Background: A cationic polyelectrolyte, polyhexamethylene biguanide hydrochloride (PHMB), is widely used for disinfectants in personal-care products for cosmetics and toiletries. Several colloidal titration methods for the determination of PHMB are very tedious and time-consuming. A Flow Injection Analysis (FIA) is a very advantageous analytical method because of its ease of handling, high reproducibility and high sampling rate. Therefore, the development of a simple and sensitive FIA method for the determination of PHMB is required. In this paper, we report the FIA method for the determination of PHMB based on the electrostatic and hydrophobic interactions between an anionic dye, Bromocresol purple (BCP) and PHMB. Methods: The FIA manifold consists of a two-channel system which is composed of a double plunger pump equipped with a sample injector, a flow-through type spectrophotometer and a strip chart recorder. A sample solution (140 μL) of PHMB was injected into a distilled and deionized water stream. This stream was subsequently merged with a reagent solution stream of a solution of 20 μM BCP adjusted to pH 8.0. The absorbance of ion associate formed between BCP and PHMB was measured at 588 nm. Results: The absorption spectra of several anionic dye (10 μM) with PHMB (50 μM) were measured at 400-750 nm by changing pH of the solution. The wavelength at the largest change in the absorbance between the mixed PHMB solution and the reagent blank, and the change of the absorbance at the wavelength was examined. When BCP is used as the anionic dye, the largest change in the absorbance was obtained at 588 nm and the change of the absorbance at the wavelength was-0.56 at pH = 8.0. From the results, BCP was selected as the anionic dye for the FIA determination of PHMB. Under the optimal FIA conditions, a linear relationship between the peak height and concentration of PHMB was obtained in the concentration range from 2.0 to 10.0 μM. The present FIA method was applied to the determination of PHMB in contact-lens detergents. Conclusion: A new spectrophotometric FIA method was developed for the determination of PHMB, based on electrostatic and hydrophobic interactions between PHMB and the anionic dye, BCP. Under the optimal flow conditions of the FIA system, a good linear relationship between peak heights and concentrations of PHMB in a concentration range from 2.0 to 10.0 μM was obtained. The present FIA method uses more simpler instrumentation, and is more rapid (sample throughput: 13 samples h-1) than other reported methods, and has the advantages of high sensitivity and no toxic organic solvent than other analytical methods. The present method will be very useful for the determination of PHMB in commercially available contact-lens detergents and also be applied to the determination of PHMB in the sample solution, which has similar matrix to the contact-lens detergent.
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