Fluorescence optical sensor for low concentrations of dissolved carbon dioxide

Abstract

The performance of an optode in measurements of low concentrations of dissolved CO₂ was characterized. The method of lipophilic extraction of ion pairs into the membrane phase according to Weigl and Wolfbeis was applied to the fluorescent pH indicator hydroxypyrenetrisulfonic acid together with the quaternary ammonium base tetraoctylammonium hydroxide. The membrane solution was applied directly to an acrylic optical fibre of 1 mm diameter. Solutions of NaHCO₃ were used as standards and H₂CO^*₃ concentrations calculated. The dynamic range for the detection of H₂CO^*₃ was 4 × 10^–6–1 × 10^–2 mol l^–1. Response times depended on the concentration range and whether the concentration was increased or decreased: τ_90% was <1 min for increasing H₂CO^*₃ concentrations in the range 10^–4–10^–2 mol l^–1. Recovery times for low concentrations, however, were τ_90% >6 min. The sensor was found to show an increasing susceptibility to temperature at the low end of the calibration graph for H₂CO^*₃. The use of a low concentration of base in the membrane resulted in the desired detection limit, but also in a lifetime limited to 1 d.