Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Fresnel Method > Page 23

The Fresnel Method

 

The relationship between the reflectance from an interface between two transparent media and their respective refractive indices is given by Fresnel's equation,

where (R) is the ratio of the intensity of the reflected light to that of the incident light and the other symbols have the meanings previously assigned to them.

                       Now,          

 

where (n1) is the refractive index of medium (1),
and (n2) is the refractive index of medium (2).

 

Consequently, if medium (2) represents the column eluent, any change in (n2) will change (R) (i.e., DR) and, thus, measurement of (DR) will detect changes in solute concentration. The first to utilize this principal of detection was in the construction of a practical detector was Conlon (4).

Conlon's device is now obsolete but it illustrates the principle of the Fresnel method of detection very simply. A diagram of Conlon's detector is shown in figure 13. The sensing element consists of a rod prism sealed into a tube through which the solvent flows. The rod  (6.8 mm in diameter and 10 cm long) is made from a glass rod, bent to the correct optical angle (just slightly less than the critical angle) and an optical flat is ground on the apex of the bend (see figure 13). The optical flat is then sealed into the window of a flow-through cell. The photocell is arranged to be one arm of a Wheatstone bridge and a reference photocell (not shown) which monitors light direct from the cell, is  situated in another arm of the bridge.