter has generated significant research and regulatory attention in recent decades.1 The health effects of inhaled particulate matter are associated with the size, shape, and chemical toxicity.2 The sampling and analysis of airborne particulate matter is complicated by the complexity of the particle size, particle interactions, †
Department of Earth Sciences. Department of Chemistry. § Current address: Texas Agricultural Experimental Station, Texas A&M University, Amarillo, TX 79106. (1) Bates, D. Inhalation Toxicol. 1995, 7, ix-xii. (2) Seinfeld, J. H. Atmospheric Chemistry and Physics of Air Pollution; John Wiley & Sons: New York, 1986. ‡
10.1021/ac001141m CCC: $20.00 Published on Web 05/12/2001
© 2001 American Chemical Society
chemical partitioning between gaseous, liquid, and solid phases, and interaction with sampling media.2,3 The combination of the coexistence of the three phases with often destructive sample analysis requires a greater overall number of samples that need to be collected, resulting in the increase of the total cost of analysis. Analytical methods for determination of the chemical composition of airborne matter require both sophisticated equipment and often very strict and time-consuming sample preparation techniques.3-7 One of the most widely studied groups of analytes found in ambient aerosols is polycyclic aromatic hydrocarbons (PAHs).8 PAHs originate from incomplete combustion and therefore are often accompanied by the presence of carbonaceous solid microparticles.2 Current sampling methods for airborne particulate matter involve the use of pumps, gravimetric filters, impingers or impactor devices, and a wide variety of light- and laser-scattering devices.3-6 Target sampling methods for PAHs demand both sampling expertise and complicated sampling equipment such as highvolume pumps, filters, and sorbent cartridges. Long sample collection times and sample preparation times combined with very strict extraction procedures are obvious disadvantages. It is also difficult to perform chemical analysis of gas or liquid phases after the solid microparticle chemical analysis. To make it possible, the solid microparticle analysis must be selective and nondestructive and should not have a negative impact on later chemical analysis. One of the very powerful techniques for on-line measurement of the size and chemical composition (including PAHs) is aerosol time-of-flight mass spectrometry.9,10 (3) Spurny, K. R., Ed. Analytical Chemistry of Aerosols; Lewis Publishers: Boca Raton, FL, 1999. (4) Lodge, J. P. American Public Health Association. Methods of air sampling and analysis; Lewis Publishers: New York, 1989. (5) National Institute of for Occupational Safety and Health. NIOSH Manual of Analytical Methods, 4th ed.; U.S. Department of Health and Human Services, Cincinnati, OH, 1994. (6) Environmental Protection Agency. Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, 2nd ed.; Center for Environmental Research Information, Cincinnati, OH, 1996. (7) Niessner, R. In Environmental Analysis: Techniques, Applications, and Quality Assurance; Barcelo, D., Ed.; Elsevier: Amsterdam, 1993. (8) Koeber, R.; Bayona, J. M.; Niessner, R. Environ. Sci. Technol. 1999, 33, 1552. (9) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1406. (10) Silva, P. J.; Prather, K. A. Environ. Sci. Technol. 1997, 31, 3074.
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Recent studies have shown that PAHs can be sampled by solidphase microextraction (SPME) fibers and analyzed by GC/MS.11,12 SPME is an attractive alternative over conventional air sampling methods. It combines fast sampling, preconcentration, and direct and complete transfer of the extracted analytes into a standard GC injector. The same SPME air sample can be subjected to diverse analytical methods without significant compromise of the standard GC analysis. The main objective of this study was to demonstrate the feasibility of using SPME fibers for airborne particulate sampling followed by both Raman microspectroscopy for single solid particle analysis and standard GC/MS analysis. This was facilitated by the recent introduction of very sensitive commercial Raman microscopes that allow fast analysis of airborne solid particles deposited on a SPME fiber during air sampling. The following report addresses only a few selected aspects of the conducted research with the emphasis on the methodology of Raman confocal microspectroscopy combined with SPME. Sampling with SPME was selected because it provides simple sample preparation, where the SPME coating serves as “glue” for fast and inexpensive sample collection.12 The importance of the confocal mode for Raman measurements was demonstrated by four types of air samples tested by the SPME technique, including cigarette smoke, oil lamp smoke, the exhaust from a cold and hot diesel engine, and suspended asbestos fibers. EXPERIMENTAL SECTION Chemicals and Supplies. All SPME fibers and devices, syringes, vials, and 16 PAH standards were purchased from Supelco (Oakville, ON, Canada). For SPME sampling, poly(dimethylsiloxane) (PDMS) 7-µm fibers were used. These fibers are commonly applied to sampling of analytes with greater partition coefficients, e.g., all PAHs.11,12 Ultrahigh-purity helium was from Praxair (Waterloo, ON, Canada). An asbestos mineral sample was obtained from the Department of Earth Sciences (University of Waterloo, ON, Canada). Transmitted light microscopic analysis of asbestos sample revealed two colors, i.e., white, which might be indicative of a chrysotile mineral phase, and brown, which might indicate the presence of amosite mineral. For tobacco smoke analysis, Matine´e Extra Mild cigarettes were used. The oil lamp was bought at a local drug store. Sample Collection and Preparation. Diesel Exhaust Smoke. All diesel exhaust samples were collected from a 1998 International 4900 series DT466E truck at the Plant Operations courtyard of the University of Waterloo. One sample was collected during the first minute immediately after engine start-up; the sample was labeled as “cold engine sample”. The second and third diesel exhaust samples were collected when the engine was idling at ∼750 rpm, with 1- and 5-min SPME fiber exposure times, respectively. Fibers were turned around during the collection time to obtain even distribution of particulate matter. A more detailed description of sample collection and preservation appeared elsewhere.12 After sample collection, SPME fibers were pulled ∼30 mm inside the needle, capped with a Teflon plug, and then transferred to the laboratory for Raman analysis. The SPME (11) Pawliszyn, J. Solid-Phase Microextraction: Theory and Practice; Wiley-VCH: New York, 1997. (12) Koziel, J. A.; Odziemkowski, M. S.; Pawliszyn, J. Anal. Chem. 2001, 73, 47.
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holder was placed and secured on a home-designed Raman microscope support and placed on the XYZ Raman microscope motorized stage. Prior to Raman measurements, the SPME fiber was pulled out of the needle. This was followed by adjusting the position of the Raman microscope holder and adjusting the XY position of the motorized stage. As a result, the SPME fiber was positioned directly under the Raman microscope lens and perpendicular to the Y axis of the microscope motorized stage. Tobacco Smoke. Tobacco smoke sampling was performed with the SPME holder secured in the Raman microscope support and positioned on the XYZ Raman microscope motorized stage. Similar to diesel sampling, an exposed SPME fiber was positioned directly under the Raman microscope lens and perpendicular to the Y axis of the microscope motorized stage. Space of ∼25 mm was created between the microscope lens and the SPME fiber by moving the motorized stage in the Z direction. The smoke from tobacco was directed, i.e., puffed, through the glass capillary tube onto the surface of the SPME fiber. Oil Lamp Smoke. Two samples of oil lamp smoke were collected. The first sample was collected by placing the SPME fiber for 10 s at 50 mm above a burning lamp. The second sample was collected for 30 s at 250 mm above the burning lamp. Similar to diesel sampling, the SPME fibers were rotated during sample collection. Asbestos Sampling. The asbestos mineral in form of fibers was placed into a septum-closed, 40-mL EPA volatile organic analysis vial. The mineral was kept in this vial for more than two years before this research. The vial was shaken several times before insertion of a SPME fiber. The SPME fiber was never in direct physical contact with the bulk asbestos mineral during sampling. After sample collection, exposed SPME fibers were placed on the Raman microscope holder under the microscope lens for analysis. Sample Analysis: Protocol. Samples of various types (many as solid particles) were collected according to details above. In general, the sample was absorbed and adsorbed on the commercial PDMS 7-µm SPME fibers. Following Raman spectroscopic measurement, the same fiber was analyzed (within a maximum of 6 h from sample collection) by desorbing the organic volatiles and subjecting them to GC/MS. Before and after sample desorption in the GC injector, each fiber was visually inspected under the microscope, to evaluate particulate matter deposition and the effectiveness of particulate removal following the 5-min desorption in the GC injector. The results of these latter analyses have been presented in a previous publication.12 The purpose of the present document is the focus on the potential of the Raman microprobe technique as a complement to the GC/MS analysis; this is particularly important for solid airborne particles, which cannot be analyzed directly by a conventional GC/MS. Raman spectra were obtained with a Renishaw 1000 microscope system with CCD detector (400 × 600 pixels) using a Melles Griot 30-mW HeNe (632.8 nm) laser. The optical throughput and sensitivity are high with the optical efficiency (the fraction of scattered light collected) being greater than 30%. The detection of a very weak signal (1 photon/s) is possible. Raman confocal spectra of uncoated (bare) fibers and fibers coated with PDMS were obtained using 50× magnification with the short focus objective lenses. In some cases, 100× magnification was employed for very small particles. Raman results were interpreted based
Figure 1. Confocal operation mode of the Renishaw 1000 system: optical configuration. (A) confocal optics; (B) CCD pixel binning. Slit width 15 µm (or less); CDD area 4 (or less) by 600 pixels.
on comparison of available literature data and data associated with the SPME samples spectra. The confocal mode of operation was achieved by decreasing both the slit width to 15 µm and the area of the CCD detector to 4 × 600 pixels as demonstrated in Figure 1. The slit is used as the primary aperture in the system and, as such, provides an additional spatial filter for the confocal measurements. The optical design of the system produces an image spot size of ∼15 µm (depending on the objective used) at the slit, and thus, it discriminates against the out-of-focus sample region. The optical alignment of the instrument was optimized using the 521-cm-1 Raman signal from a silicon wafer. The silicon wafer of 0.63-mm thickness was manufactured in the Department of Electrical and Computer Engineering at the University of Waterloo. At the confocal setting, the 50× and 100× magnification lenses resulted in 5- and 2-µm sampling depths, respectively.13 The proper confocal optical alignment and the depth of field were confirmed by plotting Si signal intensity against position through the focus, i.e., Z distance profile, of the silicon wafer. Confocal Raman microspectroscopy also dramatically diminishes undesirable fluorescence. A high fluorescence from industrial air samples was expected, and this was yet another reason confocal Raman microscopy was of critical importance for our measurements. Most attempts to use the nonconfocal (i.e., standard) mode of operation resulted in the saturation of the CCD detector by strong fluorescence background, indicating that the fluorescence in these cases originated from not only the particulate matter but also from the surroundings. Apart from the depth profiling, all the SPME/Raman experiments in this research can be classified into two classic experiments with the confocal technique, i.e., the “particle in a matrix” and the “multilayer laminate”. The former case involves analysis of individual particles collected on the surface of the SPME fiber during air sampling. The latter case is the analysis of the PDMS(13) Williams, K. P. J.; Pitt, G. D.; Batchelder, D. N.; Kip, B. J. Appl. Spectrosc. 1994, 48, 232.
Figure 2. Raman spectra of (A) bare glass fiber, laser power 3.5 mW, accumulation time 20 s, 5 accumulations and (B) glass fiber coated with the thin layer of PDMS, laser power 3.5 mW, accumulation time 25 s, 10 accumulations. Microscope lens magnification 50×.
Figure 3. White light image of the glass fiber coated with the thin layer of PDMS, magnification 50×.
coated glass fiber (or sample blank) itself. For this experiment, bare glass fibers identical to those used to manufacture 7-µm PDMS SPME fibers were rinsed with methanol and dried in air before the collection of Raman spectra. A commercial 7-µm PDMS fiber was first conditioned according to manufacturer’s recommendations before the Raman analysis of the blank. Each Raman measurement was always followed by collection of a white light image of the analyzed sample area. Laser power had to be adjusted specifically to the sample character; therefore, it is listed in figure captions. The laser power was always measured at the sample surface, i.e., under the microscope lens. RESULTS AND DISCUSSION Raman Analysis of Blank SPME Fibers. The Raman spectrum of the bare glass fiber is presented in Figure 2A. This spectrum is characterized by a very broad and strong Raman signal of glass around 450 cm-1 followed by three weak Raman signals at 605, 802, and 1062 cm-1. The white light image of the PDMS-coated fiber is presented in Figure 3. In contrast to the bare glass fiber image, which was smooth and lacked imperfections, the thin polymer coating was characterized by very small grooves visible at the bottom part of the image (Figure 3). Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
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Raman spectrum of glass fiber coated with PDMS is shown in Figure 2B. The partial vibrational assignments of PDMS have been given by Kriegsmann14 and Kovalev et al.15 The most detailed vibrational band assignment, based on IR, Raman, and isotope studies of monomers, oligomers, and PDMS, has been given by Smith and Anderson.16 There is very good agreement between the Raman spectrum of the PDMS-coated fiber and the literature for PDMS. The exception is a weak peak at 2810 cm-1 observed in our studies and not observed by others. We attribute this peak to the stretching vibration of a methyl group, since a very similar weak peak at 2820 cm-1 was observed for an ((CH3)2SiO)4 oligomer. Close inspection of the spectrum presented in Figure 2 indicates that there is a very small contribution from the underlying glass fiber. This should not be the case, since with the confocal mode of operation the sampling depth of 5 µm is significantly below the theoretical thickness of the PDMS coating, i.e., 7 µm. Thus, it is likely that the spectral contributions from the glass background were caused by either imperfections in the fiber coating thickness or, for low refractive index materials such as the PDMS coating (compared with Si wafer), a small part of the laser radiation penetrated beneath the PDMS coating. With the exception of the glass fiber signal at 605 cm-1, this contribution manifests itself mainly as a rising background signal between 250 and 500 and 780-880 cm-1. In Figure 2B, the intensities of Raman bands of the PDMS coating affected by the background signal from the glass fiber are marked with an asterisk. As expected, these bands include symmetric and asymmetric O-Si-O stretching modes of PDMS at 491 and 1061 cm-1, respectively. The other two bands affected are CH3 rocking at 862 and 790 cm-1. Raman Analysis of Particulate Matter on SPME Fibers. Diesel Exhaust Smoke. Our previous study indicated that it is feasible to use Raman microspectroscopy for analysis of single particles from diesel exhaust collected on SPME fibers.12 Cold Engine Sample. All Raman measurements have been conducted on the single SPME fiber sample and five different locations on the coating. White light images of SPME fibers after 1-min exposure to cold engine diesel exhaust smoke are presented in Figure 4. The image strongly differs from the image of the blank SPME fiber (Figure 3). The SPME fiber is covered by randomly distributed black spots with shapes that might be described as regular and irregular ovals with diameters varying from less than 1 to 5 µm. A white shiny semicircular area, at X ) 0 and Y ) 0 coordinates, results from the local burn of the sample when we attempted to collect a Raman signal with the maximum (7.2 mW) laser power. After the laser power was lowered to 0.071 mW, i.e., the lowest possible power with the 50 magnification lens, and moving the microscope stage to a new location (X ) - 40, Y ) 10), we were able to obtain the Raman signal, which was characterized by two new Raman bands at 1336 and 1600 cm-1 (Figure 5A). The shape of the spectral background in Figure 5A indicates that despite the confocal mode of measurements we were unable to totally eliminate sample fluorescence. As far as the two main peaks are concerned, Raman spectra collected at four other locations had similar character (Figure 5B and C). The appearance (14) Kriegsmann, H. Z. Electrochem. 1961, 65, 342. (15) Kovalev, I.; Shevchenko, I.; Voronkov, M.; Kozlova, N. Dokl. Akad. Nauk. USSR (Proc. Acad. Sci. USSR) 1973, 212, 101. (16) Smith, A.; Lee, D.; Anderson, D. R. Appl. Spectrosc. 1984, 38 (6), 822.
3134 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
Figure 4. White light image of SPME fibers after 5-min exposure to hot engine diesel exhaust smoke; magnification 50×. (A) Corresponding Raman spectrum is presented in Figure 5A. (B) Corresponding Raman spectrum for X ) +117, Y) +12 location is presented Figure 5C.
of two characteristic Raman bands indicates the presence of carbonaceous material as a result of incomplete diesel combustion. For an infinite graphite crystal with D46h space group symmetry, six vibrational modes are predicted but only two E2g modes are predicted to be Raman active.17 For the 488-nm laser excitation line, single-crystal and highly ordered pyrolitic graphite (HOPG) fundamental modes consist of a single graphite (so-called G band) band at 1582 cm-1 (E2g2) and another band at 42 cm-1 (E2g1).7 Increasing lattice disorder results in the appearance of two additional bands 18,19 at approximately 1360 and 1620 cm-1. These bands are often referred to as disorder-induced modes D and D′, respectively. In Figure 5, the Raman band at ∼1600 cm-1 falls between the G band at 1582 cm-1 and the D′ band at 1620 cm-1. The band-fitting procedure and Raman spectra collected at different locations (see Figure 6) demonstrated that the band observed at ∼1600 cm-1 should be attributed to the mixture of G and D′ bands, as suggested earlier by Kraft and Nickel.20 In contrast, the D band (∼1330 cm-1) in Figure 5 is shifted by almost 30 cm-1 compared to the band position obtained with 488-nm laser (17) McCreery, R. L. Carbon Electrodes; Structure Effects on Electron-Transfer Kinetics. In Electroanalytical Chemistry; Bard, A. J., Ed.: Dekker: New York, 1991; Vol. 17, p 221. (18) Dresselhaus, M. S.; Dresselhaus, G.; Sugihara, K.; Spain, I. L.; Goldberg, H. A. Graphite Fibers and Filaments; Springer Series in Materials Science 5; Springer-Verlag: Berlin, 1988. (19) Endo, M.; Kim, C.; Karaki, T.; Tamaki, T.; Nishimura, Y.; Matthews, M. J.; Brown, S. D. M.; Dresselhaus, M. S. Phys. Rev. B 1998, 58, 8991. (20) Kraft, T.; Nickel, K. G. J. Mater. Chem. 2000, 10, 671.
Figure 6. Comparison of the Raman spectrum of a glass fiber coated with the thin layer of PDMS, i.e., SPME fiber blank (C) to Raman spectrum collected from X ) +176, Y ) -5 location in Figure 4B. (A) Raman spectrum as detected, i.e., before baseline correction, laser power 1.84 mW. (B) Raman spectrum after six-degree polynomial baseline correction. R-quartz band assignments are given in parentheses after ref 24.
Figure 5. Comparison of the Raman spectrum of a glass fiber coated with the thin layer of PDMS, i.e., SPME fiber blank, to Raman spectra collected at three different locations on the SPME fiber coating that was exposed for 1 min to cold diesel exhaust. (A) Corresponding white light image presented in Figure 4A, laser power 0.071 mW. (B) Corresponding white light not presented, laser power 0.74 mW. (C) Corresponding white light image presented in Figure 4B, laser power 1.84 mW.
excitation. Vidano et al.21 carried out systematic studies of Raman spectra of different kinds of carbon materials as a function of the laser excitation wavelength, λL. They reported that the D band, observed in our studies at 1331, 1333, and 1336 cm-1, shifts to higher frequency with the increase of the laser excitation wavelength. For natural graphite, changing λL from 488 to 1064 nm results in the shift of the D band from 1357 to 1284 cm-1, while the D′ is much less affected, with a maximum shift of 17 cm-1.22 Matthews et al.23 attributed the origin of dispersive effects of the Raman D band to the coupling between D band optical phonons and the transverse acoustic branch near the K point of the Brillouin zone. For the 632.8-nm He-Ne line, used in our studies, they23 observed the D band at 1330 cm-1, which is in good agreement with the Raman spectra of carbonaceous material deposited on the SPME fiber (see Figure 5). Sampling Anomaly. The visual inspection of the white light images in Figure 4 indicate the possibility of the presence of a second, unknown phase which occurs as white irregular circles at X ) -26, Y ) +3 (Figure 4A) and X ) +176, Y ) -5 (Figure (21) Vidano, R. P.; Fishbach, D. B.; Willis, L. J.; Loehr, T. M. Solid State Commun. 1981, 39, 341. (22) Wang, Z.; Huang, X.; Xue, R.; Chen, L. J. Appl. Phys. 1998, 84, 227. (23) Matthews, M. J.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Endo, M. Phys. Rev. B 1999, 59, R6586.
4B). When the laser was focused at one of these locations, the Raman spectrum was quite different from Raman spectra presented in Figures 2 and 5. The Raman spectrum from a white shiny spot, presented in Figure 6A and B, was characterized by several weak bands at low frequency which were followed by a strong Raman signal at 463 cm-1 with a weak shoulder at 491 cm-1. The latter shoulder as well as other weak bands at 711, 2907, and 2967 cm-1 resulted from the residual signal of the PDMS coating, as evident from comparison of Figure 6A and B with Figure 6C. The asymmetric band at 1330 cm-1 and two bands at 1581 and 1602 cm-1 are attributed to graphitic carbon. A new very strong band at 463 cm-1, weak (however visible after baseline correction) bands at 203, 263, and 356 cm-1, and a band at 399 cm-1 fall exactly in the same frequencies as R-quartz.24,25 These R-quartz particles were detected only for “cold engine” air samples. The presence of R-quartz on the SPME fiber is evidently not linked to the diesel fuel combustion; most probably it is a result of a blow of quartz sand by the tailpipe exhaust gases and in consequence deposition of quartz sand onto the SPME fiber coating. Hot Engine Sample. The 1-min exposure of SPME fiber to the “hot engine” tailpipe diesel exhaust did not result in a measurable Raman signal. The extension of the exposure time to 5 min was necessary to achieve meaningful Raman measurements. In general, the SPME fiber coating was covered with much fewer black spots compared to the cold engine sample. An example of a white light image of an SPME fiber after 5 min exposure to hot engine diesel exhaust is presented in Figure 7. The corresponding Raman spectrum is shown in Figure 8A. Figure 8B is a Raman spectrum from another, randomly chosen black spot location. (24) Scott, J. F.; Porto,S. P. S. Phys. Rev. 1967, 161 (3), 903. (25) Dean, K. J.; Sherman, W. F.; Wilkinson, G. R. Spectrochim. Acta 1982, 38A (10), 1105.
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Figure 7. White light image of SPME fibers after 5-min exposure to cold engine diesel exhaust; magnification 50×.
Figure 8. (Upper trace) Raman spectrum corresponding to the white light image presented in Figure 7 at location X ) 24, Y ) 6, laser power 1.84 mW. (Lower trace) Raman spectrum obtained at another randomly chosen location where a black deposit was visible. Laser power in both cases was 1.84 mW.
Hot Engine vs Cold Engine. Air sampling of tailpipe diesel exhaust from a hot and a cold engine with SPME fibers results in the deposition of the graphitic carbon on the surface of the SPME fiber. In the case of a “cold engine”, the entire surface of the SPME fiber was covered with a thin carbon film (Figure 4B and Raman spectrum in Figure 5C). The effective Raman sampling depth is a function of the complex part of refractive index of the material under investigation. In the case of carbon or graphite, the Raman sampling depth is ∼30 nm when visible light is used.17 In Figure 5C (and the corresponding white light image presented in Figure 4B), the background signal of PDMS is visible. Therefore one might conclude that the carbon film thickness at that location is less than 30 nm. To the contrary, at the location shown in Figure 4A (see the Raman spectrum in Figure 5A, no visible signal from the PDMS coating), the carbon deposit is thicker than 0.30 µm. This observation was further confirmed by an additional experiment: by using the autofocus option of the instrument (moving the motorized stage in the Z direction), the measurement of the depth of the carbon deposit at the X ) - 40, Y ) 10 location was possible. The estimated height of the carbon deposit was ∼5 µm. This height is well above the effective Raman 3136 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001
sampling depth; therefore, it is not surprising that the background signal from the PDMS coating is not visible. In the case of “hot engine” samples, all investigated spots were characterized by carbon deposits thinner than 0.3 µm. The band fitting of Raman spectra (see Table 1) presented in Figure 5A and B revealed small differences in Raman spectra collected at different locations. This in turn, allowed us to obtain more information on the specific chemical character of these carbon deposits. These deposits were different than carbon coke and graphite materials used in lithium ion batteries.26 As mentioned above, the peak at ∼1600 cm-1 should be attributed to a mixture of the graphite band G at 1581 cm-1 and the disorderinduced D′ band at 1610 cm-1. The broad strong peak at ∼1340 cm-1 arises from defect-induced D band that is absent in the HOPG, but becomes active in microcrystalline graphite due to size effects.20 Broad peaks observed in our work at 1511, 1523, and 1525 cm-1 are required in most cases for a good fit of Raman spectra of carbon blacks20 and are typical of amorphous carbons. In amorphous diamond-like carbon, this band is conventionally attributed to sp2-bonded carbon clusters in the sp3 network.20 The origin of prebands 1, 2, and 3 is not certain, but they are commonly observed in nanocrystalline diamond films and very recently were observed as a result of hydrothermal decomposition of SiC.20 The Raman spectral features of carbon deposits most closely resemble the results obtained by Kraft and Nickel.20 In our opinion, these similarities are the result of the presence of considerable amounts of H, N, and O in their carbon samples, e.g., anthracite, and our graphitic carbon deposits. In fact, these elements might be expected in all our deposits as a result of the incomplete combustion of organic fuels. The GC/MS spectra for PAHs in diesel exhaust were presented elsewhere.11,12 Tobacco Smoke. Raman spectra of tobacco smoke that was collected with the use of an SPME fiber are presented in Figure 9. Similar to the diesel exhaust, deposited particulate can be characterized as graphitic carbon with a different degree of lattice disorder. Despite many attempts, graphitic carbon was the only species detectable with Raman microspectroscopy. Oil Lamp Smoke. The white light image of an SPME fiber after 10-s exposure to burning oil lamp smoke is presented in Figure 10A. The corresponding Raman spectrum is not shown. However, as for other samples, it was characterized by the appearance of two Raman bands characteristic of graphitic carbon. When the SPME fiber was placed ∼250 mm above the burning lamp, the white light image (see Figure 10B) had a quite characteristic appearance with very few black area spots of graphitic carbon deposits. Many new particles of very regular shape and “bubblelike” appearance were observed. One of this particles is presented in Figure 10B at X ) -113 and Y ) -54 coordinates. The Raman spectra obtained from these “bubblelike” particles is presented in Figure 11ssolid line. These spectra (in contrast to graphitic carbon particles) were characterized by the lack of fluorescence and a very rich array of Raman bands. Raman spectra were very reproducible for both Raman band positions and their relative intensities. The GC/MS analysis performed on this same SPME fiber indicated more than 30 probable analytes including organic acids, silane-type compounds, amines, and many others. (26) Irish, D. E.; Deng, Z.; Odziemkowski, M. J. Power Sources 1995, 54, 28.
Table 1. Raman Band Assignments for Poly(dimethylsiloxane). Comparison of Raman Bands Observed on SPME Fiber with Available Literature Data for Poly(dimethylsiloxane)a Smith and Anderson16 mode δ C-Si-C sym def + twist δ C-Si-C sym def δ C-Si-C wag F C-Si-C rock vs Si-O-Si str Fs Si-CH3 rock vs C-Si-C ? Fa CH3 + vs C-Si-C Fs CH3 va Si-O-Si str δs CH bend δa CH bend vs CH str vs CH str va CH str a
IR
277,328,363b 395 s, 407b s 500 w 633 w 688 w 704 m 740 vw 802 vs 864 m 1022 vs b 1087 vs b 1260 vs 1400 w, 1411 m 2905 w 2965
Kovalev et al.15
Soutzidou et al.17
this work
Raman
Raman
Raman
Raman 165 m 194 m
159 189
495 650 vw P 692 w 713 s P
395 488, 502, 515 603, 667 688 708
796 w 868 w P
158 188
161m 193 m
489 not presented
491 vs 688 m
not presented not presented not presented not presented not presented
711 s
1268 w P 1418 w
802 865 1020 1093 1260 1398, 1409, 1442
not presented not presented
2915 vs P 2967 s
2903 2962
2908 2968
790 w 862 vw 1061 vw b 1264 w 1412 w 2810 vw 2907 s 2967 m
Abbreviations: s, strong; m, medium; w, weak; v, very; b, broad; P, polarized. b Detected in crystal.
Figure 9. Raman spectra of tobacco smoke sampled with the PDMS coating. Laser power 1.84 mW. The corresponding white light image is not provided.
In the present stage of the research, with a still limited amount of data, it is very difficult to make a positive identification of the chemical composition of the captured particles. However, the presence of a very strong band at 1002 cm-1, most probably an aromatic ring breathing mode, followed by a much weaker band at 1031 cm-1, suggests a polycyclic aromatic hydrocarbon. The peaks marked in Figure 11 evidently cannot be attributed to the PDMS coating, i.e., most Raman bands fall at frequencies characteristic of PAHs, biphenyl, diphenylsilane, carbanilide, thiocarbanilide, and diphenylphosphinic acid.27 Asbestos. Asbestos is the common name given to a class of fibrous silicate minerals. The asbestos minerals are known for their interaction with the human body.28 Certain forms of asbestos (27) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grassell,i J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press Inc.: Toronto, 1991; Chapter 17.
Figure 10. White light image of SPME fibers after exposure to smoke from an oil lamp, magnification 50×. (A) 10-s exposure; fiber was placed at a distance of 50 mm above a burning lamp. (B) 30-s exposure; fiber was placed at a distance of 250 mm above a burning lamp. The corresponding Raman spectrum is presented Figure 11 as a solid line. Laser power 1.84 mW.
found in airborne dust have been shown to cause respiratory illnesses by long-term build-up in the lungs.28,29 Asbestos minerals possess different chemical compositions. However, they are all double-chain silicates with linking cations. A simplified chemical formula can written as: MenSi2mOp(OH)2r, where Me might be Mg, Na, Fe, or Ca, n ) 2, 5, or 7, m ) 1 or 4, p ) 5 or 22, and r ) 1 or 2. The Raman spectrum of the particle is characterized by the appearance of a minimum of five to six new bands that evidently (28) Guthier, G. D., Jr.; Mossman, B. T. Rev. Mineral. 1993, 28, 1. (29) Gunter, M. E. J. Geol. Educ. 1994, 42, 17.
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Figure 11. Comparison of the Raman spectrum of a glass fiber coated with the thin layer of PDMS (dotted line) to the Raman spectrum taken from the X ) -113 and Y ) -54 location in Figure 10B. Only newly detected Raman band positions are marked. Table 2. Comparison of the Average Curve Fit Values of Raman Shifts for Carbon Deposits Detected on the SPME Fiber to the Curve Fit of Anthracite (from Ref 10). Laser Excitation 632.8 nm. Band Assignments after Kraft and Nickel20a Raman shift/cm-1 band
Figure 7A
Figure 7B
Figure 8B
anthracite
G + D′
1600
1597
1596
A D pre 1 pre 2 pre 3
1525 1340 1242 1177
1523 1327
1581 (G) and 1602 (D′) 1511 1330 1222 1160 1111
1169 1103
1502 1350 1247 1169 1105
aThe band-fitting of prebands 1, 2, and 3 very strongly depends on the baseline correction procedure; the mathematical function used for baseline correction influenced the Raman band position of prebands. In this work, all baseline corrections were performed using a six-degree polynomial.
differ from Raman bands of PDMS coating. Table 2 compares these new Raman bands with available literature data for several different asbestos minerals. The asbestos mineral phase, collected on the SPME fiber coating, most closely resembles the most abundant asbestos mineral, i.e., chrysotile. The strongest Raman bands in Figure 12A were at 391 and 694 cm-1. These bands fall almost exactly at the wavenumbers of two of the strongest modes of chrysotile.30,31 The band at 391 cm-1 corresponds to a metal-O stretching mode30,31 and the band at 694 cm-1 to the O-Si stretching mode.30,31 Furthermore, for chrysotile,30,31 the OH stretching should be characterized by a complex asymmetric band around 3700 cm-1; such bands were also observed in our studies (see Figure 12B). As noted by Bard et al.,30 who used in their work exactly the same Raman instrumentation as ours and operated it in a conventional way, i.e., nonconfocal, the confocal (30) Bard, D.; Yarwood, J.; Tylee, B. J. Raman Spectrosc. 1997, 28, 803. (31) Kloprogge, J. T.; Frost, R. L.; Rintoul, L. Phys. Chem. Chem. Phys. 1999, 1, 2559.
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Figure 12. Comparison of the Raman spectrum of a glass fiber coated with the thin layer of PDMS, i.e., SPME fiber blank (lower trace) to the Raman spectrum of asbestos particle absorbed on SPME fiber. (A) Raman spectra in the low-frequency region; (B) Raman spectra in the CH and OH stretching region. Laser power 7.2 mW.
mode led to severe loss of signal. This may explain why we were unable to detect all the characteristic bands of chrysotile (see Table 3). The combination of SPME technology with commercial Raman instrumentation did not require sample preparation. For example, asbestos fibers need to be flattened to allow infrared microscopic analysis29 while scanning or transmission electron microscopy requires lengthy sample preparation. A few additional experiments were performed to address the role of SPME fibers in the air sampling process. In these experiments, instead of PDMS-coated glass fibers, a simple glass microscope slide was exposed to the diesel and oil lamp smoke. In both cases, Raman spectra analysis revealed the presence of characteristic bands of graphitic carbon; therefore, it appears that the carbonaceous material is just coated onto the SPME fiber. However, on the glass microscope slide, we never observed particulate matter such as microparticles of R-quartz crystals or polycyclic aromatic hydrocarbons. Furthermore, asbestos sampling with a bare (i.e., PDMS coating was removed) glass fiber
Table 3. Comparison of Raman Bands (Boldface) Observed on SPME Fiber after 1-min Exposure to the Air Closed in the VOA Vial Containing Asbestos Minerals with Available Raman Literature Data for Different Asbestos Minerals (after Ref 29)a
mineral amosite
anthophyllite
Si-bridging, O-Si stretching modes (cm-1)
nonbridging Si-O stretching modes (cm-1)
1093 vw 658 vs 555 vw 528 m
1020 s 968 m 903 vw
671 vs
1042 m
chrysotile
1105 (1113) vw 692 (694) vs 623 (623) w
crocidolite
1085 s 1032 m 664 s 577 vs 539 vs
969 vs 891 s 772 m 737 m
tremolite
1061 m 1028 m 672 vs
928 w
a
Si-O-Si deformation modes (cm-1) 506 w
509 w
metal-O modes (cm-1) 421 m 401 m 364 m 349 m 309 w 287 w 430 m 410 w 384 m 362 m 336 vw 304 m 260 465 m 432 vw 389 (391) vs 345 (348) s 32 1vw 304 vw 469 m 374 vs 332 m 297 s 249 s 271 414 w 393 m 369 m 349 vw 251 w 232 w 222 w
O-H stretching modes (cm-1) 3656 3639 3623
3674
3700 (3699) 3685 (3685) sh
3685 3637
3677
Abbreviations: s, strong; m, medium; w, weak; v, very; b, broad; sh, shoulder.
was also not successful. For the two cases where microparticles of R-quartz and asbestos were detected, evidently the poly(dimethylsiloxane) coating acts as an efficient absorbent for airborne microparticles. CONCLUSIONS Commercial PDMS 7-µm SPME fibers were used for sampling of airborne particulate matter followed by confocal Raman microspectroscopic analysis of single particulates. SPME fiber coating performed very well in sampling of various exhausts from organic fuel and asbestos dust samples. Sampling with SPME was very simple and fast. Raman analysis indicated that in all investigated cases the PDMS coating was stable and inert toward analytes. No interaction with the sampling media was observed. Raman bands of the PDMS coating were observed exactly at the same positions as for blanks and air samples. The sampling was easy and very effective in the field application as demonstrated by diesel exhaust sampling12 and Raman analysis. The main advantage of using confocal Raman microscopy for identification of the airborne particulate is that it is suitable for single-particle analysis of less than 1 µm in diameter. This spatial resolution is ∼1 order of magnitude better than the analysis with infrared
microscopy.29 Thus, the combination of SPME and Raman vibrational microspectroscopy, known to be a powerful method of chemical fingerprinting of molecules, is shown to be a potentially new procedure for the identification, study, and possible numeration of airborne particulate matter. A similar approach can be used for the analysis of particulates in other matrixes, e.g., water. SPME sampling allows for the use of various modern analytical tools ranging from GC/MS to vibrational laser microspectroscopy. In certain very specific situations, laser-induced desorption was observed; therefore, the combination of these analytical methods for a single (i.e., not repetitive) air sampling will require further research. Results from this research should serve as a basis for the development of other methodologies that combine SPME sampling with single-particle analysis, e.g., X-ray fluorescence. ACKNOWLEDGMENT Support for this work was obtained from Natural Sciences and Engineering Research Council of Canada. Received for review September 25, 2000. Accepted March 19, 2001. AC001141M
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