Anal. Chem. 2003, 75, 4493-4498
Photocatalytic Dehalogenation Coupled On-Line to a Reversed Micellar-Mediated Chemiluminescence Detection System: Application to the Determination of Iodinated Aromatic Compounds Terufumi Fujiwara,*,† Imdad U. Mohammadzai,‡ Hidekazu Inoue,† Yasuhide Shimizu,† and Takahiro Kumamaru§
Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan, Department of Chemistry, University of Peshawar, NWFP, Peshawar, Pakistan, and Department of Life Science, Yasuda Women’s Junior College, Yasuhigashi, Asaminami-ku, Hiroshima 731-0153, Japan
The effects of illumination time, temperature, catalyst concentration, and pH on the on-line photocatalytic dehalogenation of iodinated aromatic compounds in a nearUV-illuminated titanium dioxide (anatase type) aqueous suspension were monitored via the iodine-luminol chemiluminescence (CL) reaction in a reversed micellar medium, and a new, automated, rapid, and efficient method was developed. A water-cooled, 400-W high-pressure Hg lamp was used as an internal light source. The flow procedure involved the following: (1) photocatalytic dehalogenation/degradation of the iodinated compound by the near-UV-illuminated titanium dioxide and the production of iodide species, (2) oxidation of iodide into iodine, (3) extraction of iodine into cyclohexane, (4) membrane separation of the iodine-containing organic phase from the aqueous phase, and (5) the detection of iodine using the luminol CL reaction in the reversed micellar solution of cetyltrimethylammonium chloride in 6:5 (v/v) chloroformcyclohexane/water buffered with sodium carbonate. Results for the dehalogenation of the iodinated compounds, o-iodobenzoic acid and L-thyroxine (3,5,3′,5′-tetraiodothyronine) sodium, were compared with a standard inorganic iodide solution. After establishing the optimum chemical and instrumental conditions, detection limits of 0.8 × 10-9 and 0.2 × 10-9 M and linear calibration graphs were obtained with dynamic ranges from 0.79 × 10-7 to 7.9 × 10-7 M and from 0.20 × 10-7 to 2.0 × 10-7 M for o-iodobenzoic acid and L-thyroxine, respectively. A precision of ∼4% relative standard deviation (n ) 6) was provided at an o-iodobenzoic acid concentration of 0.79 × 10-7 M. The method developed was applied to the on-line determinations of iodinated aromatic compounds such as L-thyroxine sodium and iopamidol ((S)N,N′-bis[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-hydroxy* Corresponding author. E-mail:
[email protected]. † Hiroshima University. ‡ University of Peshawar. § Yasuda Women’s Junior College. 10.1021/ac030004x CCC: $25.00 Published on Web 07/26/2003
© 2003 American Chemical Society
1-oxopropyl)amino]-2,4,6-triiodoisophthaldiamide) in pharmaceuticals. Iodine enters living organisms through the digestive tract, and an excess of it is excreted through urine, predominantly in the form of iodide. In the food chain, iodine is present in both inorganic and organic forms.1-3 Organoiodine compounds are also used for different therapeutic purposes, for example, the sodium salt of L-thyroxine (3,5,3′,5′- tetraiodothyronine), which is an active physiological principle obtained from the thyroid gland of domesticated animals,4 is a rich source of iodine. Similarly, iopamidol ((S)-N,N′-bis[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-hydroxy-1oxopropyl)amino]-2,4,6-triiodoisophthaldiamide) is used in preclinical angiographic studies. From a bioanalytical and clinical point of view, sensitive and accurate methods are needed to monitor inorganic and organic iodine-containing species or iodinated organic compounds in biological fluids, foodstuff beverages, supplements, and pharmaceuticals. Although many methods have been reported for this purpose,5-11 the selective procedures require relatively extensive sample preparation and are timeconsuming, while complicated instruments are used when high sensitivity is desired. With respect to sensitivity, experimental simplicity, and unit cost of equipment, analytical methods based on chemiluminescence (CL) reactions have proved to have great advantages over other conventional methods such as spectrophotometry and (1) Underwood, E. J. Trace Elements in Human and Animal Nutrition, 4th ed.; Academic Press: New York, 1977 (and references therein). (2) Pennington, J. A. T. In Trace Minerals in Foods; Smith, K. T., Ed.; Marcel Dekker: New York, 1988; pp 249-289. (3) Heydorn, K. Neutron Activation Analysis for Clinical Trace Element Research; CRC Press: Boca Raton, FL, 1984; Vols. 1 and 2. (4) El-Khateeb, S. Z. Anal. Lett. 1989, 22, 2223-2232. (5) Verma, K. K.; Jain, A.; Verma, A. Anal. Chem. 1992, 64, 1484-1489. (6) Monks, C. D.; Nacapricha, D.; Taylor, C. G. Analyst 1993, 118, 623-626. (7) Tanaka, Y.; Okazaki, A.; Hozumi, K. Mikochim. Acta 1991, III, 169-173. (8) Okamoto, T.; Isozaki, A.; Nagashima, H. Bunseki Kagaku 1996, 45, 6570. (9) Ito, K. Anal. Chem. 1997, 69, 3628-3632. (10) Bichsel, Y.; Gunten, U. V. Anal. Chem. 1999, 71, 34-38. (11) Ackley, K. L.; Day, J. A.; Sutton, K. L.; Caruso, J. A. Anal. Commun. 1999, 36, 295-298.
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fluorometry.12 Sensitive determinations of halogens, halides, and hypohalites,13-16 e.g., with a detection limit of 0.1 ng mL-1 iodine,17 have been reported using the CL reactions. The use of a reversed micellar medium of the surfactant cetyltrimethylammonium chloride (CTAC) in CL analysis is also interesting because it permits one to achieve enhanced sensitivity and improved selectivity besides other advantages.18-22 A macroscopically homogeneous solution of reverse micelles, which function as microreactors,22,23 is prepared by dispersing a small amount of water in the bulk nonpolar organic medium containing surfactant molecules. The reverse micelle is a droplet of water, surrounded by surfactant polar heads, while the surfactant hydrophobic chains extend into the bulk organic phase. It has been pointed out that the micellar microreactor has the capability to facilitate the quantitative transfer of species of experimental interest into the water pool and their subsequent conversion into CL active species at the surfactantwater interface.19,24-26 Previously, it was observed that CL results upon mixing iodine with the reversed micellar solution of luminol.19 Further, the reversed micellar-mediated CL (RMM-CL) generation from the iodine-luminol reaction allowed us to develop a new method for determination of iodide or iodine in aqueous samples based on coupling of solvent extraction with the RMMCL detection, and problems associated with aqueous-phase CL detections were either eliminated or greatly reduced:27 Using a flow system, iodine was transferred from the aqueous solution into the organic solvent, onward membrane-separated, and then the postextraction step was directly coupled to the RMM-CL detection. However, the resulting extraction/RMM-CL hybrid method cannot be applied to the determination of iodinated organic compounds because iodine covalently bonded to a carbon structure does not respond to the luminol CL analysis in the same way as when it is in the inorganic form in solution. Therefore, it is necessary to release the iodine from the carbonaceous environment and bring it into an inorganic form that is suitable for the RMM-CL method (e.g., the dehalogenation of iodinated organic compounds). (12) Fujiwara, T.; Kumamaru, T. Spectrochim. Acta Rev. 1990, 13, 399-406. (13) Fernandez-Gutierrez, A.; Munoz de la Pena, A. In Molecular Luminescence Spectroscopy. Methods and Applications: Part 1; Schulman, S. G., Ed.; Wiley & Sons: New York, 1985; pp 463-546 (and references therein). (14) Chen, D.; Luque de Castro, M. D.; Valcarcel, M. Analyst 1991, 116, 10951111. (15) Burguera, J. L.; Buerguera, M. Anal. Quim. B 1982, 78, 307-310. (16) Pilipenko, A. T.; Terletskaya, A. V.; Zui, O. V. Fresenius Z. Anal. Chem. 1989, 335, 45-48. (17) Seitz, W. R.; Hercules, D. M. J. Am. Chem. Soc. 1974, 96, 4094-4098. (18) Hoshino, H.; Hinze, W. L. Anal. Chem. 1987, 59, 496-504. (19) Fujiwara, T.; Tanimoto, N.; Huang, J.-J.; Kumamaru, T. Anal. Chem. 1989, 61, 2800-2803. (20) Georges, J. Spectrochim. Acta Rev. 1990, 13, 27-45. (21) Hinze, W. L.; Srinivasan, N.; Smith, T. K.; Igarashi, S.; Hoshino, H. In Advances in Multidimensional Luminescence; Warner, I. M., McGown, L. B., Eds.; JAI Press: Greenwich, CT, 1991; Vol. 1, pp 149-206. (22) Hinze, W. L. In Organized Assemblies in Chemical Analysis; Hinze, W. L., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, pp 37-105. (23) Pileni, M. P. In Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989 (and references therein). (24) Fujiwara, T.; Tanimoto, N.; Nakahara, K.; Kumamaru, T. Chem. Lett. 1991, 1137-1140. (25) Fujiwara, T.; Theingi-Kyaw; Kumamaru, T. Anal. Sci. 1997, 13 (Suppl), 59-62. (26) Theingi-Kyaw; Kumooka, S.; Okamoto, Y.; Fujiwara, T.; Kumamaru, T. Anal. Sci. 1999, 15, 293-297. (27) Fujiwara, T.; Mohammadzai, I. U.; Inoue, H.; Kumamaru, T. Analyst 2000, 125, 759-763.
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The commonly used methods for the decomposition of organic and biological samples are as follows:28 (1) oxidation in an atmosphere of oxygen or open air (e.g., combustion tube technique) and (2) heating the sample with an excess of concentrated oxidizing mineral acid in wet-ashing, which is often quite hazardous. The main difficulties associated with decomposition techniques are loss of volatile species such as free iodine and hydrogen iodide. After completion of the wet-ashing, the reaction container or tube is allowed to cool and very carefully opened, and the contents are transferred. To recover both volatile products and involatile ash quantitatively without loss, the oxygen flask combustion for iodine was applied to estimate thyroxine sodium.4 However, the main limitation of the oxygen flask method is that often oxidation of the sample is incomplete.28 These decomposition techniques are obviously tedious, somewhat troublesome to use, and also difficult to be incorporated into a flow system. The utilization of semiconductor powders to catalyze the photoinduced decomposition of organic toxic substances is well reported.29-38 Several semiconductors, e.g., ZnO, CdS, WO3, Fe2O3, SnO2, TiO2, and RuO2,39,40 are used for the heterogeneous photoassisted catalytic degradation of organic compounds.31,40-43 The TiO2 powder is perhaps the most efficient among the semiconductors used to degrade/oxidize several toxic compounds (e.g., organophosphorus pesticides and chlorinated hydrocarbons) by converting them into less harmful species. When the TiO2 aqueous suspension is illuminated with light of wavelength
