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 (3,5,3ʹ,5ʹ-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 oiodobenzoic 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,Nʹbis[2-hydroxy-1-(hydroxymethyl)ethyl]-5-[(2-hydroxy-1-oxopropyl)amino]-2,4,6-triiodoisophthaldiamide) in pharmaceuticals.