EDX Analysis of Submicrometer

Alexander Laskin* and James P. Cowin. William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 99...
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Anal. Chem. 2001, 73, 1023-1029

Automated Single-Particle SEM/EDX Analysis of Submicrometer Particles down to 0.1 µm Alexander Laskin* and James P. Cowin

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352

Typically single-particle SEM/EDX analysis of aerosols is done on polycarbonate filters or solid carbon substrates. This has led to a widespread conclusion that EDX provides poor information on carbon, oxygen, and nitrogen content of a particle and usually could not go below 0.5µm particles. We show that use of grid-supported carbon films of 15-25-nm thickness gives exceptionally low background in the SEM/EDX analysis and allows satisfied automated analysis of particles down to 0.1-µm size, including detection of low-Z elements. In this work, six laboratory-generated 0.1-2-µm aerosols were tested for their elemental composition. The EDX analysis yields reasonably accurate quantitative results featuring all the elements present in the tested compounds, namely, C, O, N, Na, S, Al, Si, and Cl. Furthermore, the carbon film has very low backscattered electron (BSE) yield compared to that from the particle, so in the BSE mode the particle image is seen with very high contrast. This greatly improves quality and speed of the automated mapping of particles by SEM prior to EDX analysis. Scanning electron microscopy with energy-dispersed analysis of X-rays (SEM/EDX) is one of the electron probe microanalysis (EPMA) methods that have been used extensively to characterize the size, morphology, and elemental composition of ambient aerosols.1 SEM/EDX was sometimes considered too tedious and time-consuming to acquire statistically significant information on atmospheric aerosols, too insensitive to analyze submicrometer particles, and suited only for elements higher than sodium (Z > 11). But technologies have changed, and custom built fully automated SEM/EDX systems have made analysis of many thousands of particles quite feasible.2,3 Recent needs for semiconductor wafer and storage media inspection, as well as forensic applications, have resulted in commercially available automated systems. Modern X-ray detectors with ultrathin windows, or no windows, now permit routine work4-6 on elements as light as beryllium. Selecting the optimal substrate for the electron microprobebased methods is the key issue for analysis performance and (1) McMurry, P. H. Atm. Environ. 2000, 34, 1959-1999 and references sited therein. (2) Buseck, P. R.; Anderson, J. R. In Advanced Mineralogy; Marfunin, A. S., Ed.; Springer-Verlag: Berlin, 1998; Vol. 3, pp 292-312 and references sited therein. (3) Gregory, C. L.; Nullens, H. A.; Gijbels, R. H.; Van Espen, P. J.; Geuens, I.; De Keyzer, R. Anal. Chem. 1998, 70, 2551-2559. 10.1021/ac0009604 CCC: $20.00 Published on Web 01/26/2001

© 2001 American Chemical Society

especially for analysis of submicrometer particles. The ideal substrate should not contain elements with the characteristic X-ray peaks overlapping with those of the atmospheric particles. Beryllium seems to be an ideal substrate; however, its toxicity, high commercial cost, and reactivity with sulfates4 make use of beryllium impractical for atmospheric aerosols.1 The substrates used to date for the automated EDX of airborn particles have included mostly polycarbonate filters7-13 and polished glassy carbon plates.14,15 Analysis of particles smaller than 1 µm is seriously limited for particles supported on plates or filters of any kind. EDX typically probes a volume of ∼1 µm3 due to electron scattering in the sample, so the X-ray signals of smaller particles are difficult to detect against the discrete background lines from the substrate and the broad continuum (bremstrahlung) background. Recently, an innovative grazing-exit EPMA method (GE-EPMA) was successfully applied for the analysis16,17 of submicrometer particles. In this method, the X-ray detector is placed at a very sharp (“grazing”) angle to the substrate surface. This makes the detector “blind” to the X-rays emitted from the specimen depth while the X-rays emitted from the particles can be effectively detected. Another published approach was to determine the chemical composition of the particles using elemental distribution maps measured by the WDX-EPMA system.18 (4) Huang, P.-F.; Turpin, B. Atmos. Environ. 1996, 30, 4137-4148. (5) Osa´n, J.; Szalo´ki, I.; Ro, Ch.-U.; Van Grieken, R. Mikrochim. Acta 2000, 132, 349-355 (6) Ro, Ch.-U.; Osa´n, J.; Szalo´ki, I.; Oh, K. Y.; Kim, H.; Van Grieken, R. Environ. Sci. Technol. 2000, 34, 3023-3030. (7) Katrinak, K. A.; Anderson, J. R.; Buseck, P. R. Environ. Sci. Technol. 1988, 22, 321-329. (8) Anderson, J. R.; Buseck, P. R.; Patterson, Th. L.; Arimoto, R. Atmos. Environ. 1996, 30, 319-338. (9) Anderson, J. R.; Agget, F. J.; Buseck, P. R.; Germani, M. S.; Shattuck T. W. Environ. Sci. Technol. 1988, 22, 811-818. (10) Anderson, J. R.; Buseck, P. R.; Saucy, D. A.; Pacyna, J. M. Atmos. Environ. 1992, 26A, 1747-1762. (11) Post, J. E.; Buseck, P. R. Environ. Sci. Technol. 1984, 18, 35-42. (12) De Bock, L. A.; Van Malderen, H.; Van Grieken, R. E. Environ. Sci. Technol. 1994, 28, 1513-1520. (13) Artaxo, P.; Rabello, M. L. C.; Maenhaut, W.; Van Grieken, R. E. Tellus 1992, 44B, 318-334. (14) Hoffmann, P.; Dedik, A. N.; Ensling, J.; Weinbruch, S.; Weber, S.; Sinner, T.; Gutlich, P.; Ortner, H. M. Aerosol Sci. 1996, 27, 327-337. (15) Elbert, M.; Weinbruch, S.; Hoffmann, P.; Ortner, H. M. Aerosol Sci. 2000, 31, 613-632. (16) Tsuji, K.; Wagatsuma, K.; Nullens, R.; Van Grieken, R. E. Anal. Chem. 1999, 71, 2497-2501. (17) Tsuji, K.; Spolnik, Z.; Wagatsuma, K.; Zhang, J.; Van Grieken, R. E. Spectrochim. Acta 1999, B54, 1243-1251.

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The solution presented and discussed in this work is both straightforward and very effective: collect and analyze particles on a grid-supported ultrathin carbon film (0.02-µm thickness) which allows routine, automated analysis of particles down to 0.1 µm. Despite the fact that carbon film substrates have been widely used for the EDX analysis of atmospheric aerosols in transmission electron microscopy4,19-23 (TEM), we are aware of only one work of Gregory et al.3 where ultrathin carbon films were used as a substrate for the automated SEM/EDX particle analysis. However, that work was focused on the analysis of silver halide microcrystals and not on the analysis of atmosphere-related particles. In this work, we present the quantitative results of the automated SEM/EDX for aerosols of known composition deposited directly onto carbon films of 15-25-nm thickness supported by 200-mesh Cu grids. Six laboratory-generated 0.1-2-µm aerosols were tested for their elemental composition, covering the most abundant elements present in atmospheric particles, namely, C, O, N, Na, S, Al, Si, and Cl. The EDX analysis yields quantitative results surprisingly superior to our best expectations, especially for 0.1-0.5-µm particles, including detection of low-Z elements. Due to the extremely low background of the X-rays emitted from the carbon film, the major and minor elemental peaks can be clearly identified in the EDX spectra of particles down to 0.1-µm size, including carbon, nitrogen, and oxygen peaks, an important advantage. Another advantage is that the 25-nm carbon film has a very low backscattered electron (BSE) yield compared to that from the particle, so the particle image is of high contrast and provides effective mapping of particles even smaller than 0.1 µm. This greatly improves the effectiveness of the automated search for the particles by SEM prior to the EDX analysis. The effectiveness includes the high speed of the particle search (number of particles per unit time), a low fraction of “false” particles erroneously identified in the automated mode, and accurate identification of the particle coordinates, size, and shape. Last, it is shown that elemental composition of the 0.1-2-µm particles deposited on the thin films can be reasonably determined by EDX using a simple normalization method only, without corrections for the effects of particle size and shape. Practical recommendations for the use of the grid-supported thin films for the SEM/EDX as well as some minor analytical artifacts are shown and discussed. EXPERIMENTAL SECTION Instrumentation. In this work, a LEO Gemini 982 digital field emission gun scanning electron microscope (FEG-SEM) was used. The working distance was set for 10 mm. The microscope is equipped with two scintillator-type secondary electron detectors (SE-Inlens and SE detectors), solid-state backscattered electron detector, and energy-dispersive X-ray spectrometer. The SE-Inlens detector is integrated above the objective lens of the microscope; the chamber SE detector is mounted on the side wall of the (18) Weinbruch, S.; Wentzel, M.; Kluckner, M.; Hoffmann, P.; Ortner, H. M. Microchim. Acta 1997, 125, 137-141. (19) Posfai, M.; Anderson, J. R.; Buseck, P. R.; Sievering, H. J. Geophys. Res. 1999, 104 (D17), 21685-21693. (20) Posfai, M.; Xu, H,; Anderson, J. R.; Buseck, P. R. J. Geophys. Res. Lett. 1998, 25, 1907-1910. (21) McInnes, L.; Covert, D.; Baker, B. Tellus 1997, 49B, 300-313. (22) McInnes, L. M.; Covert, D. S.; Quinn, P. K.; Germani, M. S. J. Geophys. Res. 1994, 99 (D4), 8257-8268. (23) Mouri, H.; Nagao, I.; Okada, K.; Koga, S.; Tanaka, H. Tellus 1999, 51B, 603-611.

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Figure 1. Aerosol sample preparation setup. The sample powder is placed in dry bubbler and sonicated. Particles are picked up by the airflow and then impacted on the film-coated grid.

microscope chamber. The BSE detector is an annular 10-mm-o.d. ultralow solid-state detector (K. E. Developments Ltd., Cambridge, U.K.). The BSE detector is designed to collect maximal signals at a working distance of 8-10 mm and detects electrons above 400 eV. The detector is placed directly above the specimen, concentric and close to the final lens. The EDX spectrometer is a Pentafet 6901 spectrometer (Oxford Instruments Ltd., Bucks, U.K.) with a lithium-drifted silicon detector of an active area of 10 mm2 and super Atmosphere Thin Window (ATW2) which allows X-rays detection from the elements with atomic number higher than that of berrylium (Z > 4). The EDX detector is located at the takeoff angle of 45°. Materials and Sample Preparation. The EDX elemental analysis tests presented in this study were done for the submicrometer particles prepared from six chemical compounds. All materials were obtained from Aldrich Chemical Co. and used without further purification. Their listed purities are as follows: sodium chloride 99.99%; sodium sulfate 99.99%; aluminum nitride 98%; silicon nitride 99.9%; anthraquinone-2,6-disulfonic acid, disodium salt (C14H6O2(SO3Na)2, ADS) 95%; 2,6-naphthalenedisulfonic acid, disodium salt (C10H6(SO3Na)2, NDS) 97%. The choice of these substances was motivated by two reasons. First, the tests were designed to demonstrate analysis performance for the quantitative detection of most abundant elements in atmospheric aerosols by varying their relative concentrations with the special effort devoted to the analysis of the light elements C, N, and O. Second, except for NaCl, all the substances listed above are either submicrometer powders or can be easily powdered to submicrometer size in mortar. Samples of particles for SEM/EDX analysis were prepared in the same manner for all tested compounds except NaCl. Sample preparation setup is schematically shown in Figure 1. The filtered air first is pumped through a standard bubbler operated dry and then through a one-stage impactor. The bubbler is placed into the ultrasonic bath. The powder of the tested compound is inserted into the bubbler and then sonicated. The airflow picks up particles from the bubbler (mostly of the submicrometer size) and brings them to the impactor. The particles were impacted on the 15-25-nm-thick Carbon Type-B film (Ted Pella, Inc.) supported by a Cu 200-mesh grid, which was fixed to the impaction surface. The impactor used in this study has an orifice of 0.457mm diameter and at the operational flow rate of 1.3 L/min provides

near-unity deposition probability for particles large than 0.27 µm in diameter. Particles with a diameter down to almost 0.1 µm were also found in the aerosol samples, probably as a joint result of their high number concentration and diffusive deposition probability. At these operational conditions, the carbon film was found to be damaged in few central grid cells, with the most of the grid area still suitable for the analysis. Sodium chloride particles were obtained by drying of liquid droplets deposited on the film-coated grid. The liquid droplets were nebulized from a 0.1 M solution of NaCl in water. Automated Individual Particle Analysis. The LEO 982 microscope is equipped with commercially available DiskInspector hardware and software (Oxford Instruments, Ltd.) This is an automated particle analysis system, developed for the specific needs of the semiconductor industry and forensic laboratories to locate and analyze particles sparsely dispersed on flat substrates. The system allows setting of specific areas to be fully inspected in the unattended run. It automatically “tiles” the sample area with fields of view at the working magnification, and then the area is automatically inspected on a field-by-field basis. In each field of view, particles are recognized by an increase of the detector signal above a selectable threshold level. For the chemical substances presented in this work, particles were imaged using mostly the BSE signal; however, imaging of ADS and NDS particles was acquired using mixed SE-Inlens and BSE signals, as will be discussed later. As soon as the contour of the particles in the single field of view is detected, the electron beam rasters continuously over the particle projection area and the EDX spectra are acquired for the detected particles. The system was configured to detect and analyze particles with the equivalent diameter of the projection area higher than 0.1 µm. Magnification of 1800× was used, X-ray spectra were acquired for 10 s of live time over a 0-20-kV energy range, at the beam current of 150 pA and an accelerating voltage of 20 kV. The relative dead times of the EDX spectrometer never exceeded 30%. The X-ray collection time was kept short (10 s) as needed to examine thousands of particles in an overnight run. The choice of the working conditions will be discussed in the next section. For quantitation of the EDX results, DiskInspector software utilizes a simple normalization method which does not include correction24,25 for the effects of particle size and shape. The apparent particle composition is determined first from the measured intensities of the X-ray spectral peaks relative to intensities of thick, flat standards, given by

CiN ) Ci/

∑C

i

The X-ray intensities of the tested compounds are evaluated using a filtered least-squares fitting procedure to fit experimental spectra with the set of stored X-ray “library” peaks (profiles). Except for the nitrogen profile, the set of default “library” profiles provided by Oxford Instruments is used. Standard default profiles were prepared26 using a JEOL 6400 SEM at an accelerating voltage of 20 kV from the analysis of the following flat standards: Quartz (SiO2) for Si and O, KCl for Cl, Al2O3 for Al, FeS2 for S, CaCO3 for C, and albite (NaAlSi3O8) for Na. The default nitrogen profile was not evaluated experimentally by the manufacturer, but set as an arbitrary profile, which is considerably in error for our needs. The current version of the DiskInspector software does not include an option to replace or reestablish the default profiles. To compensate for this, we scaled the EDX-measured nitrogen concentrations for the AlN aerosol with the scaling factor of 2.5 in order to fit the corresponding nominal concentration. The obtained scaling factor was then verified against EDX data for the Si3N4 aerosol, which nominal elemental composition was also correctly predicted using the same scaling factor of 2.5. Elemental compositions in this work are reported as normalized weight percents for all tested compounds.

where Ci and Ci st are the compositional weight percent of element i in the tested compound and in the standard and Ii and Ii st are the corresponding intensities of the most abundant X-ray spectral line for given element, respectively. Then the elemental composition is reported in the normalized form, given by

RESULTS AND DISCUSSION As has been pointed out in the introduction, the major difficulty in the EPMA of small particles is their high transparency for the primary electron beam. For submicrometer-size particles, particularly those composed from light elements, the electron beam penetrates the entire particle with substantial lateral scattering of electrons transmitted and escaped through the particle sides. The transmitted/scattered electrons can create a substantial X-ray signal if they strike nearby objects. Typically, to control this problem, the film-coated grid is placed over a blind hole in an electron-absorbent material such as graphite.27 However, this causes a large carbon X-ray peak to appear, interfering with the carbon content of the particles. Instead, the grid was placed over a hole in a piece of copper foil with nothing but vacuum below the hole. The electrons transmitted either through the film or through the particle pass freely all the way down to the chamber floor. Electrons and X-rays scattered and emitted from the chamber floor are spread out into a wide solid angle and thus are hardly detected by electron or X-ray detectors. In addition, the copper foil of the sample holder is wide enough (∼4 cm2 area) to provide effective screening of the BSE and SE-Inlens detectors from the electrons scattered from the floor. This setup provides almost 100% transmission efficiency for the electron beam through the areas of the film without particles and results (a) in very low (dark) background electron signal, especially in the BSE mode and (b) in exceptionally low X-ray background. For the automated work over an extended period of time, the BSE signal is preferred over the SE signal because of its high signal stability. However, “low atomic number” compounds,

(24) Armstrong, J. T. In Electron Probe Quantitation; Heinrich, K. F. J., Newbury, D. E., Eds.; Plenum Press: New York and London, 1991; pp 261-315. (25) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Roming, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed.; Plenum Press: New York, 1992; Chapters 8 and 9.

(26) Betts, M. Oxford Instruments, Ltd., Microanalysis group, 45950 Hotchkiss St., Fremont, CA 94539, private communication. (27) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Roming, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed.; Plenum Press: New York, 1992; Chapter 11.

Ci ) (Ii/Ii st)Ci st

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Figure 3. EDX spectrum of 0.2-µm ADS particle supported by a carbon film compared to that from the carbon film alone.

Figure 2. BSE (a) and SE-Inlens (b) images of NaCl and ADS particles in the same single field view of the microscope. NaCl particles (square shaped) are effectively imaged in both BSE and Se-Inlens detection modes. ADS particles (irregular shaped) are imaged in SE-Inlens mode much better than in BSE mode, especially particles below 0.3-µm size.

particularly those with high carbon concentration, are poorly seen in the BSE mode due to low backscattered electron yield of light elements. Figure 2 shows an example of the same single-field image of NaCl and ADS particles deposited together on the same substrate. The images are obtained using the BSE (a) and the SE-Inlens (b) detectors and taken at the same beam and working distance conditions as were used for the automated analysis (150 pA and 10 mm). Higher magnification of 20000× is used for clarity here. As can be seen, NaCl particles (square shaped) can be effectively imaged in both BSE and SE-Inlens modes, while ADS particles (irregularly shaped) are seen rather poorly in the BSE mode due to their relative “low atomic number” elemental composition. Many of the 0.2 µm and smaller ADS particles are perfectly seen in the SE-Inlens mode (Figure 2b); however, they do not give even a single electron in the BSE mode (Figure 2a). Higher imaging contrast for the ADS particles can be obtained at higher beam currents; however, higher currents result in higher intensities of the emitted X-ray and lead to coincidence problems in the EDX spectrometer. For EDX analysis of light elements C, N, and O, the processing time of the EDX spectrometer should be set as long as possible providing higher energy resolution to 1026 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

distinguish between their close-coupled X-ray peaks. On the other hand, higher processing time drops down the maximal count rate acceptable for the EDX spectrometer. Therefore, beam current must be optimized in order to keep the highest energy resolution of the EDX spectrometer and not to exceed 20-30% of the relative dead times. In this work, the beam current was kept as low as 150 pA, allowing proper work of the EDX spectrometer at highest (64 µs) processing time. The imaging of “low atomic number” compounds, namely, ADS and NDS, was optimized using mixed BSE and SE-Inlens signals. In this approach, the SE-Inlens signal provides adequate particle contrast over the carbon film background and addition of the BSE signal gives higher stability of the mixed signal compared to the signal from the SE-Inlens detector only. High electron transparency of the 0.02-µm carbon film makes EDX analysis feasible at very low intensities of the X-ray signals for all elements, surprisingly including carbon. Figure 3 shows the EDX spectrum collected from a typical 0.2-µm ADS particle compared to that from the carbon film itself (10-s X-ray collection). Even for such small particles, 10 s of the live time is enough to collect 1000-2000 photon counts per each characteristic peak, which corresponds to ∼5000 counts from the entire particle. The integrals of all the ADS elemental peaks shown in Figure 3 are at the level of 100-200 counts/s including a carbon peak (∼100 counts/s) which is raised significantly above the background level of the carbon film (∼15-30 counts/s). Thus, for 10 s average a relative error due to the counting statistics can be estimated as ∼3% (10000.5/1000) for all the elements in the ADS particle. We note that the background of the particle spectra has a complicated nature and can be seriously affected by the particle composition and size. Indeed, appearance of a copper peak in the ADS particle spectrum comes from electrons scattered sideways by the particle. The scattered electrons reach the bars of the copper grid, hit them, and generate characteristic copper X-rays. However, the high transparency of the film helps minimize this effect. This is illustrated by the EDX analysis of the particles that contain no carbon, namely, NaCl particles. Figure 4 shows the quantitative EDX results obtained in the automated analysis of ∼1000 of NaCl particles. Plot a presents the arbitrary compositional weight percents of C, Cu, and sodium chloride (Na + Cl) obtained from the original particle spectra and

Figure 5. Normalized elemental composition of Na2SO4 particles in weight percent, measured by EDX (dots), normalized after ignoring Cu. Dashed lines correspond to the nominal Na2SO4 elemental composition. Error bars show standard deviations from the mean values in each size bin.

Figure 4. Normalized elemental composition of NaCl particles in weight percent, measured by EDX. Part a shows NaCl arbitrary concentration with respect to the level of copper and carbon backgrounds, normalized to the sum of Na, Cl, Cu, and C concentrations. Part b shows composition of NaCl particles measured by EDX (dots) normalized after ignoring Cu and C. Dashed lines correspond to the nominal NaCl elemental composition.

normalized to the total signal from C, Cu, Na, and Cl. Plot b presents the normalized Na and Cl weight percents obtained after excluding of carbon and copper from the original results. The extent of the copper contribution in the EDX measured results for NaCl particles (Figure 4a) can be viewed as an indicator of the sideway scattering of the electrons by the particles. Very large particles (∼10 µm) behave like bulk samples and give minimal sideward scattering. This results in the low copper background. As the particle size decreases down to the electron penetration volume (∼1 µm3), the amount of the sideward scattered electrons increases and the corresponding copper background increases as well. Further decrease of the particle size makes the particle more and more transparent to the electron beam, so the electrons mostly traverse the particle downward with little spreading of the beam sideward. The corresponding copper background decreases and reaches its lowest value for 0.5-0.6-µm particles. The rise of the copper background for very small NaCl particles (0.1-0.5 µm) is due to the fast drop in the intensities of the X-rays emitted from particles compared to those emitted from the copper grid as a result of the sideward electron scattering. However, as can be seen in Figure 4, while the level of the copper background goes

Figure 6. Normalized elemental composition of AlN particles in weight percent, measured by EDX (dots), normalized after ignoring Cu. Dashed lines correspond to the nominal AlN elemental composition. Error bars show standard deviations from the mean values in each size bin.

up and down as a function of particle size, the carbon background contributes negligibly in the EDX results of NaCl particles for almost the entire size range covered by the analysis. The exceptions are some single analyses of the smallest particles (0.10.3 µm), where intensities of the X-rays emitted from the particle may be as low as the level of the X-rays generated in the carbon film (∼15-30 counts/s). The EDX analysis of thousands of Na2SO4, AlN, and Si3N4 particles revealed the same results; the relative contribution of the carbon background was always within a few weight percent for any tested compound with the exception of several single analyses of 0.1-0.3-µm particles. Figures 5-9 show the composition of Na2SO4, AlN, Si3N4, ADS, and NDS aerosols measured by the automated EDX and normalized after ignoring copper. The results are plotted showing both the average composition and the Gaussian spread in each particle bin, binned after individual analysis of large number of particles. Each bin represents an average result for 100-300 individual measurements. About 1000 particles were analyzed totally for each Na2SO4, AlN, and Si3N4 aerosol sample. The composition of ADS Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

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Figure 7. Normalized elemental composition of Si3N4 particles in weight percent, measured by EDX (dots), normalized after ignoring Cu. Dashed lines correspond to the nominal Si3N4 elemental composition. Error bars show standard deviations from the mean values in each size bin.

Figure 9. Normalized elemental composition of NDS particles in weight percent, measured by EDX (dots), normalized after ignoring Cu. Dashed lines correspond to the nominal NDS elemental composition. Error bars show standard deviations from the mean values in each size bin.

Figure 8. Normalized elemental composition of ADS particles in weight percent, measured by EDX (dots), normalized after ignoring Cu. Dashed lines correspond to the nominal ADS elemental composition. Error bars show standard deviations from the mean values in each size bin.

and NDS aerosols is presented after individual analysis of ∼3000 particles, which were analyzed in three independent samples for each compound. We have excluded from the analysis results for some erroneously identified particles originating from the follow1028 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

ing: (1) When the bars of the copper grid are seen in the single field of view they can be erroneously treated as particles. Normally the image of the bar is much larger than any reasonable particle size; thus setting the cutoff size at 10 µm effectively rejects copper bars from the automated analysis. However, when the bar is seen just near the edge of the single field of view, its projection area may be within the cutoff criterion; in this case, the piece of the grid will be still detected and analyzed as a particle. (2) DiskInspector software first maps the particle in each single field of view, and then the subsequent EDX spectra are acquired for each detected particle one by one. As a result of the image drift, X-ray analysis will sometimes miss very small particles, especially the particles being analyzed last in a single field of view. The drift problem becomes to be more serious for the fields of view with a high concentration of particles. (3) During many hours of the automated run, the mixed Se-Inlens/BSE signal from the carbon film will drift somewhat. As a result, Se-Inlens/BSE signal may exceed the threshold value and cause the software to treat some substrate region as a particle. To eliminate these erroneous artifacts, we excluded from the results all the particles that showed total concentration of carbon and copper together greater than 90 wt %. The fraction of the excluded particles was different for different aerosols. For NaCl, only a few percent of particles were

excluded, while for the ADS and NDS, the fraction of the excluded particles increased to 30-35% due to critical setting of the threshold level in these cases. The results shown in Figures 4-9 are largely self-evident; the EDX-measured compositions are fairly accurate even down to the 0.1-0.2-µm size range for all aerosols presented in this work. As can be expected, the ADS and NDS particles are analyzed with less success than the noncarbonaceous aerosols. These organic particles are difficult to analyze due to their susceptibility to beam damage, low contrast in the SEM images, low atomic mass, and possible contamination of the carbon signal from the film background. Nevertheless, the analyses we achieve are semiquantitatively ((15% units of scatter) useful down to about 0.1-0.2 µm, where the background signal from the carbon film might play a serious role. Despite the fact that sulfur-containing compounds are known4 to be sensitive to the electron beam, the EDX analyses of Na2SO4, ADS, and NDS particles show no major beam damage for these particles at the present operation conditions. This is probably due to our ability to use only 10 s of the X-ray acquisition time, low beam current, and the automated rastering of the beam over the particle during the X-ray analysis. As can be seen from Figures 4-9, the experimentally obtained concentration profiles show some systematic deviations from the corresponding lines of nominal concentrations. For all the substances presented in this work, the apparent concentration of low atomic number elements tends to be slightly underestimated while the apparent concentration of elements with relatively high atomic number tends to be slightly overestimated. The best match of the experimental profiles with the nominal concentrations is usually obtained near 1 µm of particle size. In this work, the EDXmeasured particle compositions are not corrected for sensitivity variations related to X-ray and electron penetration/scattering effects that are particle size dependent. For submicrometer particles, the corresponding size and shape corrections24,28 typically cause some (5-(15% of systematic deviations from nominal concentrations. When better accuracy is required, these calculations can and should be applied. The largest uncertainties in the EDX results are obtained for carbonaceous ADS and NDS aerosols that are affected by the contribution of carbon background, which leads to (15 wt % of scatter in the measured concentrations (Figures 8 and 9). Even so, the analyses we achieved in this study are still semiquantitatively useful for characterization and classification of atmospheric aerosols in the size range of 0.1-2.5 µm including detection of low-Z elements. Due to very low carbon peak background, carbonaceous sulfur-bearing particles such as ADS and NDS with a nominal C/Na/S/O composition of 41/11/

16/32 and 37/14/20/29 wt %, respectively (Figures 8 and 9), can be clearly distinguished from the noncarbonaceous sodium sulfate particles with a C/Na/S/O composition of 0/32/23/45 wt % (Figure 5). However, demonstrated accuracy of the analysis is still not enough to distinguish between similar carbonaceous particles such as ADS and NDS, if they were analyzed in the same mixture. Nevertheless, the use of the carbon films rather than traditional polycarbonate filters as substrates expends drastically the ability of SEM/EDX to distinguish, analyze, and classify submicrometer particles including semiquantitative analysis of their carbon content.

(28) Ro, Ch.-U.; Osa´n, J.; Van Grieken, R. Anal. Chem. 1999, 71, 1521-1528.

AC0009604

CONCLUSION The advantage of the use of the thin supported carbon films for the automated SEM/EDX single particle analysis can be summarized as follows. 1. Particles deposited on the thin carbon films are seen with superior contrast in both BSE and SE modes of detection. The imaging of particles composed from the low atomic number elements may be optimized using the mixed BSE/SE signal. This greatly improves performance and speed of the particle imaging prior the EDX analysis. 2. Thin carbon films yield very low carbon peak X-ray background and low broadband background in the X-ray particle spectra. This allows the unique ability for quantitative EDX measurement of the particle composition including analysis of low-Z elements: oxygen, nitrogen, and particularly carbon. 3. For the size range of 0.1-2.0 µm, the elemental composition of the particles deposited on the carbon films can be reasonably estimated from the EDX spectra using the simplest quantitative approach. Altogether, the suggested approach seems to be very promising for significant improvement of the automated single-particle analysis in the electron microprobe-based techniques, which usually provide no information on low-Z elements below ∼0.5 µm. ACKNOWLEDGMENT This work was supported by Laboratory Directed Research and Development funds of Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. The authors appreciate the cooperation of J. S. Young, who provided important guidance and an access to the LEO 982 scanning electron microscope used in this study. Received for review August 14, 2000. Accepted November 27, 2000.

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