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Fast Digestion Procedure for Determination of Catalyst Residues in La- and Ni-Based Carbon Nanotubes Sergio Roberto Mortari,† Carmem Regina Cocco,† Fabiane Regina Bartz,‡ Valderi L. Dresssler,‡ ´ rico Marlon de Moraes Flores*,‡ and E Cieˆncias Tecnolo´gicas, Centro Universita´rio Franciscano-UNIFRA, Rua dos Andradas, 1614, Santa Maria/RS, Brazil, Departamento de Quı´mica, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil, and Instituto Nacional de Cieˆncia e Tecnologia de Bioanalı´tica, Campinas, SP, Brazil A procedure based on microwave-induced combustion (MIC) was applied for carbon nanotube (CNT) digestion and further determination of La and Ni by inductively coupled plasma optical emission spectrometry (ICP OES). Samples (up to 400 mg) were completely combusted at 20 bar of oxygen, and a reflux step was applied to improve the analyte absorption. Combustion was finished in less than 50 s, and analytes were absorbed in diluted acid solution. Absorbing solutions ranging from 1 to 12 mol L-1 for HCl and from 1 to 14 mol L-1 HNO3 were tested. Accuracy for both analytes was evaluated using certified reference materials and analyte spikes. Neutron activation analysis was also used to check accuracy for La. Agreement was better than 96% for La and Ni using a 4 mol L-1 absorbing solution of HNO3 or HCl and 15 min of reflux. The residual carbon content was lower than 0.5%. Up to eight samples could be digested simultaneously in 36 min, that makes the throughput using MIC more suitable when it is compared to the digestion by dry ashing as recommended by other procedures. The obtained limits of detection using MIC were lower than those using dry ashing, and a single absorbing solution, e.g., diluted HNO3, can be used for simultaneous determination of La and Ni by ICP OES. Nowadays, despite other preparation procedures the synthesis of commercially available carbon nanotubes (CNTs) has been mainly performed by chemical vapor deposition, arc discharge, and high pressure carbon monoxide (HiPco method). However, CNTs based on these procedures are frequently contaminated by residues of metals used in the synthesis process.1-3 The use of acid washing, the most common procedure for CNT purification, does not efficiently remove the catalyst residues, and a variable * Corresponding author. Fax: +55 55 3220-9445. E-mail: flores@ quimica.ufsm.br. † Centro Universita´rio Franciscano-UNIFRA. ‡ Universidade Federal de Santa Maria and Instituto Nacional de Cieˆncia e Tecnologia de Bioanalı´tica. (1) Chen, F. Appl. Phys. Lett. 2003, 83, 4601. (2) Pumera, M. Langmuir 2007, 23, 6453–6458. (3) Kim, H. M.; Kim, K.; Lee, C. Y.; Joo, J.; Cho, S. J.; Yoon, H. S.; Pejakovic, D. A.; Yoo, J. W.; Epstein, A. J. Appl. Phys. Lett. 2004, 845, 89–591.
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content of metals unfortunately still remains after this cleaning process.4 CNTs with metal contaminants can make the complete understanding of their properties difficult, frequently causing contradictions related to toxicity and risk assessments.5-7 On the other hand, electronic and other related properties can be affected by the presence of metals in CNTs.8-10 Therefore, there is a strong necessity for the development of analytical methods for the metallic impurity control, in special catalyst residues, with accuracy and limit of detection (LOD) suitable to the determination of contaminants in low levels. However, despite that the interest in new applications of CNTs has grown at a very fast rate in the past decade, there is a lack of reliable analytical procedures to determine metallic impurities, specially for Ni and La. General analysis of CNTs can be performed by Raman spectroscopy, X-ray fluorescence spectroscopy (XRF), thermal gravity analysis (TGA), transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron scattering (XPS).11-15 However, some of these techniques provide only information about general impurities and, in some cases, for indirect metal content (e.g., TGA). On the other hand, XRF is a powerful technique, but it presents some limitations if quantitative determination in low levels is required.16 In the case of XPS, results for some elements can be underestimated resulting in (4) Ge, C.; Lao, F.; Li, W.; Li, Y.; Chen, C.; Qiu, Y.; Mao, X.; Li, Bai; Chai, Z.; Zhao, Y. Anal. Chem. 2008, 80, 9426–9434. (5) Isobe, H.; Tanaka, T.; Maeda, R.; Noiri, E.; Solin, N.; Yudasaka, M.; Iijima, S.; Nakamura, E. Angew. Chem. 2006, 45, 6676–6680. (6) Smart, S. K.; Cassady, A. I.; Lu, G. Q.; Martin, D. J. Carbon 2006, 44, 1034–1047. (7) Kagan, V. E.; Bayir, H.; Shvedova, A. A. Nanomedicine: NBM 2005, 1, 313–316. (8) Ajayan, P. M. Chem. Rev. 1999, 99, 1787–1799. (9) McEuen, P. L.; Fuhrer, M. S.; Park, H. K. IEEE Trans. Nanotechnol. 2002, 1, 78–85. (10) Jorio, A.; Dresselhaus, G., Dresselhaus, M. S., Eds. Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties and Applications. Springer-Verlag: Berlin and Heidelberg, 2008. (11) Bustero, I.; Ainara, G.; Isabel, O.; Roberto, M.; Ine´s, R.; Amaya, A. Microchim. Acta 2006, 15, 239–247. (12) Itkis, M. E.; Perea, D. E.; Jung, R.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 3439–3448. (13) Li, H.; Zhao, N.; He, C.; Shi, C.; Du, X.; Li, J. Mater. Sci. Eng., A 2008, 473, 355–359. (14) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; McLaughlin, J.; Brown, N. M. D. Carbon 2005, 43, 153–161. (15) Bussy, C.; Cambedouzou, J.; Lanone, S.; Leccia, E.; Heresanu, V.; Pinault, M.; Mayne-l’Hermite, M.; Brun, N.; Mory, C.; Cotte, M.; Doucet, J.; Boczkowski, J.; Launois, P. Nano Lett. 2008, 8, 2659–2663. 10.1021/ac100429v 2010 American Chemical Society Published on Web 04/21/2010
errors in quality evaluation of CNT.17 Neutron activation analysis (NAA) has some advantages in view of its high sensitivity and selective determination for many elements, but the necessity of a nuclear reactor and other related drawbacks impair its availability for routine analysis. Inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry (ICPMS) have been used for element determination in a variety of samples.18-21 In spite of the high performance of these techniques, samples must be previously decomposed in order to obtain a suitable solution for further analysis. A CNT, like other carbon structured materials (e.g., graphite), is very difficult to bring into solution by conventional acid wet digestion due to its stable composition. Some works have used a dry ashing method for digestion of CNT (550 °C by 3 h) and further analysis by ICPMS.22 Strong et al.17 performed the determination of metal impurities in CNTs by ICPMS after partial digestion using aqua regia. However, as the digestion was incomplete, the undissolved fraction must be analyzed by XRF. Underestimated results were found for some metals, and it was suggested that a shell carbon surrounded the metal catalyst avoiding an efficient attack by aqua regia. Recently, several digestion methods were evaluated for further determination of metal contaminants in CNTs by analysis using ICPMS.4 Carbon nanotubes were digested using a dry ashing method followed by an acid extraction step, wet digestion, or a combination of dry ashing (up to 750 °C) with wet digestion in open vessels. In spite that a suitable solution could be obtained using microwave-assisted digestion, some drawbacks, such as acid consumption and excessive time of digestion, were reported as limitations of this procedure. Authors recommended the use of a procedure based on a combination of dry ashing with acid digestion in open vessels as a convenient way to digest CNTs before ICPMS analysis. However, the use of dry ashing for trace analysis is nowadays avoided due to the risk of analyte losses and specially to contamination23,24 (no information about dry ashing time was provided by the authors).4 The same comments can be made regarding the use of wet digestion performed in open vessels for the long digestion time (in this case, up to 8 h). Even after wet digestion, an additional step for acid evaporation was necessary. In addition, the overall proposed procedure is timeconsuming, and it is suitable only for low sample masses (between 10 and 20 mg) that could result in high relative standard deviation if samples are not homogeneous. Therefore, the procedure proposed by Ge et al. involving the combination of two digestion (16) Ramesh, B. P.; Blau, W. J.; Tyagi, P. K.; Misra, D. S.; Ali, N.; Gracio, J.; Cabral, G.; Titus, E. Thin Solid Films 2006, 494, 128–132. (17) Strong, K. L.; Anderson, D. P.; Lafdi, K.; Kuhn, J. N. Carbon 2003, 41, 1477–1488. (18) No ¨lte, J. ICP Emission Spectrometry; Wiley-VCH: Weinheim, Germany, 2003. (19) Montaser, A., Ed. Inductively Coupled Plasma Mass Spectrometry; WileyVCH: New York, 1998. (20) Moraes, D. P.; Mesko, M. F.; Mello, P. A.; Paniz, J. N. G.; Dressler, V. L.; Knapp, G.; Flores, E. M. M. Spectrochim. Acta, Part B 2007, 62, 1065– 1071. (21) Charlton, B.; Fisher, A. S.; Goodall, P. S.; Hinds, M. W.; Lancaster, S.; Salisbury, M. J. Anal. At. Spectrom. 2007, 22, 1517–1560. (22) Lam, C.-W.; James, J. T.; McCluskey, R.; Hunter, R. L. Toxicol. Sci. 2004, 77, 126–134. (23) Barin, J. S.; Flores, E. M. M.; Knapp, G. In Trends in Sample Preparation; Arruda, M. A. Z., Ed.; Nova Science Publishers: Hauppauge, 2006; pp 73114. (24) Flores, E. M. M.; Mesko, M. F.; Knapp, G.; Barin, J. S. Spectrochim. Acta, Part B 2007, 62, 1051–1064.
procedures (dry ashing and wet digestion) and a further acid evaporation step could be very troublesome and perhaps not suitable for routine analysis, mainly if a high throughput is required. Combustion methods are considered suitable for carbon-based materials, and they can be very attractive for trace analysis if closed vessels are used.24 Recently, a new method, combining the advantages of combustion and pressurized wet digestion in closed vessels, has been proposed. This method, named microwaveinduced combustion (MIC), was successfully applied for digestion of organic samples and further determination of a variety of analytes.25-30 With this system, the used system was considered safe for routine operation, the use of reagents is generally reduced, and very low residual carbon content (RCC), a parameter used to evaluate digestion efficiency, can be obtained in addition to a relatively low digestion time (in general eight samples can be simultaneously digested in about 30 min). In addition, the use of diluted solutions makes the obtained digest suitable for further analysis using different techniques.25–30 On the basis of the limitations of classical methods discussed before, in the present work, the use of MIC is proposed in the first time for total digestion and further determination of nickel and lanthanum in multiwalled CNTs using inductively coupled plasma optical emission spectrometry. These elements are the most common contaminants in CNT, and they can affect the physical properties of CNTs, making their determination necessary using reliable analytical procedures in order to ensure a suitable quality control. Certified reference samples of similar composition, analyte spikes, and also neutron activation analysis were used in order to evaluate the accuracy of the proposed procedure. EXPERIMENTAL SECTION Instrumentation. A Model Multiwave 3000 microwave sample preparation system (Anton Paar, Graz, Austria) equipped with up to eight high-pressure quartz vessels was used in this study for sample digestion. Vessels had an internal volume of 80 mL, and the maximum operation pressure was 80 bar. This equipment was used for conventional high-pressure acid digestion and for the proposed combustion procedure (MIC). Pressure was monitored in each vessel for all the runs. The software version was v1.27Synt, and the microwave system was previously modified to run with a maximum pressure rate of 0.3 MPa s-1 (and not 0.08 MPa s-1 as in the original software). This change was necessary to prevent eventual interruption of the microwave irradiation before all the samples start the combustion. Commercial quartz holders were used to place the sample into the quartz vessel. The quartz holder was designed in order to reduce the cool (25) Duarte, F. A.; Pereira, J. S. F.; Barin, J. S.; Mesko, M. F.; Dressler, V. L.; Flores, E. M. M.; Knapp, G. J. Anal. At. Spectrom. 2009, 24, 224–227. (26) Flores, E. M. M.; Mesko, M. F.; Moraes, D. P.; Pereira, J. S. F.; Mello, P. A.; Barin, J. S.; Knapp, G. Anal. Chem. 2008, 80, 1865–1870. (27) Mello, P. A.; Giesbrecht, C. K.; Alencar, M. S.; Moreira, E. M.; Paniz, J. N. G.; Dressler, V. L.; Flores, E. M. M. Anal. Lett. 2008, 41, 1623–1632. (28) Pereira, J. S. F.; Diehl, L. O.; Duarte, F. A.; Santos, M. F. P.; Guimara˜es, R. C. L.; Dressler, V. L.; Flores, E. M. M. J. Chromatogr. 2008, 1213, 249– 252. (29) Barin, J. S.; Bartz, F. R.; Dressler, V. L.; Paniz, J. N. G.; Flores, E. M. M. Anal. Chem. 2008, 80, 9369–9374. (30) Pereira, J. S. F.; Mello, P. A.; Moraes, D. P.; Duarte, F. A.; Dressler, V. L.; Knapp, G.; Flores, E. M. M. Spectrochim. Acta, Part B 2009, 64, 554–558.
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Table 1. Operational Parameters for ICP OES Determination ICP OES Settings power (W) 1400 RF generator (MHz) 27 Ar plasma flow rate (L min-1) 12 Ar auxiliary flow rate (L min-1) 1.0 Ar nebulizer flow rate (L min-1) 1.0 spray chamber Scott double pass nebulizer cross flow Analytical Wavelength (nm) La 408.672 Ni 231.604
surfaces that might cause carbon deposits during the combustion. The holder had a conical quartz protector mounted in the upper part to shield the polytetrafluorethylene lid of the quartz vessel from the flame during combustion. A Model Spectro Ciros CCD simultaneous spectrometer was used for Ni and La determinations by ICP OES with axial view configuration (Spectro Analytical Instruments, Kleve, Germany). A Scott double pass type nebulization chamber coupled to a crossflow nebulizer was used throughout. Plasma operating conditions and wavelengths used for La and Ni determination are listed in Table 1 and were optimized or used as recommended by the instrument manufacturer.31 Argon 99.996 (White Martins-Praxair, SP, Brazil) was used for plasma generation, for nebulization, and as auxiliary gas. Residual carbon content was determined by ICP OES, and measurements were performed according to the conditions previously described.32 A conventional muffle furnace (Heraeus, Germany) was used to digest CNT samples by the dry ash method. Reagents. Milli-Q water (18.2 MΩ cm, Millipore, Billerica, USA) was used to prepare all solutions. Analytical-grade reagents were used (Merck, Darmstadt, Germany) throughout. Concentrated nitric acid (65%) was purified by distillation in a subboiling system (Milestone, model DuoPur, Sorisole, Italy). Working analytical solutions for La and Ni were prepared before use by serial dilution of stock reference solutions containing 1000 mg L-1 (Spex plasma solutions, Metuchen, USA). Ammonium nitrate was dissolved in water, and this solution was used as an igniter for the combustion procedure. A small disk of filter paper (15 mm of diameter, 12 mg) with low ash content (Black Ribbon Ashless, Schleicher and Schuell GmbH, Dassel, Germany) was used to aid the combustion process. The filter paper was previously cleaned by washing using 10% (m/v) HNO3 solution for 20 min in an ultrasound bath and dried in an oven for 2 h at 60 °C. Glass materials were soaked in 10% (m/v) HNO3 for 24 h and thoroughly washed with water before using. The studies were carried out using commercial multiwalled carbon nanotubes (MWCNT) produced using La and Ni catalysts. The following CNTs were analyzed: MWCNT-1 (o.d. 20-40 nm, i.d. 5-10 nm, length 1-2 µm), MWCNT-2 (o.d. 10 ± 3 nm, i.d. 2-7 nm, length 5-15 µm), and MWCNT-3 (o.d. 20-40 nm, i.d. 5-10 nm, length 5-15 µm). Samples were (31) Spectro Ciros CCD-Software version 01/March 2003, Spectro Analytical Instruments GmbH & Co. KG, Kleve, Germany. (32) Flores, E. M. M.; Mesko, M. F.; Moraes, D. P.; Pereira, J. S. F.; Mello, P. A.; Barin, J. S.; Knapp, G. Anal. Chem. 2008, 80, 1865.
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pressed as pellets (diameter of 5 or 13 mm and mass between 20 and 400 mg) using a hydraulic press set at 3 ton during 4 min (Specac, Orpington, UK). The following certified reference materials (CRM) were used in this work: IRMM BCR 40 (trace elements in coal), NIST SRM 1632b (trace elements in coal), NIST SRM 1632c (trace elements in coal), and SARM 19 (Coal, O. F. S.). These CRMs were pressed and digested using the same proposed procedure selected for CNT samples. Proposed MIC Procedure and Microwave-Assisted and Dry Ashing Digestion. For the proposed combustion procedure, pressed samples were weighed (20-400 mg) directly on the filter paper. After weighing, samples were placed in the quartz holder. Fifty microliters of 6 mol L-1 ammonium nitrate solution was added to the filter paper. Quartz vessels were previously charged with 6 mL of absorbing solution as follows: nitric acid (1, 2, 4, 8, 12, or 14 mol L-1) or hydrochloric acid (1, 2, 4, 8, or 12 mol L-1). After closing the vessels and capping the rotor, vessels were pressurized with 20 bar of oxygen. Then, the rotor was placed inside the microwave oven, and the selected microwave heating program was started: (i) 1400 W for 60 s (ignition/combustion step), (ii) 1400 W for 15 min (reflux step), and (iii) 0 W for 20 min (cooling step). In this work, each run was performed with a minimum of four vessels. After this procedure, pressure was released for all vessels. Clear solutions were obtained, and they were diluted with water and transferred to 50 mL polypropylene vials and analyzed by ICP OES. If necessary, further dilution was performed with water. For the comparison of results, a procedure using microwaveassisted digestion in closed vessels was carried out using the following conditions: 100 mg of sample and 60 min at 1400 W (maximum temperature and pressure were 250 °C and 80 bar, respectively) using 6 mL of concentrated HNO3, concentrated HCl, a mixture of concentrated HNO3 and HCl (5 and 1 mL, respectively), or a mixture of concentrated HNO3 and H2O2 (5 and 1 mL, respectively). After cooling (20 min), digests were diluted with water to 50 mL. Cleaning of vessels and holders used in the proposed MIC procedure and the high-pressure microwave digestion procedure was carried out with 6 mL of concentrated HCl and HNO3 (1:1) with the microwave power set at 1400 W for 5 min and 0 W for 20 min for cooling. The dry ashing procedure was carried out using a conventional muffle furnace with electronic control of the temperature setting. Sample was weighed (100 mg) and transferred to Pt crucibles. Temperature was raised with a heating rate of 5 °C min-1 until 550 °C for 3 h. Residual ashes were dissolved in 5 mL of concentrated HCl by heating at 80 °C during 2 h. The ash dissolution procedure was repeated 3 times, and addition of HCl solution was performed in order to avoid the dryness of the solution. Finally, digests were diluted with water and analyzed by ICP OES. Caution: Take into account health care when working with CNTs; the usual precautions for handling chemicals should be followed. Work with CNTs should always be (if possible) performed in fume hoods, avoiding contact with the eyes and skin. RESULTS AND DISCUSSION Initial studies were performed in order to evaluate the safety aspects of the MWCNT combustion process. The temperature achieved during the combustion was higher than 1350 °C for 400
Figure 1. Recoveries for La (0) and Ni (9) determination after MIC digestion with a reflux step of 15 min using HCl (A) and HNO3 (B) absorbing solutions.
Figure 2. (A) MWCNT digested using concentrated HNO3 microwaveassisted digestion (250 °C and 80 bar, 60 min). (B) MWCNT digested using MIC procedure: 400 mg of sample, 6 mL of 4 mol L-1 HNO3 as absorbing solution, and 15 min of reflux time.
mg of CNT using the initial oxygen pressure at 20 atm. Combustion was complete in 50 s, and during this step, a white, bright light was emitted that also confirms the high temperature achieved. However, despite the high temperature, no damages were observed in quartz holders and vessels. The maximum pressure achieved during the combustion of 200 and 400 mg of CNT samples using 20 atm as initial oxygen pressure was 25 and 32 atm, respectively. It is important to notice that maximum pressure was only about 40% of the maximum working pressure recommended by the manufacturer (80 atm). Then, the proposed MIC procedure can be considered safe for CNT combustion, even with a sample mass as high as 400 mg. When MIC was used without a reflux step, a small portion of white solid residue was observed on the quartz holder surface. This residue was identified as Ni and La oxides that were generated during combustion. However, with a reflux step of only 15 min using nitric acid or hydrochloric acid (arbitrarily selected as 6 mol L-1), this residue was completely dissolved and a clear solution was obtained. Therefore, in order to ensure a complete leaching of residues, a reflux step was used for all the further tests. The use of concentrated acids for sample digestion should, if possible, always be avoided due to safety reasons and also to minimize the blank values, reagent consumption, and the consequent effluent generation. With respect to this, previous works using MIC for high carbon-content samples have shown the efficiency of a reflux step using diluted solutions for achieving better analyte recoveries.20,24 Then, diluted and also concentrated
Figure 3. Transmission electron microscopy images (TEM, Zeiss EH-900 electron microscope, 80 kV) of the sample “MWCNT-3” after acid digestion.
solutions of HCl and HNO3 were tested in this work as absorbing solutions with the use of a reflux step. For these tests, spikes of Ni and La reference solutions were added to CNT samples. In Figure 1 are shown the recoveries for La and Ni after a MIC procedure and using a reflux step of 15 min (mass of CNT was 400 mg). For Ni, the use of diluted solutions (1 or 2 mol L-1 for HCl or HNO3, respectively) up to the more concentrated solutions were considered suitable as absorbing solutions. Recoveries better than 97% were obtained using diluted solutions. Therefore, despite that more concentrated solutions for HCl and HNO3 could also be employed, their use was not necessary. It can be considered as an advantage, taking into account the consumption of reagents, level of blanks and, consequently, the generation of laboratory residues. However, for La at least 4 mol L-1 HCl or HNO3 should be used to obtain quantitative recoveries (>97%). Therefore, these results show that a suitable absorbing solution for both analytes could be 4 mol L-1 HCl or HNO3. Obviously, if only Ni should be determined, more diluted solutions could be used. In addition, as the digests were further diluted with water to 50 mL, the final acidity should be less than 0.5 mol L-1, that is a convenient solution for further determination by ICP OES. Analytical Chemistry, Vol. 82, No. 10, May 15, 2010
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Table 2. Concentration of Ni and La (µg g-1) in Certified Reference Materials of Coal (Values in Brackets Represent the Interval of Confidence) CRM
Ni
La
BCR 40 SRM 1632b SRM 1632c SARM 19
25.4 ± 1.6 6.10 ± 0.27 9.32 ± 0.51 16 (13-20)
5.1 (informed value) 27 (26-29)
Table 3. Content of La and Ni in MWCNT Determined by ICP OES after Digestion Using the Proposed MIC Procedure (n ) 5) sample
La (µg g-1)
Ni (µg g-1)
1 2 3
12264 ± 576 4277 ± 132 72.4 ± 1.8
5540 ± 87 13230 ± 821 3648 ± 69
Digestion using a conventional high-pressure microwave assisted procedure was also evaluated using concentrated HCl or HNO3 or their mixtures. However, for all the tests and even using concentrated acids, the digestion was not complete and a high amount of residual matrix remained as a black suspension (Figure 2A). This result can be explained in view of the high resistance of CNTs to acid attack using concentrated acids even under high temperature and pressure (250 °C and 80 bar, respectively, by 60 min).4 Therefore, this digestion procedure could not be used for further determination of Ni and La by ICP OES. Even if the resultant suspension was filtered and the solution was analyzed by ICP OES, the recovery for Ni and La presented a maximum value of 85%. On the other hand, using the proposed MIC procedure, a clear solution was obtained after the reflux step (Figure 2B) Residual carbon content was evaluated for highpressure microwave and MIC digests. It was >65% and
