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 differences 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 that spectroscopic refractive index detection has the potential to eliminate the thermal noise that is connected with conventional refractive index detection. The design and testing of a spectroscopic refractometer 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 measure the deflection at several wavelengths simultaneously. When thermal variations of 4-5 degrees C are evoked, the noise observed corresponds to 4 x 10(-3)degrees C, while the gain obtained in the signal-to-noise ratio is a factor 40. It is also shown that the sensitivity and selectivity of the method can be controlled by spectrochemical modification of the refractive index spectrum of the solvent. In this mode, similar or better signal-to-noise ratios can also he obtained at wavelengths where there is little or no difference in refractive index dispersion between the analyte and the solvent. Spectroscopic refractive index detection combines low noise with universal and concentration sensitive response and should therefore have a considerable potential in liquid chromatography and process analysis, particularly in applications where it is difficult to accurately control the temperature. Since spectroscopic refractive index detection is well suited for miniaturization, it should be of special interest for small refractometric sensors and in microcolumn separations.