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 change 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 investigated in order to determine the concentration of amines of low molecular mass in a continuous-flow system. For this purpose, solvent polymeric optode membranes selective for amines were modified by using a reflective TiO2 pigment or an additional white PTFE layer (adlayer). different sensor layers were investigated, namely: (a) optode membranes incorporating ETHT 4001 in plasticized PVC coated with a microporous white PTFE film (membranes M1), (b) membranes incorporating ETHT 4001 with titanium dioxide dispersed in plasticized PVC (membranes M2), and (c) membranes incorporating a copolymer from ETHT 4012, methyl methacrylate and butyl acrylate coated with PTFE (membranes M3). ypes 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 chromoreactand in the transflectance mode. All three approaches were compared in terms of reproducibility, dynamic range, detection limit, and response time. The free-diffusion membranes M1 and M2 responded faster (30 to 45 s) than membrane M3 (2 to 14 min), which incorporated the reactand covalently linked to the copolymer. Surprisingly, the detection limit of M3 was lower than that of M1 and M2. Due to M3ʹs 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, detection limit and response time of the amine-selective optodes. Thus, it is possible to convert any sensor chemistry based on indicator dyes incorporated into a polymer matrix (optode membranes) into reflectance-based fiber optic sensors without affecting the sensorʹs performance.