Photocatalytic dehalogenation coupled on-line to a reversed micellar-mediated chemiluminescence detection system: application to the determination of iodinated aromatic compounds

The effects of illumination time, temperature, catalyst concentration, and pH on the on-line photocatalytic dehalogenation of iodinated aromatic compounds in a near-UV-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) chloroform-cyclohexane/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 x 10(-9) and 0.2 x 10(-9) M and linear calibration graphs were obtained with dynamic ranges from 0.79 x 10(-7) to 7.9 x 10(-7) M and from 0.20 x 10(-7) to 2.0 x 10(-7) M for o-iodobenzoic acid and L-thyroxine, respectively. A precision of approximately 4% relative standard deviation (n = 6) was provided at an o-iodobenzoic acid concentration of 0.79 x 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'-bis2-hydroxy-1-(hydroxymethyl)ethyl]-5-(2-hydroxy-1-oxopropyl)amino]-2,4,6-triiodoisophthaldiamide) in pharmaceuticals.