Continuous optical monitoring of aqueous amines in transflectance mode

Derivatives of the chromoreactand 4-trifluoroacetylazobenzene were shown to allow continuous monitoring of amines in aqueous solutions. The optical ch ange was measured by absorbance spectroscopy. The method did not satisfy in view of applications to real food specimens or waste water. In this paper, an optical sensor system based on transflectance measurements is investiga ted in order to determine the concentration of amines of low molecular mass in a continuous-flow system. For this purpose, solvent polymeric optode me mbranes selective for amines were modified by using a reflective TiO2 pigme nt or an additional white PTFE layer (adlayer). Three different sensor layers were investigated, namely: (a) optode membran es incorporating ETHT 4001 in plasticized PVC coated with a microporous whi te PTFE film (membranes M1), (b) membranes incorporating ETHT 4001 with tit anium dioxide dispersed in plasticized PVC (membranes M2), and (c) membrane s incorporating a copolymer from ETHT 4012, methyl methacrylate and butyl a crylate coated with PTFE (membranes M3). Both types of reflective materials were applied to membranes placed within a continuous-flow module. The transducer consisted of a fiber-optical probe , which allowed monitoring of variations in the spectrum of a chromoreactan d in the transflectance mode. All three approaches were compared in terms o f reproducibility, dynamic range, detection limit, and response time. The f ree-diffusion membranes M1 and M2 responded faster (30 to 45 s) than membra ne M3 (2 to 14 min), which incorporated the reactand covalently linked to t he copolymer. Surprisingly, the detection limit of M3 was lower than that o f M1 and M2. Due to M3's high mechanical stability, its reproducibility was superior to M1 and M2. In general, the use of reflective material did not affect such sensor characteristics as the selectivity, dynamic range, detec tion limit and response time of the amine-selective optodes. Thus, it is po ssible to convert any sensor chemistry based on indicator dyes incorporated into a polymer matrix (optode membranes) into reflectance-based fiber opti c sensors without affecting the sensor's performance. (C) 2000 Elsevier Sci ence B.V. AU rights reserved.