Anal. Chem. 2006, 78, 1345-1348
Application of Ceramic Carbon Materials for Solid-Phase Extraction of Organic Compounds Lihong Shi,†,‡ Xiaoqing Liu,†,‡ Haijuan Li,†,‡ Wenxin Niu,†,‡ and Guobao Xu*,†
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China
Ceramic carbon materials were developed as new sorbents for solid-phase extraction of organic compounds using chlorpromazine as a representative. The macroporosity and heterogeneity of ceramic carbon materials allow extracting a large amount of chlorpromazine over a short time. Thus, the highly sensitive and selective determination of chlorpromazine in urine sample was achieved by differential pulse voltammograms after only 1-min extraction. The total analysis time was less than 3 min. In comparison with other electrochemical and electrochemiluminescent methods following 1-min extraction, the proposed method improved sensitivity by about 2 and 1 order of magnitude, respectively. The fast extraction, diversity, and conductivity of ceramic carbon materials make them promising sorbents for various solid-phase extractions, such as solid-phase microextraction, thin-film microextraction, and electrochemically controlled solidphase extraction. The preliminary applications of ceramic carbon materials in chromatography were also studied. Solid-phase extraction (SPE) is the most popular sample preparation method and is very actively used in the field of separation science. Many SPE sorbents have been developed, such as silica-based materials, carbon-based sorbents, ion-pair and ionexchange sorbents, immunoaffinity extraction sorbents, molecularly imprinted polymers, metal-loaded sorbents, and mixed-mode sorbents.1-7 Ceramic carbon materials (CCMs) are silica-carbon composite materials that are prepared by mixing carbon powder with sol-gel-derived ceramic binder. Compared with other silicabased materials, CCMs have the advantages of conductivity, stability, macroporosity, heterogeneity, and versatility.8-11 Al* Corresponding author. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry. ‡ Graduate School of the Chinese Academy of Sciences. (1) Thurman, E. M.; Mills, M. S. Solid-Phase Extraction: Principles and Practice; Wiley-Interscience: New York, 1998. (2) Hennion, M. C. J. Chromatogr., A 1999, 856, 3-54. (3) Pawliszyn, J. Solid-Phase Microextraction: Theory and Practice; Wiley-VCH: New York, 1997. (4) Fritz, J. S. Analytical Solid-Phase Extraction; Wiley-VCH: New York, 1999. (5) Simpson, N. J. K. Solid-Phase Extraction: Principles, Strategies and Applications; Marcel Dekker: New York, 1998. (6) Huck, C. W.; Bonn, G. K. J. Chromatogr., A 2000, 885, 51-72. (7) Hennion, M. C. J. Chromatogr., A 2000, 885, 73-95. (8) Lev, O.; Wu, Z.; Bharathi, S.; Glezer, V.; Modestov, A.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 9, 2354-2375. (9) Rabinovich, L.; Lev, O. Electroanalysis 2001, 13, 265-275. (10) Walcarius, A. Chem. Mater. 2001, 13, 3351-3372. 10.1021/ac051894e CCC: $33.50 Published on Web 01/12/2006
© 2006 American Chemical Society
though CCMs have been popular materials in electrochemistry for more than 10 years, their applications in other fields were rarely reported. An interesting application of CCMs is for extraction of some metal ion. The extraction process of metal ions reported was rather slow probably because the CCMs used are hydrophobic and metal ions are hydrophilic.12,13 Inspiringly, Lev’s group has suggested the diffusion of organic compounds in CCMs is very likely faster than that in silica films because of macroporosity and heterogeneity of CCMs.9 Moreover, it is possible to synthesize CCMs with a controlled wetted section up to 1 mm.9 The fast diffusion of organic compounds at CCMs, the thick wetted section of CCMs, together with the diversity and stability of CCMs indicate that CCMs are very likely attractive sorbents for fast and abundant extraction of organic compounds. Chlorpromazine is an important drug commonly used for treating various mental and personality disorders, and thus, its quantitative measurement in dosage form and physiological fluids gained numerous interest.14 For example, spectrophotometry,15,16 chemiluminescence,17 HPLC,18 electrochemistry,19 and absorption spectrometry20 have been utilized for the determination of chlorpromazine. Among these approaches, the approach based on adsorptive preconcentration at carbon paste electrodes has proved to be impressive for its sensitivity, selectivity, and simplicity.21-23 The ceramic carbon electrodes (CCEs) made from CCMs have many advantages over carbon paste electrodes, such as better stability, easier renewal, and faster diffusion. Superior properties are often expected by substituting CCE for carbon paste electrode.9 Moreover, it is easy to monitor SPE of electroactive organic compounds to CCEs. (11) Oskam, G.; Searson, P. C. J. Phys. Chem. B 1998, 102, 2464-2468. (12) Wang, J.; Pamidi, P. V. A.; Nascimento, V. B.; Angnes, L. Electroanalysis 1997, 9, 689-692. (13) Ji, Z. Q.; Guadalupe, A. R. Electroanalysis 1998, 11, 167-174. (14) Wilson, C. O.; Gisvold, O.; Doerge, R. F. Text Book of Organic Medicinal and Pharmacuetical Chemistry, 7th ed.; Lippincott: Philadelphia, 1977; p 384. (15) Revanasiddappa, H. D.; Ramappa, P. G. Talanta 1996, 43, 1291-1296. (16) Daniel, D.; Gutz, I. G. R. J. Pharm. Biomed. Anal. 2005, 23, 281-286. (17) Huang, Y.; Chen, Z. Talanta 2002, 57, 953-959. (18) Kollmorgen, D.; Kraut, B. J. Chromatogr., B 1998, 707, 181-187. (19) Petit, C.; Murakami, K.; Erdem, A.; Kilinc, E.; Borondo, G. O.; Liegoeis, J. F.; Kauffmann, J. M. Electroanalysis 1998, 10, 1241-1248. (20) El-Ansary, A. L.; El-Hawary, W. F.; Issa, Y. M.; Ahmed, A. F. Anal. Lett. 1999, 32, 2255-2269. (21) Wang, J.; Freiha, B. A. Anal. Chem. 1983, 55, 1285-1288. (22) Xu, G. B.; Dong, S. J. Anal. Chem. 2000, 72, 5308-5312. (23) Wang, J.; Freiha, B. A. Anal. Chem. 1984, 56, 849-852.
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Therefore, we demonstrated SPE of organic compounds to CCMs using electroactive chlorpromazine as a model analyte and CCEs as sorbents. The SPE of chlorpromazine was monitored by an electrochemical method. Fast SPE of organic compounds to CCEs was shown by effective extraction of chlorpromazine to CCEs within 1 min without taking any special measures, such as applying a suitable potential at the electrodes during extraction or adding suitable dopants. Furthermore, highly sensitive and selective determination of chlorpromazine was achieved by the combination of SPE to CCEs, medium exchange, and differential pulse voltammograms. Because many sorbents can be used not only in SPE but also in chromatography, the applicability of CCMs for liquid chromatography was also tested. EXPERIMENTAL SECTION Chlorpromazine, dopamine, methyltrimethoxysilane, and graphite powder were purchased from Aldrich. All other reagents were of reagent grade. Solutions were prepared with water purified in a Millipore system. The urine sample was obtained from one healthy volunteer who took no medications and was in good condition. Electrochemical measurements were performed in a conventional three-electrode cell with a CH Instrument model 832B (CHI Inc.). The fabrication of CCE is described as follows based on previous reports.24,25 A solution of 15 µL of methyltrimethoxysilane, 45 µL of methanol, and 1 µL of hydrochloric acid (11 mol L-1) catalyst was mixed ultrasonically for 2 min to ensure uniform mixing, then 50 mg of graphite powder was added, and the mixture was thoroughly mixed for an additional 3 min. The final mixture was packed into glass tubes (with 2-mm inner diameter, 5-mm depth) and dried at room temperature for 48 h. A fresh working electrode surface was obtained by polishing the prepared CCE before each measurement. The determination of chlorpromazine included two main steps, the SPE in a 4-mL stirred sample solution for a given time period and the differential pulse voltammetric detection in the electrolytic blank solution of 50 mmol L-1 phosphate buffer using a scan rate of 10 mV s-1 at an amplitude of 50 mV. Liquid chromatography was carried out using a homemade chromatograph that was equipped with a pump, an injector, and an electrochemical detector (CH Instrument model 832B). The working electrode used in these experiments was an Au electrode with a diameter of 1.0 mm. Columns were prepared by a drypacking technique; they were 100 mm long with a 5-mm bore. The ceramic carbon column-packing material was obtained by powdering the CCMs prepared above. The flow rate was 0.3 mL min-1. The eluent was 50 mmol L-1 acetate buffer (pH 4.5). RESULTS AND DISCUSSION Electrochemistry of Chlorpromazine Following SPE. The SPE process at CCEs is a combination of extraction at carbon powder and at a sol-gel-derived ceramic binder. Figure 1a shows the differential pulse voltammograms in 50 mmol L-1 pH 7.0 phosphate buffer following 60-s extraction from a 4-mL stirred 5 × 10-7 mol L-1 chlorpromazine solution to the CCEs. In com(24) Tsionsky, M.; Gun, J.; Glezer, V.; Lev, O. Anal. Chem. 1994, 66, 17471753. (25) Wang, P.; Yuan, Y.; Jing, X.; Zhu, G. Talanta 2001, 53, 863-869.
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Figure 1. Differential pulse voltammograms at CCE (a) in supporting electrolyte after 60-s extraction from a stirred 50 mmol L-1 pH 7.0 phosphate buffer containing 5 × 10-7 mol L-1 chlorpromazine and (b) in 50 mmol L-1 pH 7.0 phosphate buffer containing 5 × 10-7 mol L-1 chlorpromazine without preconcentration.
parison with the direct measurement in sample solution (Figure 1b), the SPE resulted in a significant enhancement of the current response. The basic electrochemical properties of accumulated chlorpromazine at CCEs were investigated by cyclic voltammetry. Similar to the case at a carbon paste electrode, accumulated chlorpromazine at the CCE exhibits an irreversible oxidation behavior in 50 mmol L-1 pH 7.0 phosphate buffer.22 A positive peak occurred at 0.74 V at a scan rate of 100 mV s-1, and the peak potential shifts by ∼60 mV with increasing scan rate from 10 to 200 mV s-1. A linear relationship can be established between the peak current and the scan rate (correlation coefficient of 0.999, n ) 7) in the range of 10-200 mV s-1, indicating that it is a typical surface process. In this study, a transfer of the working electrode from the sample solution to an electrolytic blank solution was used for improving selectivity. Consequently, the sensitivity of the proposed method depended not only on pH of the sample solution but also on pH of the electrolytic blank solution. To obtain good sensitivity, the effect of pH of the electrolytic blank solution on differential pulse voltammetric detection was first studied. The current response for chlorpromazine increases as the pH of the electrolytic blank solution increases from 4 to 7 and decreases at a solution pH of >7 following 60-s extraction from 4-mL stirred pH 7.0 chlorpromazine solution to the CCEs. Thus, a pH value of 7.0 was used for further measurements. The dependence of current on pH of the electrolytic blank solution was likely attributed to the deprotonation of chlorpromazine (pKa ) 9.35) at higher pH and the dependence of surface properties of the CCE on pH.26 Optimization of Experimental Conditions for SPE of Chlorpromazine. To estimate the extraction rate of chlorpromazine at CCEs, the influence of preconcentration time on the peak current was studied. As shown in Figure 2, the peak current increases rapidly with preconcentration time and then levels off at ∼60 s. The extraction of chlorpromazine at CCEs is much faster than that at carbon paste electrodes.21-23 The rapid and highly effective accumulation of chlorpromazine is attributed to the macroporosity, heterogeneity, and hydrophobicity of CCEs.9,27 (26) Kitamura, K.; Takenaka, M.; Yoshida, S.; Ito, M.; Nakamura, Y.; Hozumi, K. Anal. Chim. Acta 1991, 242, 131-135. (27) Wang, J.; Park, D. S.; Pamidi, P. V. A. J. Electroanal. Chem. 1997, 434, 185-189.
Figure 2. Influence of the extraction time on the peak current of 5 × 10-6 mol L-1 chlorpromazine.
Figure 3. Influence of pH of 5 × 10-6 mol L-1 chlorpromazine solution on the peak current at CCE following 60-s extraction.
As mentioned above, the SPE of chlorpromazine is also affected by the pH of chlorpromazine sample solutions. The maximum response is obtained in pH 7.0 sample solution (Figure 3). The results are most likely due to the increase in the concentration of the neutral form of chlorpromazine with increasing chlorpromazine solution pH, which results in a higher affinity of chlorpromazine toward CCMs.28-30 At a chlorpromazine solution pH higher than 7, the surface of CCE may become less hydrophobic, and their sorptive properties may be compromised.31 Therefore, a pH value of 7 was used for the further measurements. Calibration Curve. The calibration plot for chlorpromazine determination is linear in the range of 5.0 × 10-9-1.0 × 10-6 mol L-1 under optimum conditions (slope 0.66 nA L/nmol; intercept 38 nA; correlation coefficient 0.998, n ) 8). The detection limit is 3.5 × 10-9 mol L-1 at a signal-to-noise ratio of 3. The relative standard deviation is 5.2% for six replicate determinations of 5 × 10-7 mol L-1 chlorpromazine. The total analysis time is less than 3 min. To our knowledge, it is the most sensitive method for the (28) Cheng, H. Y.; Sackett, P. H.; McCreery, R. L. J. Am. Chem. Soc. 1978, 100, 962-967. (29) Cheng, H. Y.; Sackett, P. H.; McCreery, R. L. J. Med. Chem. 1978, 21, 948-952. (30) Yang, H. H.; McCreery, R. L. Anal. Chem. 1999, 71, 4081-4087. (31) Alhooshani, K.; Kim, T.-Y.; Kabir, A.; Malik, A. J. Chromatogr., A 2005, 1062, 1-14.
determination of chlorpromazine. In comparison with other electrochemical and electrochemiluminescent methods following 1-min extraction, the proposed method improved sensitivity by about 2 and 1 order of magnitude, respectively.21,22 The high sensitivity of the proposed method is ascribed to fast diffusion of chlorpromazine and the large adsorbing area of CCMs that facilitate extraction of large amounts of chlorpromazine over a short time. The remarkable sensitivity of the proposed method suggests that CCMs are very promising materials for fast and effective SPE. Interference Study and Sample Analysis. Solutions of 5 mmol L-1 ascorbic acid, alanine, isolecucine, and serine, 2 mmol L-1 glutamic acid, threonine, phenylalanine and oxalate, and 0.5 mmol L-1 tyrosine, tryptophan, asparagine, and cystine did not interfere with the determination of 1 µmol L-1 chlorpromazine. The investigated concentrations of ascorbic acid, oxalate, and amino acids were higher than their concentrations in normal urine samples, respectively.32,33 The tolerable limit of a foreign species was taken as a relative error less than 5%. To further demonstrate the practicality of the proposed method, the concentration dependence and precision studies were performed utilizing urine samples after storage at room temperature for 5 h. Prior to use, the urine samples were diluted (1:4) with the supporting electrolyte. The differential pulse voltammogram of diluted urine samples, spiked with 1 × 10-6 mol L-1 chlorpromazine, exhibited one main anodic peak of chlorpromazine at 0.66 V and a very weak peak of the adsorbed uric acid at 0.36 V.34 Because of the marked difference of the two potentials, uric acid did not interfere with the determination of chlorpromazine in urine samples. The voltammetric peak current varies linearly with chlorpromazine concentration over the range of 2.0 × 10-8-1.0 × 10-6 mol L-1 (slope 0.50 nA L/nmol; intercept 53 nA; correlation coefficient 0.998, n ) 7). The detection limit is 1.0 × 10-8 mol L-1 at a signal-to-noise ratio of 3. The relative standard deviation of six repeated determinations of diluted urine sample spiked with 5 × 10-7 mol L-1 chlorpromazine is 6.9%. This result shows that the proposed method allows selective determination of chlorpromazine. The selectivity observed is likely due to the following reasons. First, polar interfering compounds were effectively rejected by the CCE because of its permselective properties based on polarity. Second, the medium-exchange approach (a transfer of the working electrode from the sample solution to electrolytic blank solution) was adopted in the present method. So interferences from species that cannot be accumulated onto the electrode have been minimized. Finally, the oxidation potential of electroactive substances, such as uric acid, is far away from that of chlorpromazine. Chromatographic Applications of CCMs. A good chromatographic column-packing material requires a high surface area and good structural strength.35 CCMs, the combination of carbon and silica materials, have been proved to match these properties.9 So we made a preliminary investigation of the potential chromatographic application of CCMs. The separation of dopamine and (32) Rubinstein, I.; Martin, C. R.; Bard, A. J. Anal. Chem. 1983, 55, 15801582. (33) Medical Biochemistry; The first medical college of Shanghai; People’s Medical Press: Beijing, 1979; pp 281-309. (34) Chaney, E. N.; Baldwin, R. P. Anal. Chem. 1982, 54, 2556-2560. (35) Knox, H. H.; Kaur, B.; Millward, G. R. J. Chromatogr. 1986, 352, 3-25.
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Figure 4. Separation of dopamine and ascorbic acid on a CCM packing column. Applied potential, 0.75 V. Peak 1, 1 mmol L-1 ascorbic acid; peak 2, 1 mmol L-1 dopamine.
ascorbic acid was selected as a model considering the critical importance of simultaneous determination of dopamine and ascorbic acid in the field of neurochemistry and biomedical chemistry. Because dopamine and ascorbic acid have similar oxidation potential at bare electrodes, one of the methods for their simultaneous determination is based on electrochemical detection coupled with separation techniques.36-39 Figure 4 shows that dopamine and ascorbic acid were well separated on a CCM packing column. The efficient separation of the two compounds confirmed the feasibility of CCMs in chromatographic application. CONCLUSIONS This work shows that CCMs are able to extract organic compounds such as chlorpromazine rapidly and effectively. The (36) Ranganathan, S.; Kuo, T. C.; McCreery, R. L. Anal. Chem. 1999, 71, 35743580. (37) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893-900. (38) Wang, J.; Walcarius, A. J. Electroanal. Chem. 1996, 407, 183-187. (39) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180-3186. (40) Bruheim, I.; Liu, X. C.; Pawliszyn, J. Anal. Chem. 2003, 75, 1002-1010. (41) Wu, J. C.; Mullett, W. M.; Pawliszyn, J. Anal. Chem. 2002, 74, 48554859.
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combination of SPE to CCEs, medium exchange, and differential pulse voltammograms allows fast, sensitive, and selective determination of chlorpromazine. Such a combination can be readily extended to directly analyzing other electroactive organic compounds in simple samples. The determination of complex samples can be achieved by coupling SPE at CCMs with other separation techniques such as chromatography and capillary electrophoresis. CCM sorbents have seven main advantages. First, there is much freedom in choosing the composition of CCMs. Second, CCM sorbents can be readily cast to a wide range of configurations such as powder, film, rod, and disk and thus are applicable to various SPEs.40 Third, macroporosity and heterogeneity of CCMs allow fast extraction of target analytes. Fourth, a controlled porous section up to 1 mm can be wetted in solutions, permitting abundant extraction of target analytes. Fifth, it is possible to control sorption and desorption at CCMs by electrochemical methods.41 Sixth, CCMs offer the possibility for in situ electrochemically derivative SPE. Finally, both extraction and electrochemical detection at the same CCM is possible. The advantages above indicate that CCMs are nice sorbents for a variety of SPE, such as solid-phase microextraction, thin-film extraction, and electrochemical derivatization SPE. Preliminary study shows that CCMs also is an alternative packing material for chromatography. Work on solid-phase microextraction of organic compounds is ongoing in our laboratory. ACKNOWLEDGMENT We thank Professor Wang EK and Dong SJ for helpful suggestions. This project was supported by the National Natural Science Foundation of China (20505016 and 20427003), Changchun Institute of Applied Chemistry, and State Key Laboratory of Electroanalytical Chemistry.
Received for review December 16, 2005. AC051894E
October
24,
2005.
Accepted
