Chemically selective optode membranes casted on multiple internal reflection elements of high refractive index

Measurements of calcium ion activities with lately available Schott glass `LaSF35' as the multiple internal reflection element are presented. Use of the evanescent wave in absorption mode as well as corresponding data for the refractive index are shown. Looking from the backside at the membrane eliminates optical sample interferences and the crucial assumption of a homogeneous bulk optode can be verified by profiling into a controlled depth of the membrane.