Reflective detectors

The refractive index detector based on the Fresnel principle is relatively rear. There was only one or two commercial models and they are not in the production any more. Figure below shows the optical schematic of this type of detector. Here, the light beam is reflected from the liquid-glass  interface in the detecting photocell. As the introduction of sample into one cell causes light to be refracted at a different angle. The deflection of the light beam from the photoresistor cause the appearance of the electrical signal. Here, too, this difference between sample cell signal and reference-cell signal is output to a recorder or data handling system as peak.

The major advantage of this type of detector is a very high sensitivity since the optics allow a higher concentration of signal in a particular RI range than is possible in other wide-range detectors. Other advantages include the ability to operate at extremely low flow rates with very low-volume cells, easy cell accessibility, and low cost. Its disadvantages are the incredible sensitivity to the flow and pressure fluctuations, and the need for changing prisms to accommodate either high or low RI solvents and the need to manually adjust the optical path when making solvent changes.

Optical schematic of the Fresnel-type refractive index detector.

The refractive index of a analyte is a function of its concentration. Change in concentration is reflected as a change in the RI.  A refractive index detector   for liquid chromatography should be sensitive to changes as small as 10-7 RI units (corresponding to a concentration change of 1 ppm). Presence of dissolved air, changes in solvent composition, improper mixing and column bleed will contribute to baseline drift. Eluent  pressure change of 15 psi.will cause the change of 1 x 10-6 RI unit and 1C temperature variation will be equivalent to the change of 600 x 10-6 RI units. Thus it is obvious that both of these parameters must be closely controlled, especially temperature. To operate at high sensitivities, a RI detector must usually be thermostated ( 0.01C), actually the using of the water bath connected to the detector head does not give required temperature stability, alternately, passive thermostabilisation with massive metallic block usually gives much better results.