Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Thermal Lens Detector > Page 32

The Thermal Lens Detector

If a laser is focused on an absorbing substance, the refractive index of the material can be modified in such a way that the medium behaves as a lens. The thermal lens effect was first reported by Gorden et al. (25,26) in 1964 but since that time the phenomenon has been investigated by a number of workers. Thermal lens formation results from extremely weak laser light adsorption The excited-state molecules subsequently decay back to ground state causing localized temperature increases to occur in the sample. Since the refractive index of the medium depends on the temperature, the ensuing spatial variation of refractive index produces an effect which appears equivalent to the formation of a lens within the medium.

For most liquids, the temperature coefficient of refractive index is negative and consequently, the insertion of a liquid in the laser beam produces a concave lens that results in beam divergence. Buffet and Momis (27) used the thermal lens effect to develop a small volume detector, a diagram of which is shown in figure 20.


Figure 20 The Layout of a Thermal Lens Detector


The device consists of a heating laser, the light from which is passed directly through the sample via two lens and a half mirror. Another laser, the probe laser, passes light in the opposite direction through one lens, through the sample to the half mirror where the light is reflected onto a photocell. A filter and a pinhole screen is placed between the mirror and the photo-cell to remove the heating laser light. When an absorbing solute is eluted from the column through the cell, a thermal lens is produced causing the probe light to diverge, and the intensity of the light passing through the pinhole and on to the photocell is reduced.