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Rapid Detection of Trace Heavy Metals using Laser Breakdown Time-of-Flight Mass Spectrometry
Available online at www.sciencedirect.com
Procedia Environmental Sciences 18 (2013) 329 – 337
2013 International Symposium on Environmental Science and Technology (2013 ISEST)
Rapid detection of trace heavy metals using laser breakdown time-offlight mass spectrometry Zhen Zhen Wanga,b, Yoshihiro Deguchib,*, Jun Jie Yana, Ji Ping Liua a School of En Graduate School of Advanced Technology and Science, The University of Tokushima, Tokushima 770-8501, Japan
b
Abstract It has been highly recognized heavy metals pollution concerns the environment, as well as human health. Mercury (Hg) pollution has greatly increased and been considered as a global pollutant because of its long residence time in surrounding. This paper describes the rapid detection of mercury using laser breakdown time-of-flight mass spectrometry at high sensitivity without fragmentation interference from other species. Two irradiation wavelengths 1064nm and 532nm were employed under various experimental conditions. The second harmonic 532nm performs excellent measurement results. The influence of pressure on mercury signal intensity displays a liner growth when increasing the pressure. These results also show as the laser power increased, nitrogen signal intensity increased, but mercury signal intensity increased first and then decreased. Experiment with different buffer gases clarified the recombination of Hg ions and electrons when increasing the laser power, resulting in the decrease of mercury signal intensity. According to these measurement results, the method of enlarging focus area and reducing laser power by tilting the focus lens was applied to decrease the recombination rate to enhance the detection limit. It is demonstrated that the detection limit with 1ppb can be acquired facilely. © 2013 2013 The byby Elsevier B.V. © TheAuthors. Authors.Published Published Elsevier B.V. Selection and/or peer-reviewunder under responsibility responsibility ofofBeijing ofTechnology. Technology. Selection and peer-review Beijing Institute Institute of Keywords: Laser breakdown; Time-of-flight mass spectrometry (TOFMS); Rapid detection; Heavy metals; Trace element
1. Introduction With the development of economy, the environmental pollution is becoming increasingly severe and much more attention should be paid to the issue [1-3]. In recent years, various heavy metals pollution incidents, such as the pollution of Pb, Cd, Cr, Hg, As and so on, result in serious influences on environment and human health. The major sources of heavy metal contamination include industry, traffic and domestic waste, which pollute the soil, water and atmosphere. The atmosphere as an important part of the urban environment suffers from human interference. It is imperative to detect heavy metals exiting in complex substances in the atmosphere. As mercury is widely used in various industries, mercury pollution or its potentiality is one of the serious environmental problems concerning human health. There are mainly three existing forms of mercury, including metallic, inorganic and organic mercury, which can deposit to atmosphere, terrestrial and aquatic ecosystems endangering human health directly or indirectly [4-7]. The toxicity of mercury is highly dependent on its chemical form, organic mercury being more toxic than the inorganic forms. Several papers have reported the measurement of mercury in different applications [8-10]. Because of the low concentration of mercury in surrounding, enrichment and separation are essential to analyse the contamination using conventional methods, however these procedures usually take long time. On the other hand, due to the restriction of detection limit these methods cannot obtain satisfactory results in some applications.
* Corresponding author. Tel.: +81-88-656-7375; fax: +81-88-656-9082. E-mail address: [email protected].
1878-0296 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Beijing Institute of Technology. doi:10.1016/j.proenv.2013.04.043
330
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337
Laser-induced breakdown spectroscopy (LIBS) is an analytical detection technique based on atomic emission spectroscopy to measure elemental composition. With the development of laser and detection systems, LIBS has been applied in various fields, including combustion, metallurgy, foods and human [11-14], etc. Especially, the advantages of the method are more significant in the areas of combustion, metallurgy, and the harsh environment. However, LIBS applicability cannot reach the trace species detection needs because of its detection limit. Time-of-Flight Mass Spectrometry (TOFMS) is an appealing technique for qualitative and quantitative analysis of atom and molecule with the features of increased sensitivity and rapid analysis. The detection limit of measured species using TOFMS is often ppb or less. TOFMS has been applied to measure hydrocarbons and nanoparticle constituents with low concentration range of ppb - ppt [15-19], as well as waste disposal and treatment plant [20,21]. There are other developed TOFMS applications in several fields, such as medical and pharmaceutical fields [22,23]. The method of laser ionization using a vacuum ultraviolet wavelength can result in a signal-photon ionization (soft ionization) process with a minimum amount of fragmentation. This usually makes the measurement system much more complicated. Resonance-enhanced multiphoton ionization can cover the shortcomings of soft ionization using resonance states as steps to the ionization energy levels. Considering the features of TOFMS, fragmentation and signal intensity are the important factors for sensitive measurement. In order to eliminate or minimize the interference of fragmentation to heavy metals, the method of laser breakdown combined with ionization time-of-flight mass spectrometry was employed in our study. The laser is introduced to break down the measured species and ionize to detect ion signals using TOFMS. 2. Theory In LIBS process, a laser beam is focused onto a small area, producing hot plasma. The material contained in plasma is atomized, and light is emitted corresponding to a unique wavelength for each element. A detector records the emission signals from excited atomic and ionic species. Here, laser breakdown combined with ionization to atomize the molecules and then ionize. In TOFMS system, a measurement sample is introduced into a vacuum chamber and it is atomized and ionized by laser irradiation. The electric field potential is simultaneously applied for acceleration of ions. The accelerated ions enter the drift region with no potential difference and undergo uniform motion. Differing from the emission detection of LIBS, an ion detector records the signals of ionized species and ion counter takes over to digitize and display the results. Due to their kinetic energy. The following formula is established by the energy conservation law [24]:
zV
m L 2 t
2
(1)
In the above expression, z is the ionic valence, V is the acceleration electric field potential, m is the ion mass, L is the ion distance of flight, t is the time of flight. TOFMS distinguishes the ions of different atoms or molecules based on their arrival time to ion detector. The following relationship between the ion mass and the time of flight can be expressed from Eq. (1).
m
2zV 2 t L2
(2)
3. Experimental apparatus The schematic diagram of the experimental apparatus in this study is shown in Fig.1(a), consisting of sample containers, nanosecond lasers, jet chamber, test chamber, flight chamber, ion detector and auxiliary equipment. One nanosecond laser employed is a pulse energy Nd:YAG laser (Quanta-Ray Pro, 10Hz and Quantel Brilliant b, 10Hz) operating at fundamental radiation 1064nm and second harmonic 532nm with different pulse energy. Another nanosecond laser is a pulse energy Nd:YAG laser (LS-2137U, 10Hz) operating at fourth harmonic 266nm. For the measurement of heavy metals, the jet chamber was used to ensure heavy metals go straight ahead to test chamber. The temperature of gas inlet and outlet pipes was controlled at 150 . There are two turbo pumps (Pfeiffer vacuum, MVP 055-3) belonging to test chamber and flight chamber respectively. The pressure in the test chamber is an important parameter as an indicator under different experimental conditions.
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337
Measured heavy metals are generally present in various mixtures or as the molecules with other elements, such as particles and hydrocarbons from exhaust in industries. In this system, there are three main samples including mercury, hydrocarbons and fine particles of fly ash. As for particles, Andersen cascade impactor was used to separate the particles according to particle diameter, which employs the impacting method with multistage and porous jet nozzles and can measure the granularity distribution of aerosol according to aerodynamics. Diameter of particles gradually decreases from top down. The influence of different wavelengths 1064nm and 266nm breakdown was demonstrated using hydrocarbons, the mixture of p-C7H6Cll2, C7H8, C6H5C2H3, p-C8H10, p-C6H4(C2H5)2 and C6H3(CH3)3, introduced from another pipe to jet chamber. The gaseous mixture of Hg with buffer gas, such as N2, Ar and He, was fed into the jet chamber m from the constant-temperature bath according to the vaporizing temperature of mercury. Diluent gas N2 was used to reduce the concentration of Hg introduced to the jet chamber. The dilution ratio was defined on the basis of the flow rate ratio of buffer gas to diluent gas. The concentration of Hg was determined using mercury gas detector tube, sampling from the gas outlet, as shown in Fig.1(a).
Fig.1. Experiment setup (a) Schematic diagram of TOFMS; (b) Vertical position of focus lens; (c) Tilted position of focus lens.
The objective of the present experiment is to rapidly detect trace heavy metal Hg using laser breakdown time-off flight mass spectrometry at a high sensitivity. The lasers with wavelengths of 1064nm, 532nm, and 266nm were employed to break down the samples and ionize under different pulse energy and pressure conditions to compare the features of breakdown and ionization process. In order to enhance the detection limit, the focus area was enlarged by tilting the focus lens. Two positions of focus lens are shown in Fig.1(b) and Fig.1(c). Several buffer gases, N2, Ar and He, were also used to examine the ionization process. 4. Results and discussion 4.1. Influence off fragments using 1064nm, 532nm, and 266nm laser breakdown processes In practice, trace heavy metals of low concentration are usually enclosed with hydrocarbons and particles or form compounds. It is necessary to distinguish the target from intermixture without any interference. The method of laser breakdown time-off flight f mass spectrometry was employed to eliminate or minimize the fragmentation of the intermixture. Fig.2 shows the measurement results of hydrocarbon mixture of p-C7H6Cll2, C7H8, C6H5C2H3, p-C8H10, pC6H4(C2H5)2 and C6H3(CH3)3 using 266nm and 1064nm breakdown. Laser breakdown time-off flight mass
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spectrometry with wavelength 266nm can produce fragmentation, especially for large fragile molecules which will be pre-dissociated using 266nm breakdown. Compared to the result of 266nm breakdown with a lot of fragmentation, the result of 1064nm breakdown is very distinct without any interference in the mass region of 30-300m/z. The laser operated at 532nm and 1064nm was also introduced to break down the hydrocarbon mixture of p-C8H10 and N2 with 10ppm concentration, the results are shown in Fig.3. The mass range of 30-300m/z is also very clear in both cases. It is demonstrated the breakdown of molecules was complete without fragmentation.
Fig.2. Mass spectra of breakdown hydrocarbons using different wavelengths (a) 266nm breakdown; (b) 1064nm breakdown.
Fig. 3. Mass spectra of breakdown hydrocarbons using different wavelengths (a)1064nm breakdown; (b)532nm breakdown.
A Applying laser breakdown time-off flight f mass spectrometry to measure trace heavy metal Hg, the samples employed were the mixtures of mercury, fine particles, and hydrocarbon to prove the applicability of this method to practical fields, one consisting of mercury, nitrogen and hydrocarbon p-C8H10, another one consisting of mercury, helium and fine particles. Different atoms or molecules can be distinguished according to the mass proportionating to the square of the time of flight. It is facile to detect Hg+ signal without fragmentation even if the low concentration of Hg in the mixtures. Fig.4 shows the measurement results of Hg in hydrocarbon using 1064nm breakdown. Hg+ signal can be detected without fragmentation of hydrocarbon because of the laser breakdown process as shown in Fig.4(a). There are several Hg+ signal lines, such as 204, 202, 201, 200, 199 and 198amu, representing the isotopes of Hg, as shown in Fig.4(b). The element can be measured precisely, as the method depends on the mass of atom and molecule with high sensitivity.
Fig.4. Measurement results of Hg using 1064nm breakdown (a) Hg in hydrocarbon; (b) Hg isotopes.
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337
In the particles of fly ash, the content of Si is much higher than other species. Fig.5 displays accessible Si signal using 532nm breakdown employing He as the buffer gas. In both cases of 1064nm and 532nm breakdown, Hg+ signal from mixtures was revealed without the interference.
Fig.5. Measurement result of Hg in particles using 532nm breakdown.
4.2. Pressure dependence of 1064nm and 532nm breakdown The pressure dependence of Hg+ signal intensity was measured according to the pressure in test chamber. Fig.6 shows the pressure dependence with different wavelengths. Under 1064nm breakdown condition, Hg was introduced to the jet chamber with the dilution ratio of 2.4. The dilution ratio was the flow rate ratio of buffer gas N2 to diluent gas N2 in Fig.1(a). As Fig.6(a) shows, the measurement result presents the signal intensity increased when increasing the pressure. In the case of 532nm breakdown, the dilution ratio was 0.08. The pressure dependence of Hg+ signal intensity is linear in accordance with the result of 1064nm breakdown, as shown in Fig.6(b). When increasing the pressure in test chamber, the number density of ions increases to enhance the signal intensity in both cases. The concentration of Hg in the case of 1064nm breakdown was higher than that of 532nm breakdown; however the signal intensity was lower under the same pressure condition.
Fig. 6. Dependence of Hg+ signal intensity on pressure in test chamber (a) 1064nm breakdown; (b) 532nm breakdown.
4.3. Laser power dependence of 1064nm and 532nm breakdown The power dependence in the cases of 1064nm and 532nm breakdown was detected at the pressure 0.5Pa. The variations of Hg+ signal intensity based on the laser power are presented in Fig.7, which indicate Hg+ signal intensity increases first and then decreases when increasing the laser power. According to the multiphoton ionization, the signal intensity increases as the laser power increases. Under the experimental condition, the laser power exceeding 850mJ/p of 1064nm and 40mJ/p of 532nm caused reduction of Hg+ signal intensity. This phenomenon may have occurred because of the recombination of Hg ions and elections under high laser power condition. As increasing the laser power, more sample of N2 and Hg was broken down and ionized to produce dense ions and electrons. Because of the high recombination rate of Hg ions and electrons, Hg+ signal intensity decreased with high laser power. Hg with different buffer gases was introduced to the test chamber to verify the phenomenon of recombination, such as Ar and He besides N2. Fig.8 displays the dependence of Hg+ signal intensity on the laser power employing
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different buffer gases. The laser power corresponding to the maximum Hg+ signal intensity increased when using Ar as the buffer gas. In the case of He, Hg+ signal intensity increased as the laser power increased under the experimental condition. The peak shifted due to different recombination rate resulted by different ionization potential of N2, Ar and He, as shown in Table 1. Because the ionization potential of He is the highest one among these buffer gases, the minimum He atoms can be ionized to produce electrons. Therefore the recombination rate of Hg ions and electrons was the lowest under same laser power condition.
Fig.7. Dependence of Hg+ signal intensity on laser power (a) 1064nm breakdown; (b) 532nm breakdown.
Fig.8. Power dependence using different buffer gases . Table 1. Ionization potential species
Ionization potential (eV)
N2
9.8
N
14.5
Ar
15.8
He
24.6
4.4. Enhancement of the detection limit Above analysis indicates the measurement results using 532nm breakdown are much more desirable than 1064nm breakdown. It is reasonable to enhance the detection limit employing the laser operated at 532nm. The signal intensity was positive correlated with the pressure in test chamber, so the sample can be measured at low pressure 0.05Pa. Due to the influence of recombination of Hg ions and elections on Hg + signal intensity, one possible method to enhance the detection limit is to enlarge focus area and decreasing the laser power by tilting the focus lens. Two positions of the focus lens were determined to compare the Hg+ signal intensity, including the parallel position to the window and tilted position. The angles of the focus lens were 0o and 10o respectively. In different positions, the dependence of signal intensity on the laser power was also measured, as shown in Fig.9. When tilting the focus lens to enlarge focus area, more sample was exposed in the laser beam with diminished laser power, resulting in reduction of ions and elections density. Because of the recombination of Hg ions and elections, the curves of power dependence shifted to
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337
higher laser power area. When further enlarging the focus area, Hg+ signal and N+ signal suffer from a sharp decline. The appropriate position of focus lens must be determined according to measurement results. When enlarging the focus area, the laser power corresponding to maximum Hg+ signal intensity shifted to higher energy area, as Fig.9(a) shows. Fig.9(b) shows the variation of N+ signal intensity through tilting the focus lens. In Fig.9(b), N+ signal became saturated around the intensity 1a.u. the laser power corresponding to N+ signal intensity also increased as tilting the focus lens. These findings suggest the recombination rate of Hg ions and electrons can be reduced with appropriate focus condition. Fig.10 shows the mass spectra at different positions of focus lens. Data in Table 2 confirm this improvement because the ratio of Hg + to N + increased in position 2 to reduce the influence of N2 on Hg+ signal intensity.
Fig.9. Power dependence using 532nm breakdown at different positions of focus lens (a) Hg+ signal; (b) N+ signal.
Fig.10. Mass spectra at different positions of focus lens with laser power 100mJ/p (a) position-1; (b) position-2. Table 2. The intensity ratio of Hg+/N+ at different positions of focus lens.
Power (mJ/p)
Hg+/N+ Position-1
Position-2
20
3.49
---
40
0.36
3.19
60
0.11
1.81
80
0.06
0.52
100
0.02
0.32
120
0.01
0.20
150
0.01
0.14
The concentration dependence of Hg+ signal intensity was measured using different dilution ratio samples at pressure 0.05Pa, as shown in Fig.11. Hg+ signal intensity was directly proportional to the concentration, indicated by the dilution ratio of buffer gas to diluent gas. The concentration was examined using mercury gas detector tube with
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the dilution ratio of 0.02, sampling point as shown in Fig.1(a). The detection limit of Hg was about 1ppb. As the linear pressure dependence of Hg+ signal intensity, the detection limit will be enhanced when increasing the pressure.
Fig. 11. Dependence of Hg+ signal intensity on concentration.
5. Conclusions Laser breakdown TOFMS method was developed and applied to detect heavy metal Hg in intermixture. This method features rapid detection without complex sample preparation and better detection limit compared with LIBS. (1) In order to measure heavy metals from intermixture, such as hydrocarbons and particles, laser breakdown process is essential to eliminate or minimize fragmentation from other species. Measurement results employing 532nm and 1064nm breakdown display without interference f of fragmentation, compared to the laser breakdown process using 266nm. (2) The influence of pressure in test chamber on Hg+ signal intensity shows the linear growth when the pressure increases. As two wavelengths were employed to break down and ionize the sample, the results using 532nm breakdown present higher signal intensity than that using 1064nm breakdown. (3) Because of the recombination of Hg ions and electrons, power dependence of Hg+ signal intensity increases first and then decreases using 1064nm and 532nm breakdown. Among the buffer gases, N2, Ar and He, the lowest recombination rate using He as buffer gas with highest ionization potential determined the phenomenon of recombination of Hg ions and electrons. (4) Due to the influence of recombination, the method of enlarging focus area and decreasing the laser power reduce the recombination rate of Hg ions and electrons. It is a proven means to enhance the detection limit with appropriate focus area. It is demonstrated the method is significantly desirable to measure heavy metals in intermixture. Further study will apply this method to these practical applications, such as environmental monitoring system and power plants. References [1]
Gauderman WJ, Gilliland GF, Vora H, Avol E, Stram D, McConnell R, et al. Association between air pollution and lung function growth in southern California children. Am J Respir Crit Care Med 2002; 166:76 84. [2] Arden Pope III C, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J. Air & Waste Manage 2006; 56:709 742 [3] Kampa M, Castanas E. Human health effects of air pollution. Environmental Pollution 2008; 151: 362-367. [4] Ipolyi I, Massanisso P, Sposato S, Fodor P, Morabito R. Concentration levels of total and methylmercury in mussel samples collected along the coasts of Sardinia Island (Italy). Analytica Chimica Acta 2004; 505: 145 151. [5] Obrist D. Atmospheric mercury pollution due to losses of terrestrial carbon pools? Biogeochemistry 2007; 85:119 123. [6] Lapanje A, Drobne D, Nolde N, Valant J, Muscet B, Leser V, et al. Long-term Hg pollution induced Hg tolerance in the terrestrial isopod Porcellio scaber (Isopoda, Crustacea). Environmental Pollution 2008; 153: 537-547. [7] Li P, Feng XB, Qiu GL, Shang LH, Li ZG. Mercury pollution in Asia: A review of the contaminated sites. Journal of Hazardous Materials 2009; 168: 591 601. [8] Meij R. A sampling method based on activated carbon for gaseous mercury in ambient air and flue gases. Water, Air, and Soil Pollution 1991; 56:117-129. [9] Landis MS, Stevens RK, Schaedlich F, Prestbo EM. Development and characterization of an annular denuder methodology for the measurement of divalent inorganic reactive gaseous mercury in ambient air. Environ. Sci. Technol 2002; 36: 3000-3009. [10] Lee SJ, Seo YC, Jang HN, Park KS, Baek JI, An HS, et al. Speciation and mass distribution of mercury in a bituminous coal-fired power plant. Atmospheric Environment 2006; 40: 2215 2224. [11] Ctvrtnickova T, Mateo MP, Yañez A, Nicolas G. Laser induced breakdown spectroscopy application for ash characterization for a coal fired power plant. Spectrochim. Acta Part B 2010; 65: 734-737.
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337 [12] Boué-Bigne F. Laser-induced breakdown spectroscopy applications in the steel industry: Rapid analysis of segregation and decarburization. Spectrochim. Acta Part B 2008; 63: 1122 1129. [13] Pouzar M T M P A. LIBS analysis of crop plants. J. Anal. Atom. Spectrom. 2009; 24: 953 957. [14] Thareja RK, Sharma AK, Shukla S. Spectroscopic investigations of carious tooth decay. Med. Eng. Phys. 2008; 30: 1143 1148. [15] Gittins CM, Castaldi MJ, Senkan SM, Rohlfing EA. Real-time quantitative analysis of combustion-generated polycyclic aromatic hydrocarbons by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry. Anal. Chem. 1997; 69: 286-293. [16] Heger HJ, Zimmermann R, Dorfner R, Beckmann M, Griebel H, Kettrup A, et al. On-line emission analysis of polycyclic aromatic hydrocarbons down to pptv concentration levels in the flue gas of an incineration pilot plant with a mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometer. Anal. Chem. 1999; 71:46-57. [17] Bente M, Sklorz M, Streibel T, Zimmermann R. Thermal desorption-multiphoton ionization time-of-flight mass spectrometry of individual aerosol particles: a simplified approach for online single-particle analysis of polycyclic aromatic hydrocarbons and their derivatives. Anal. Chem. 2009; 81: 2525 2536. [18] Gullett BK, Touati A, Oudejans L, Ryan SP. Real-time emission characterization of organic air toxic pollutants during steady state and transient operation of a medium duty diesel engine. Atmospheric Environment 2006; 40: 4037 4047. [19] Deguchi Y, Tanaka N, Tsuzaki M, Fushimi A, Kobayashi S, Tanabe K. Detection of components in nanoparticles by resonant ionization and laser breakdown time-of-flight mass spectrometry. Environ. Chem. 2008; 5:402-412. [20] Dobashi S, Yamaguchi Y, Izawa Y, Deguchi Y, Wada A, Hara M. Laser mass spectrometry: rapid analysis of polychlorinated biphenyls in exhaust gas of disposal plants. Journal of Environment and Engineering 2007; 2:25-34. [21] Dobashi S, Yamaguchi Y, Izawa Y, Wada A, Hara M. Safety management by use of laser mass spectrometry of polychlorinated biphenyls (PCBs) in the processed gas and work environment of a PCB disposal plant. J Mater Cycles Waste Manag 2009; 11:148 154. [22] Pignone M, Greth KM, Cooper J, Emerson D, Tang J. Identification of mycobacteria by matrix-assisted laser desorption ionization time-offlight mass spectrometry. Journal of Clinical Microbiology 2006; 44: 1963 1970. [23] Hazen TH, Martinez RJ, Chen YF, Lafon PC, Garrett NM, Parsons MB, et al. Rapid identification of vibrio parahaemolyticus by whole-cell matrix-assisted laser desorption ionization time of flight mass spectrometry. Applied and Environmental Microbiology 2009; 75: 6745 6756. [24] Deguchi Y. Industrial applications of laser diagnostics, New York: CRS Press, Taylor & Francis; 2011, p. 209 216.
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Procedia Environmental Sciences 18 (2013) 329 – 337
2013 International Symposium on Environmental Science and Technology (2013 ISEST)
Rapid detection of trace heavy metals using laser breakdown time-offlight mass spectrometry Zhen Zhen Wanga,b, Yoshihiro Deguchib,*, Jun Jie Yana, Ji Ping Liua a School of En Graduate School of Advanced Technology and Science, The University of Tokushima, Tokushima 770-8501, Japan
b
Abstract It has been highly recognized heavy metals pollution concerns the environment, as well as human health. Mercury (Hg) pollution has greatly increased and been considered as a global pollutant because of its long residence time in surrounding. This paper describes the rapid detection of mercury using laser breakdown time-of-flight mass spectrometry at high sensitivity without fragmentation interference from other species. Two irradiation wavelengths 1064nm and 532nm were employed under various experimental conditions. The second harmonic 532nm performs excellent measurement results. The influence of pressure on mercury signal intensity displays a liner growth when increasing the pressure. These results also show as the laser power increased, nitrogen signal intensity increased, but mercury signal intensity increased first and then decreased. Experiment with different buffer gases clarified the recombination of Hg ions and electrons when increasing the laser power, resulting in the decrease of mercury signal intensity. According to these measurement results, the method of enlarging focus area and reducing laser power by tilting the focus lens was applied to decrease the recombination rate to enhance the detection limit. It is demonstrated that the detection limit with 1ppb can be acquired facilely. © 2013 2013 The byby Elsevier B.V. © TheAuthors. Authors.Published Published Elsevier B.V. Selection and/or peer-reviewunder under responsibility responsibility ofofBeijing ofTechnology. Technology. Selection and peer-review Beijing Institute Institute of Keywords: Laser breakdown; Time-of-flight mass spectrometry (TOFMS); Rapid detection; Heavy metals; Trace element
1. Introduction With the development of economy, the environmental pollution is becoming increasingly severe and much more attention should be paid to the issue [1-3]. In recent years, various heavy metals pollution incidents, such as the pollution of Pb, Cd, Cr, Hg, As and so on, result in serious influences on environment and human health. The major sources of heavy metal contamination include industry, traffic and domestic waste, which pollute the soil, water and atmosphere. The atmosphere as an important part of the urban environment suffers from human interference. It is imperative to detect heavy metals exiting in complex substances in the atmosphere. As mercury is widely used in various industries, mercury pollution or its potentiality is one of the serious environmental problems concerning human health. There are mainly three existing forms of mercury, including metallic, inorganic and organic mercury, which can deposit to atmosphere, terrestrial and aquatic ecosystems endangering human health directly or indirectly [4-7]. The toxicity of mercury is highly dependent on its chemical form, organic mercury being more toxic than the inorganic forms. Several papers have reported the measurement of mercury in different applications [8-10]. Because of the low concentration of mercury in surrounding, enrichment and separation are essential to analyse the contamination using conventional methods, however these procedures usually take long time. On the other hand, due to the restriction of detection limit these methods cannot obtain satisfactory results in some applications.
* Corresponding author. Tel.: +81-88-656-7375; fax: +81-88-656-9082. E-mail address: [email protected].
1878-0296 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Beijing Institute of Technology. doi:10.1016/j.proenv.2013.04.043
330
Zhen Zhen Wanga et al. / Procedia Environmental Sciences 18 (2013) 329 – 337
Laser-induced breakdown spectroscopy (LIBS) is an analytical detection technique based on atomic emission spectroscopy to measure elemental composition. With the development of laser and detection systems, LIBS has been applied in various fields, including combustion, metallurgy, foods and human [11-14], etc. Especially, the advantages of the method are more significant in the areas of combustion, metallurgy, and the harsh environment. However, LIBS applicability cannot reach the trace species detection needs because of its detection limit. Time-of-Flight Mass Spectrometry (TOFMS) is an appealing technique for qualitative and quantitative analysis of atom and molecule with the features of increased sensitivity and rapid analysis. The detection limit of measured species using TOFMS is often ppb or less. TOFMS has been applied to measure hydrocarbons and nanoparticle constituents with low concentration range of ppb - ppt [15-19], as well as waste disposal and treatment plant [20,21]. There are other developed TOFMS applications in several fields, such as medical and pharmaceutical fields [22,23]. The method of laser ionization using a vacuum ultraviolet wavelength can result in a signal-photon ionization (soft ionization) process with a minimum amount of fragmentation. This usually makes the measurement system much more complicated. Resonance-enhanced multiphoton ionization can cover the shortcomings of soft ionization using resonance states as steps to the ionization energy levels. Considering the features of TOFMS, fragmentation and signal intensity are the important factors for sensitive measurement. In order to eliminate or minimize the interference of fragmentation to heavy metals, the method of laser breakdown combined with ionization time-of-flight mass spectrometry was employed in our study. The laser is introduced to break down the measured species and ionize to detect ion signals using TOFMS. 2. Theory In LIBS process, a laser beam is focused onto a small area, producing hot plasma. The material contained in plasma is atomized, and light is emitted corresponding to a unique wavelength for each element. A detector records the emission signals from excited atomic and ionic species. Here, laser breakdown combined with ionization to atomize the molecules and then ionize. In TOFMS system, a measurement sample is introduced into a vacuum chamber and it is atomized and ionized by laser irradiation. The electric field potential is simultaneously applied for acceleration of ions. The accelerated ions enter the drift region with no potential difference and undergo uniform motion. Differing from the emission detection of LIBS, an ion detector records the signals of ionized species and ion counter takes over to digitize and display the results. Due to their kinetic energy. The following formula is established by the energy conservation law [24]:
zV
m L 2 t
2
(1)
In the above expression, z is the ionic valence, V is the acceleration electric field potential, m is the ion mass, L is the ion distance of flight, t is the time of flight. TOFMS distinguishes the ions of different atoms or molecules based on their arrival time to ion detector. The following relationship between the ion mass and the time of flight can be expressed from Eq. (1).
m
2zV 2 t L2
(2)
3. Experimental apparatus The schematic diagram of the experimental apparatus in this study is shown in Fig.1(a), consisting of sample containers, nanosecond lasers, jet chamber, test chamber, flight chamber, ion detector and auxiliary equipment. One nanosecond laser employed is a pulse energy Nd:YAG laser (Quanta-Ray Pro, 10Hz and Quantel Brilliant b, 10Hz) operating at fundamental radiation 1064nm and second harmonic 532nm with different pulse energy. Another nanosecond laser is a pulse energy Nd:YAG laser (LS-2137U, 10Hz) operating at fourth harmonic 266nm. For the measurement of heavy metals, the jet chamber was used to ensure heavy metals go straight ahead to test chamber. The temperature of gas inlet and outlet pipes was controlled at 150 . There are two turbo pumps (Pfeiffer vacuum, MVP 055-3) belonging to test chamber and flight chamber respectively. The pressure in the test chamber is an important parameter as an indicator under different experimental conditions.
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Measured heavy metals are generally present in various mixtures or as the molecules with other elements, such as particles and hydrocarbons from exhaust in industries. In this system, there are three main samples including mercury, hydrocarbons and fine particles of fly ash. As for particles, Andersen cascade impactor was used to separate the particles according to particle diameter, which employs the impacting method with multistage and porous jet nozzles and can measure the granularity distribution of aerosol according to aerodynamics. Diameter of particles gradually decreases from top down. The influence of different wavelengths 1064nm and 266nm breakdown was demonstrated using hydrocarbons, the mixture of p-C7H6Cll2, C7H8, C6H5C2H3, p-C8H10, p-C6H4(C2H5)2 and C6H3(CH3)3, introduced from another pipe to jet chamber. The gaseous mixture of Hg with buffer gas, such as N2, Ar and He, was fed into the jet chamber m from the constant-temperature bath according to the vaporizing temperature of mercury. Diluent gas N2 was used to reduce the concentration of Hg introduced to the jet chamber. The dilution ratio was defined on the basis of the flow rate ratio of buffer gas to diluent gas. The concentration of Hg was determined using mercury gas detector tube, sampling from the gas outlet, as shown in Fig.1(a).
Fig.1. Experiment setup (a) Schematic diagram of TOFMS; (b) Vertical position of focus lens; (c) Tilted position of focus lens.
The objective of the present experiment is to rapidly detect trace heavy metal Hg using laser breakdown time-off flight mass spectrometry at a high sensitivity. The lasers with wavelengths of 1064nm, 532nm, and 266nm were employed to break down the samples and ionize under different pulse energy and pressure conditions to compare the features of breakdown and ionization process. In order to enhance the detection limit, the focus area was enlarged by tilting the focus lens. Two positions of focus lens are shown in Fig.1(b) and Fig.1(c). Several buffer gases, N2, Ar and He, were also used to examine the ionization process. 4. Results and discussion 4.1. Influence off fragments using 1064nm, 532nm, and 266nm laser breakdown processes In practice, trace heavy metals of low concentration are usually enclosed with hydrocarbons and particles or form compounds. It is necessary to distinguish the target from intermixture without any interference. The method of laser breakdown time-off flight f mass spectrometry was employed to eliminate or minimize the fragmentation of the intermixture. Fig.2 shows the measurement results of hydrocarbon mixture of p-C7H6Cll2, C7H8, C6H5C2H3, p-C8H10, pC6H4(C2H5)2 and C6H3(CH3)3 using 266nm and 1064nm breakdown. Laser breakdown time-off flight mass
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spectrometry with wavelength 266nm can produce fragmentation, especially for large fragile molecules which will be pre-dissociated using 266nm breakdown. Compared to the result of 266nm breakdown with a lot of fragmentation, the result of 1064nm breakdown is very distinct without any interference in the mass region of 30-300m/z. The laser operated at 532nm and 1064nm was also introduced to break down the hydrocarbon mixture of p-C8H10 and N2 with 10ppm concentration, the results are shown in Fig.3. The mass range of 30-300m/z is also very clear in both cases. It is demonstrated the breakdown of molecules was complete without fragmentation.
Fig.2. Mass spectra of breakdown hydrocarbons using different wavelengths (a) 266nm breakdown; (b) 1064nm breakdown.
Fig. 3. Mass spectra of breakdown hydrocarbons using different wavelengths (a)1064nm breakdown; (b)532nm breakdown.
A Applying laser breakdown time-off flight f mass spectrometry to measure trace heavy metal Hg, the samples employed were the mixtures of mercury, fine particles, and hydrocarbon to prove the applicability of this method to practical fields, one consisting of mercury, nitrogen and hydrocarbon p-C8H10, another one consisting of mercury, helium and fine particles. Different atoms or molecules can be distinguished according to the mass proportionating to the square of the time of flight. It is facile to detect Hg+ signal without fragmentation even if the low concentration of Hg in the mixtures. Fig.4 shows the measurement results of Hg in hydrocarbon using 1064nm breakdown. Hg+ signal can be detected without fragmentation of hydrocarbon because of the laser breakdown process as shown in Fig.4(a). There are several Hg+ signal lines, such as 204, 202, 201, 200, 199 and 198amu, representing the isotopes of Hg, as shown in Fig.4(b). The element can be measured precisely, as the method depends on the mass of atom and molecule with high sensitivity.
Fig.4. Measurement results of Hg using 1064nm breakdown (a) Hg in hydrocarbon; (b) Hg isotopes.
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In the particles of fly ash, the content of Si is much higher than other species. Fig.5 displays accessible Si signal using 532nm breakdown employing He as the buffer gas. In both cases of 1064nm and 532nm breakdown, Hg+ signal from mixtures was revealed without the interference.
Fig.5. Measurement result of Hg in particles using 532nm breakdown.
4.2. Pressure dependence of 1064nm and 532nm breakdown The pressure dependence of Hg+ signal intensity was measured according to the pressure in test chamber. Fig.6 shows the pressure dependence with different wavelengths. Under 1064nm breakdown condition, Hg was introduced to the jet chamber with the dilution ratio of 2.4. The dilution ratio was the flow rate ratio of buffer gas N2 to diluent gas N2 in Fig.1(a). As Fig.6(a) shows, the measurement result presents the signal intensity increased when increasing the pressure. In the case of 532nm breakdown, the dilution ratio was 0.08. The pressure dependence of Hg+ signal intensity is linear in accordance with the result of 1064nm breakdown, as shown in Fig.6(b). When increasing the pressure in test chamber, the number density of ions increases to enhance the signal intensity in both cases. The concentration of Hg in the case of 1064nm breakdown was higher than that of 532nm breakdown; however the signal intensity was lower under the same pressure condition.
Fig. 6. Dependence of Hg+ signal intensity on pressure in test chamber (a) 1064nm breakdown; (b) 532nm breakdown.
4.3. Laser power dependence of 1064nm and 532nm breakdown The power dependence in the cases of 1064nm and 532nm breakdown was detected at the pressure 0.5Pa. The variations of Hg+ signal intensity based on the laser power are presented in Fig.7, which indicate Hg+ signal intensity increases first and then decreases when increasing the laser power. According to the multiphoton ionization, the signal intensity increases as the laser power increases. Under the experimental condition, the laser power exceeding 850mJ/p of 1064nm and 40mJ/p of 532nm caused reduction of Hg+ signal intensity. This phenomenon may have occurred because of the recombination of Hg ions and elections under high laser power condition. As increasing the laser power, more sample of N2 and Hg was broken down and ionized to produce dense ions and electrons. Because of the high recombination rate of Hg ions and electrons, Hg+ signal intensity decreased with high laser power. Hg with different buffer gases was introduced to the test chamber to verify the phenomenon of recombination, such as Ar and He besides N2. Fig.8 displays the dependence of Hg+ signal intensity on the laser power employing
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different buffer gases. The laser power corresponding to the maximum Hg+ signal intensity increased when using Ar as the buffer gas. In the case of He, Hg+ signal intensity increased as the laser power increased under the experimental condition. The peak shifted due to different recombination rate resulted by different ionization potential of N2, Ar and He, as shown in Table 1. Because the ionization potential of He is the highest one among these buffer gases, the minimum He atoms can be ionized to produce electrons. Therefore the recombination rate of Hg ions and electrons was the lowest under same laser power condition.
Fig.7. Dependence of Hg+ signal intensity on laser power (a) 1064nm breakdown; (b) 532nm breakdown.
Fig.8. Power dependence using different buffer gases . Table 1. Ionization potential species
Ionization potential (eV)
N2
9.8
N
14.5
Ar
15.8
He
24.6
4.4. Enhancement of the detection limit Above analysis indicates the measurement results using 532nm breakdown are much more desirable than 1064nm breakdown. It is reasonable to enhance the detection limit employing the laser operated at 532nm. The signal intensity was positive correlated with the pressure in test chamber, so the sample can be measured at low pressure 0.05Pa. Due to the influence of recombination of Hg ions and elections on Hg + signal intensity, one possible method to enhance the detection limit is to enlarge focus area and decreasing the laser power by tilting the focus lens. Two positions of the focus lens were determined to compare the Hg+ signal intensity, including the parallel position to the window and tilted position. The angles of the focus lens were 0o and 10o respectively. In different positions, the dependence of signal intensity on the laser power was also measured, as shown in Fig.9. When tilting the focus lens to enlarge focus area, more sample was exposed in the laser beam with diminished laser power, resulting in reduction of ions and elections density. Because of the recombination of Hg ions and elections, the curves of power dependence shifted to
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higher laser power area. When further enlarging the focus area, Hg+ signal and N+ signal suffer from a sharp decline. The appropriate position of focus lens must be determined according to measurement results. When enlarging the focus area, the laser power corresponding to maximum Hg+ signal intensity shifted to higher energy area, as Fig.9(a) shows. Fig.9(b) shows the variation of N+ signal intensity through tilting the focus lens. In Fig.9(b), N+ signal became saturated around the intensity 1a.u. the laser power corresponding to N+ signal intensity also increased as tilting the focus lens. These findings suggest the recombination rate of Hg ions and electrons can be reduced with appropriate focus condition. Fig.10 shows the mass spectra at different positions of focus lens. Data in Table 2 confirm this improvement because the ratio of Hg + to N + increased in position 2 to reduce the influence of N2 on Hg+ signal intensity.
Fig.9. Power dependence using 532nm breakdown at different positions of focus lens (a) Hg+ signal; (b) N+ signal.
Fig.10. Mass spectra at different positions of focus lens with laser power 100mJ/p (a) position-1; (b) position-2. Table 2. The intensity ratio of Hg+/N+ at different positions of focus lens.
Power (mJ/p)
Hg+/N+ Position-1
Position-2
20
3.49
---
40
0.36
3.19
60
0.11
1.81
80
0.06
0.52
100
0.02
0.32
120
0.01
0.20
150
0.01
0.14
The concentration dependence of Hg+ signal intensity was measured using different dilution ratio samples at pressure 0.05Pa, as shown in Fig.11. Hg+ signal intensity was directly proportional to the concentration, indicated by the dilution ratio of buffer gas to diluent gas. The concentration was examined using mercury gas detector tube with
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the dilution ratio of 0.02, sampling point as shown in Fig.1(a). The detection limit of Hg was about 1ppb. As the linear pressure dependence of Hg+ signal intensity, the detection limit will be enhanced when increasing the pressure.
Fig. 11. Dependence of Hg+ signal intensity on concentration.
5. Conclusions Laser breakdown TOFMS method was developed and applied to detect heavy metal Hg in intermixture. This method features rapid detection without complex sample preparation and better detection limit compared with LIBS. (1) In order to measure heavy metals from intermixture, such as hydrocarbons and particles, laser breakdown process is essential to eliminate or minimize fragmentation from other species. Measurement results employing 532nm and 1064nm breakdown display without interference f of fragmentation, compared to the laser breakdown process using 266nm. (2) The influence of pressure in test chamber on Hg+ signal intensity shows the linear growth when the pressure increases. As two wavelengths were employed to break down and ionize the sample, the results using 532nm breakdown present higher signal intensity than that using 1064nm breakdown. (3) Because of the recombination of Hg ions and electrons, power dependence of Hg+ signal intensity increases first and then decreases using 1064nm and 532nm breakdown. Among the buffer gases, N2, Ar and He, the lowest recombination rate using He as buffer gas with highest ionization potential determined the phenomenon of recombination of Hg ions and electrons. (4) Due to the influence of recombination, the method of enlarging focus area and decreasing the laser power reduce the recombination rate of Hg ions and electrons. It is a proven means to enhance the detection limit with appropriate focus area. It is demonstrated the method is significantly desirable to measure heavy metals in intermixture. Further study will apply this method to these practical applications, such as environmental monitoring system and power plants. References [1]
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