INTEGRATED OPTICAL NIR-EVANESCENT WAVE ABSORBENCY SENSOR FOR CHEMICAL-ANALYSIS

A new long-path integrated optical (IO) sensor for the detection of no n-polar organic substances is described. The sensing layer deposited o n a planar multimode IO structure is built by a suitable silicone poly mer with lower refractive index (RI). It acts as a hydrophobic matrix for the reversible enrichment of non-polar organic contaminants from w ater or ail. Light from the near-infrared (NIR) range is coupled into the planar structure and the evanescent wave part of the light field p enetrating into the silicone layer interacts with the enriched organic species. Is a result. light is absorbed at the characteristic frequen cies of the corresponding C-H, N-H or O-R overtone and combination ban d vibrations of the analytes, To perform evanescent field absorbance ( EFA) measurements, the arc-shaped strip waveguide structure of 172 mm inter-action length was adapted to a tungsten-halogen lamp and an InGa As diode arrag spectrograph over gradient index fibers. Dimethyl-co-me thyl(phenyl)polysiloxanes with varying degrees of phenylation were pre pared and used as sensitive coating materials for the 10 structure. Li ght attenuation in the are-shaped waveguides is high and typical inser tion losses in the range of 14-18 dB were obtained. When the coated se nsors were brought in contact with aqueous samples, the light transmis sion decreases. which is due to the formation of H2O micro-emulsions i n the silicone superstrates. Nevertheless. after reaching constant lig ht transmissions, absorbance spectra of aqueous trichloroethene sample s were successfully collected. For gas measurements, where water cross sensitivity problems are absent, the sensitivity of the lO device for trichloroethene was tested as a function of the RT of the silicone su perstrate. The slope of the TCE calibration function increases by a fa ctor of 10 by using a poly(methylphenylsiloxane) layer with a RI of 1. 449 instead of poly(dimethylsiloxane) (RI:1.41). A comparison of the I O-EFA and an earlier developed fiber-optic EFA sensor for trichloroeth ene measurements in the gas phase showed an increase in sensitivity pe r unit length of the waveguide by a factor of up to 120.