FLUORESCENCE OPTICAL SENSOR FOR LOW CONCENTRATIONS OF DISSOLVED CARBON-DIOXIDE

The performance of an optode in measurements of low concentrations of dissolved CO2 was characterized. The method of lipophilic extraction o f ion pairs into the membrane phase according to Weigl and Wolfbeis wa s applied to the fluorescent pH indicator hydroxypyrenetrisulfonic aci d together with the quaternary ammonium base tetraoctylammonium hydrox ide. The membrane solution was applied directly to an acrylic optical fibre of 1 mm diameter. Solutions of NaHCO3 were used as standards and H2CO3 concentrations calculated. The dynamic range for the detection of H2CO3: was 4 x 10(-6)-1 x 10(-2) mol l(-1). Response times depend ed on the concentration range and whether the concentration was increa sed or decreased: tau(90%) was <1 min for increasing H2CO3 concentrat ions in the range 10(-4)-10(-2) mol l(-1). Recovery times for low conc entrations, however, were tau(90%) >6 min. The sensor was found to sho w an increasing susceptibility to temperature at the low end of the ca libration graph for H2CO3. The use of a low concentration pf base in the membrane resulted in the desired detection limit, but also in a li fetime limited to 1 d.