A SPECTROSCOPIC REFRACTOMETER FOR TEMPERATURE-INDEPENDENT REFRACTIVE-INDEX DETECTION

This paper presents the working principles of spectroscopic refractive index detection. The method is based on measurement of the difference s in the wavelength dispersion of the refractive index of a solute and a solvent The basic theory of the method is outlined, and his shown t hat spectroscopic refractive index detection has the potential to elim inate the thermal noise that is connected with conventional refractive index detection. The design and testing of a spectroscopic refractome ter are described. The device is based on light deflection in a liquid prism and utilizes a deuteriun lamp and a CCD detector in order to me asure the deflection at several wavelengths simultaneously. When therm al variations of 4-5 degrees C are evoked, the noise observed correspo nds to 4 x 10(-3)degrees C, while the gain obtained in the signal-to-n oise ratio is a factor 40. It is also shown that the sensitivity and s electivity of the method can be controlled by spectrochemical modifica tion of the refractive index spectrum of the solvent. In this mode, si milar or better signal-to-noise ratios can also he obtained at wavelen gths where there is little or no difference in refractive index disper sion between the analyte and the solvent. Spectroscopic refractive ind ex detection combines low noise with universal and concentration sensi tive response and should therefore have a considerable potential in li quid chromatography and process analysis, particularly in applications where it is difficult to accurately control the temperature. Since sp ectroscopic refractive index detection is well suited for miniaturizat ion, it should be of special interest for small refractometric sensors and in microcolumn separations.