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Analysis of surface composition and internal structure of fly ash particles using an ion and electron multibeam microanalyzer
Applied Surface Science 203±204 (2003) 762±766
Analysis of surface composition and internal structure of ¯y ash particles using an ion and electron multibeam microanalyzer T. Sakamotoa,*, K. Shibatab, K. Takanashib, M. Owaria,b, Y. Niheic a
Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Institute of Industrial Science, The University of Tokyo, Tokyo 113-0033, Japan c Faculty of Science and Technology, Science University of Tokyo, Tokyo, Japan
Abstract An ion and electron multibeam microanalyzer was developed and applied to analysis of coal ¯y ash particles. Employing ordinary TOF-SIMS function, it was found that the surface of the ¯y ash particles mainly consisted of Ca, C, Si, and Na. A special analysis technique with a combination of ``shave-off'' cross-sectioning and TOF-SIMS mapping of the cross section was adopted to a single ¯y ash particle in order to reveal the internal structure. It was found that the particle had a cenosphere structure. TOF-SIMS mapping of the cross-sectioned particle clari®ed that the particle had the following layers, outermost layer (Na, Si, Ca-rich), shell (Na-rich), inner shell (Na, Si, Al-rich). # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; 3D-analysis; FIB; Fly ash; Cenosphere
1. Introduction Detailed information on both the surface and internal structures of microparticles, e.g. combustion ¯y ash particles, plays an important role in the development of new analytical techniques for toxic compounds on and beneath the surfaces. In recent years, analysis of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) or polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs) becomes an urgent issue. Current analytical procedure employs Soxhlet method to extract organic pollutants from ¯y ash particles prior to clean-up, concentration and GC±MS analysis. However, Soxhlet extraction is
*
Corresponding author. Tel.: 81-3-5841-2975; fax: 81-3-5841-2976. E-mail address: [email protected] (T. Sakamoto).
time-consuming and produces a large quantity of organic waste ¯uids. Recently, a novel extraction technique ``supercritical ¯uid extraction'' (SFE) has been investigated as a substitute for Soxhlet extraction. Since supercritical CO2 is used as a solvent, SFE produces no organic ef¯uents and realizes extremely high-speed extraction of organic compounds from solid matrices. It was however pointed out that the extraction ef®ciency of organic compounds from real ¯y ash particles is frequently different from that from spiked standard samples [1]. This difference implies that the extraction ef®ciency is in¯uenced by not only the strength of adsorption of organic compounds to particles, but surface and internal structures, e.g. pores or covering layers. The aim of our study is the application of our novel SIMS analytical technique to clarify both surface composition and internal structure of microparticles.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 7 8 - 4
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
In this paper, the result of surface and cross-sectional analysis of combustion ¯y ash particles using an ion and electron multibeam microanalyzer is described. 2. Experimental Fly ash particles collected with an electrostatic precipitator in a coal thermal power plant were used as the sample. The sample was known to contain PCDDs and PCDFs concentration at 0.20 ng-TEQ/g by means of conventional Soxhlet extraction and GC± MS analysis. For SIMS measurements, the particles were ®xed on an in plate without any pretreatment except grinding with a mortar. An ion and electron mutibeam microanalyzer [2,3] developed by our group was used for all the measurements. The apparatus is equipped with two FIB columns (FI-1000 Eiko-Engineering and IOG25 Ionoptika) and a ®eld emission electron beam column (AP-70900, JEOL). As for analyzers, a two-stage re¯ectron time-of-¯ight mass spectrometer (TOFMS, laboratory made) and a cylindrical mirror analyzer (CMA, PHI) are installed in the apparatus. With a combination of the pulsed-FIB (FI-1000) and TOFMS, microarea mapping analysis of surfaces can be performed. In addition to this ordinary TOF-SIMS function, the apparatus has a special operation mode named ``shave-off'' cross-sectioning [4]. In the shaveoff mode, a horizontal scanning FIB is moved very slowly to vertical direction only for one scanning frame. During shave-off scan, a ¯at cross section continuously is developed at the vertical scan position of the FIB. Employing this method, a ¯at cross section at precisely controlled position can be obtained continuously or intermittently. TOF-SIMS measurements of the ¯y ash particles within a 100 mm 100 mm area were performed with the pulsed-FIB operated at 15 keV energy, 1 nA dc current, 10 kHz pulse repetition rate, 40 ns pulse width, and an incident angle of 458. Mass resolutions were estimated to be 300 at m/z 40 and 500 at m/z 113. Cross-sectioning and secondary electron image observation were performed with the other FIB (IOG-25) operated at 25 keV energy, 500 pA (for cross-sectioning) and 100 pA (imaging) currents. After the cross-sectioning, TOF-SIMS mapping was
763
performed with the pulsed-FIB at the same condition mentioned above except for an energy of 20 keV. 3. Results and discussion 3.1. Surface compositions of particles The ®rst measurements aimed to investigate the surface composition of the ¯y ash particles. Two TOF-SIMS spectra of the particles around a few micrometer size located within a 100 mm 100 mm area were acquired sequentially. Obtained TOF-SIMS spectrum with an FIB dose around the static-limit (2:4 1013 ions/cm2) is shown in Fig. 1(a). Many secondary ion mass peaks of elements and small organic fragments, e.g. CHn were detected suggesting existence of organic compounds on the surfaces. The other spectrum shown in Fig. 1(b) was acquired after FIB bombardment of 3:7 1016 ions/cm2 in direct current mode. By the FIB bombardment, it was thought that surface organic compounds had been destructed. Therefore, spectrum (b) involves mostly inorganic secondary ion signals. Evidently, signals due to organic fragment ions (CHn, C2H5, CH3Na, m/ z 53, 54, etc.) were greatly reduced in spectrum (b). As for the inorganic composition, the concentration ratio of major elements was estimated from spectrum (b) as summarized in Table 1. The ratio was calculated using relative sensitivity factors for FIB-SIMS [5]. This result shows that the ¯y ash particles consisted of mainly Ca, C, Si, and Na. Since chloride, oxide and hydroxide peaks were also detected, these elements should exist as these compounds.
Table 1 Concentration ratio of major elements detected from surfaces (sum of concentrations of these eight elements was assumed to be 100%) Concentration Li C Na Mg Al Si K Ca
0.03 32 10 3 4 11 4 36
764
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
Fig. 1. TOF-SIMS spectra from ¯y ash particles acquired with around the static-limit (a) and over the limit (b) of Ga FIB dose. Mass peaks whose intensities were decreased in spectrum (b) are labeled with slanted letters, whereas increased or sustained peaks with underlines.
3.2. Cross-sectioning of a particle In the next experiment, we focused our attention on the internal structure of ¯y ash particle. A ¯y ash particle with a diameter of 28 mm was cross-sectioned up to half of the particle by shave-off method for 80 min. FIB-induced secondary electron images of the cross-sectioned particle are shown in Fig. 2 (a) and (b).
It is clear that this particle had a hollow structure called ``cenoshpere''. The thickness of its shell was estimated to be 5 mm. The shell consisted of an inner shell (part i) with a thickness of 4 mm and an outer layer of deposits (part ii) with 1 mm thickness. Furthermore, there was a very thin bright layer (part iii) suggesting the existence of constituents with high secondary electron yields.
Fig. 2. FIB-induced secondary electron images of a cross-sectioned ¯y ash particle, (a) front view, and (c) top view.
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
765
Fig. 3. Secondary electron image, total secondary ion image and TOF-SIMS maps of the cross-sectioned cenosphere particle. (Field of view: 60 mm 60 mm, number of pixels: 64 64).
3.3. TOF-SIMS analysis of the cross section TOF-SIMS mapping was performed on the crosssectioned cenosphere particle. Obtained TOF-SIMS maps are shown in Fig. 3. In the total ion image, intense signals were detected mainly from the circular region corresponding to the shell. Secondary ion signals of displayed elements were also intense at the shell except for Ca and CaOH. This result means that Ca and its salt mainly exists not in the shell but on the outermost deposition layer. To clarify main constituents of (i) inner surface, (ii) spherical shell, and (iii) deposition layer, the mass spectra of region of interest (ROI) were extracted in the next. Within the mapped region, we selected three ROIs as shown in Fig. 4. Ratio of atomic concentrations of each ROI estimated using RSFs are summarized in Table 2. Each layer showed its characteristic composition. The inner surface, which was observed bright in secondary electron image (Fig. 2(a)), was mainly composed of Na, Al, and Si. It is a feature that this part contained Al at higher concentration which must be resulted in the higher brightness of inner surface in secondary electron image (Fig. 2 (a)). The cross section of the shell mostly composed of Na. As for the deposition layer, Ca concentration was much higher than the other regions. This result is consistent with averaged mass spectrum of ¯y ash surfaces shown in Fig. 1. Considering 57 CaOH map in Fig. 3, Ca atoms exist in a form of hydroxide or oxide.
Fig. 4. Regions of interest: (a) inner surface, (b) shell, and (c) deposit.
Table 2 Ratios of atomic concentrations estimated from ROIs in Fig. 4
Inner surface Shell cross section Deposit cross section
Na
Mg
Al
Si
K
52 81 29
0.4 1.4 2.9
14 27 3.6 4.2 9.8 2.3 8.5 23 4.0
Ca 2.7 1.3 33
766
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
Summarizing the result, it is concluded that the cenosphere particle can be divided into three layers: (i) Al-containing inner surface, (ii) spherical shell mainly consist of Na with a thickness of 4 mm, and (iii) oxide or hydroxide compounds of Na, Ca, and Si-rich deposition layer.
Acknowledgements The authors would like to thank Dr. S. Oishi (National Institute of Advanced Industrial Science and Technology) for sample preparation. References
4. Conclusion An ion and electron multibeam microanalyzer developed by our group was applied to surface and internal composition analysis of ¯y ash particles. Both averaged surface composition and the internal structure of a cenosphere particle were revealed by the experiments. These kinds of information will give important knowledge for quantitative extraction and data interpretation of organic compounds from ¯y ash particles.
[1] E. BjoÈrklund, T. Nilsson, S. Bùwadt, K. Pilorz, L. Mathiasson, S.B. Hawthorne, J. Biochem. Biophys. Methods 43 (2000) 295. [2] M. Owari, T. Sakamoto, Zh. H. Cheng, M. Takahashi, Y. Nihei, ECASIA 97, in: I. Olefjord, L. Nyborg, D. Briggs (Eds.), Wiley, Chichester, 1997, p. 1085. [3] T. Sakamoto, Zh.H. Cheng, M. Takahashi, Y. Kuramoto, M. Owari, Y. Nihei, Jpn. J. Appl. Phys. 37 (1998) 2051. [4] H. Satoh, M. Owari, Y. Nihei, J. Vaccine Sci. Technol. B6 (1988) 915. [5] H. Satoh, M. Owari, Y. Nihei, Jpn. J. Appl. Phys. 32 (1993) 3616.
Analysis of surface composition and internal structure of ¯y ash particles using an ion and electron multibeam microanalyzer T. Sakamotoa,*, K. Shibatab, K. Takanashib, M. Owaria,b, Y. Niheic a
Environmental Science Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Institute of Industrial Science, The University of Tokyo, Tokyo 113-0033, Japan c Faculty of Science and Technology, Science University of Tokyo, Tokyo, Japan
Abstract An ion and electron multibeam microanalyzer was developed and applied to analysis of coal ¯y ash particles. Employing ordinary TOF-SIMS function, it was found that the surface of the ¯y ash particles mainly consisted of Ca, C, Si, and Na. A special analysis technique with a combination of ``shave-off'' cross-sectioning and TOF-SIMS mapping of the cross section was adopted to a single ¯y ash particle in order to reveal the internal structure. It was found that the particle had a cenosphere structure. TOF-SIMS mapping of the cross-sectioned particle clari®ed that the particle had the following layers, outermost layer (Na, Si, Ca-rich), shell (Na-rich), inner shell (Na, Si, Al-rich). # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; 3D-analysis; FIB; Fly ash; Cenosphere
1. Introduction Detailed information on both the surface and internal structures of microparticles, e.g. combustion ¯y ash particles, plays an important role in the development of new analytical techniques for toxic compounds on and beneath the surfaces. In recent years, analysis of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) or polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs) becomes an urgent issue. Current analytical procedure employs Soxhlet method to extract organic pollutants from ¯y ash particles prior to clean-up, concentration and GC±MS analysis. However, Soxhlet extraction is
*
Corresponding author. Tel.: 81-3-5841-2975; fax: 81-3-5841-2976. E-mail address: [email protected] (T. Sakamoto).
time-consuming and produces a large quantity of organic waste ¯uids. Recently, a novel extraction technique ``supercritical ¯uid extraction'' (SFE) has been investigated as a substitute for Soxhlet extraction. Since supercritical CO2 is used as a solvent, SFE produces no organic ef¯uents and realizes extremely high-speed extraction of organic compounds from solid matrices. It was however pointed out that the extraction ef®ciency of organic compounds from real ¯y ash particles is frequently different from that from spiked standard samples [1]. This difference implies that the extraction ef®ciency is in¯uenced by not only the strength of adsorption of organic compounds to particles, but surface and internal structures, e.g. pores or covering layers. The aim of our study is the application of our novel SIMS analytical technique to clarify both surface composition and internal structure of microparticles.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 7 8 - 4
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
In this paper, the result of surface and cross-sectional analysis of combustion ¯y ash particles using an ion and electron multibeam microanalyzer is described. 2. Experimental Fly ash particles collected with an electrostatic precipitator in a coal thermal power plant were used as the sample. The sample was known to contain PCDDs and PCDFs concentration at 0.20 ng-TEQ/g by means of conventional Soxhlet extraction and GC± MS analysis. For SIMS measurements, the particles were ®xed on an in plate without any pretreatment except grinding with a mortar. An ion and electron mutibeam microanalyzer [2,3] developed by our group was used for all the measurements. The apparatus is equipped with two FIB columns (FI-1000 Eiko-Engineering and IOG25 Ionoptika) and a ®eld emission electron beam column (AP-70900, JEOL). As for analyzers, a two-stage re¯ectron time-of-¯ight mass spectrometer (TOFMS, laboratory made) and a cylindrical mirror analyzer (CMA, PHI) are installed in the apparatus. With a combination of the pulsed-FIB (FI-1000) and TOFMS, microarea mapping analysis of surfaces can be performed. In addition to this ordinary TOF-SIMS function, the apparatus has a special operation mode named ``shave-off'' cross-sectioning [4]. In the shaveoff mode, a horizontal scanning FIB is moved very slowly to vertical direction only for one scanning frame. During shave-off scan, a ¯at cross section continuously is developed at the vertical scan position of the FIB. Employing this method, a ¯at cross section at precisely controlled position can be obtained continuously or intermittently. TOF-SIMS measurements of the ¯y ash particles within a 100 mm 100 mm area were performed with the pulsed-FIB operated at 15 keV energy, 1 nA dc current, 10 kHz pulse repetition rate, 40 ns pulse width, and an incident angle of 458. Mass resolutions were estimated to be 300 at m/z 40 and 500 at m/z 113. Cross-sectioning and secondary electron image observation were performed with the other FIB (IOG-25) operated at 25 keV energy, 500 pA (for cross-sectioning) and 100 pA (imaging) currents. After the cross-sectioning, TOF-SIMS mapping was
763
performed with the pulsed-FIB at the same condition mentioned above except for an energy of 20 keV. 3. Results and discussion 3.1. Surface compositions of particles The ®rst measurements aimed to investigate the surface composition of the ¯y ash particles. Two TOF-SIMS spectra of the particles around a few micrometer size located within a 100 mm 100 mm area were acquired sequentially. Obtained TOF-SIMS spectrum with an FIB dose around the static-limit (2:4 1013 ions/cm2) is shown in Fig. 1(a). Many secondary ion mass peaks of elements and small organic fragments, e.g. CHn were detected suggesting existence of organic compounds on the surfaces. The other spectrum shown in Fig. 1(b) was acquired after FIB bombardment of 3:7 1016 ions/cm2 in direct current mode. By the FIB bombardment, it was thought that surface organic compounds had been destructed. Therefore, spectrum (b) involves mostly inorganic secondary ion signals. Evidently, signals due to organic fragment ions (CHn, C2H5, CH3Na, m/ z 53, 54, etc.) were greatly reduced in spectrum (b). As for the inorganic composition, the concentration ratio of major elements was estimated from spectrum (b) as summarized in Table 1. The ratio was calculated using relative sensitivity factors for FIB-SIMS [5]. This result shows that the ¯y ash particles consisted of mainly Ca, C, Si, and Na. Since chloride, oxide and hydroxide peaks were also detected, these elements should exist as these compounds.
Table 1 Concentration ratio of major elements detected from surfaces (sum of concentrations of these eight elements was assumed to be 100%) Concentration Li C Na Mg Al Si K Ca
0.03 32 10 3 4 11 4 36
764
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
Fig. 1. TOF-SIMS spectra from ¯y ash particles acquired with around the static-limit (a) and over the limit (b) of Ga FIB dose. Mass peaks whose intensities were decreased in spectrum (b) are labeled with slanted letters, whereas increased or sustained peaks with underlines.
3.2. Cross-sectioning of a particle In the next experiment, we focused our attention on the internal structure of ¯y ash particle. A ¯y ash particle with a diameter of 28 mm was cross-sectioned up to half of the particle by shave-off method for 80 min. FIB-induced secondary electron images of the cross-sectioned particle are shown in Fig. 2 (a) and (b).
It is clear that this particle had a hollow structure called ``cenoshpere''. The thickness of its shell was estimated to be 5 mm. The shell consisted of an inner shell (part i) with a thickness of 4 mm and an outer layer of deposits (part ii) with 1 mm thickness. Furthermore, there was a very thin bright layer (part iii) suggesting the existence of constituents with high secondary electron yields.
Fig. 2. FIB-induced secondary electron images of a cross-sectioned ¯y ash particle, (a) front view, and (c) top view.
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
765
Fig. 3. Secondary electron image, total secondary ion image and TOF-SIMS maps of the cross-sectioned cenosphere particle. (Field of view: 60 mm 60 mm, number of pixels: 64 64).
3.3. TOF-SIMS analysis of the cross section TOF-SIMS mapping was performed on the crosssectioned cenosphere particle. Obtained TOF-SIMS maps are shown in Fig. 3. In the total ion image, intense signals were detected mainly from the circular region corresponding to the shell. Secondary ion signals of displayed elements were also intense at the shell except for Ca and CaOH. This result means that Ca and its salt mainly exists not in the shell but on the outermost deposition layer. To clarify main constituents of (i) inner surface, (ii) spherical shell, and (iii) deposition layer, the mass spectra of region of interest (ROI) were extracted in the next. Within the mapped region, we selected three ROIs as shown in Fig. 4. Ratio of atomic concentrations of each ROI estimated using RSFs are summarized in Table 2. Each layer showed its characteristic composition. The inner surface, which was observed bright in secondary electron image (Fig. 2(a)), was mainly composed of Na, Al, and Si. It is a feature that this part contained Al at higher concentration which must be resulted in the higher brightness of inner surface in secondary electron image (Fig. 2 (a)). The cross section of the shell mostly composed of Na. As for the deposition layer, Ca concentration was much higher than the other regions. This result is consistent with averaged mass spectrum of ¯y ash surfaces shown in Fig. 1. Considering 57 CaOH map in Fig. 3, Ca atoms exist in a form of hydroxide or oxide.
Fig. 4. Regions of interest: (a) inner surface, (b) shell, and (c) deposit.
Table 2 Ratios of atomic concentrations estimated from ROIs in Fig. 4
Inner surface Shell cross section Deposit cross section
Na
Mg
Al
Si
K
52 81 29
0.4 1.4 2.9
14 27 3.6 4.2 9.8 2.3 8.5 23 4.0
Ca 2.7 1.3 33
766
T. Sakamoto et al. / Applied Surface Science 203±204 (2003) 762±766
Summarizing the result, it is concluded that the cenosphere particle can be divided into three layers: (i) Al-containing inner surface, (ii) spherical shell mainly consist of Na with a thickness of 4 mm, and (iii) oxide or hydroxide compounds of Na, Ca, and Si-rich deposition layer.
Acknowledgements The authors would like to thank Dr. S. Oishi (National Institute of Advanced Industrial Science and Technology) for sample preparation. References
4. Conclusion An ion and electron multibeam microanalyzer developed by our group was applied to surface and internal composition analysis of ¯y ash particles. Both averaged surface composition and the internal structure of a cenosphere particle were revealed by the experiments. These kinds of information will give important knowledge for quantitative extraction and data interpretation of organic compounds from ¯y ash particles.
[1] E. BjoÈrklund, T. Nilsson, S. Bùwadt, K. Pilorz, L. Mathiasson, S.B. Hawthorne, J. Biochem. Biophys. Methods 43 (2000) 295. [2] M. Owari, T. Sakamoto, Zh. H. Cheng, M. Takahashi, Y. Nihei, ECASIA 97, in: I. Olefjord, L. Nyborg, D. Briggs (Eds.), Wiley, Chichester, 1997, p. 1085. [3] T. Sakamoto, Zh.H. Cheng, M. Takahashi, Y. Kuramoto, M. Owari, Y. Nihei, Jpn. J. Appl. Phys. 37 (1998) 2051. [4] H. Satoh, M. Owari, Y. Nihei, J. Vaccine Sci. Technol. B6 (1988) 915. [5] H. Satoh, M. Owari, Y. Nihei, Jpn. J. Appl. Phys. 32 (1993) 3616.