The Thermal Lens Detector

There are a number of detectors that have been developed that are not classed as refractive index detectors, but their response is either based directly on refractive index measurement, or a function of some physical property of the mobile-phase system that is related to the refractive index. One such example of these is the thermal lens detector.

When a laser is focused on to an absorbing substance, the refractive index may be changed and modify the medium in such a way that it behaves as a lens. This phenomenon was first reported by Gorden et al. in 1964 and subsequently the effect has been examined by a number of workers. The formation of the thermal lens is caused by the absorption of laser light which may be extremely weak. The excited-state molecules subsequently decay to the ground state, with a resulting localized temperature increase in the sample. As the refractive index of the medium depends on the temperature, the spatial variation of the refractive index in the medium produces the phenomenon which appears to be equivalent to the formation of a lens within the medium. The temperature coefficient of refractive index is, for most liquids, negative; consequently, the insertion of a liquid in the laser beam produces a concave lens that results in beam divergence. The thermal lens effect has been used by Buffet and Momis to develop a small volume detector. Their system is shown in Figure 8.

The device consists of a heating laser, from which light is passed directly through the sample via two lenses and a half mirror. Another laser, the probe laser, passes light in the opposite direction, through one lens, then through the sample to the half mirror where the light is reflected on to a photocell. Between the mirror and the photocell is a filter to remove the heating laser light and a small pinhole aperture. When an absorbing solute arrives in the cell, a thermal lens is produced which causes the probe light to diverge, and consequently the intensity of the light passing through the pinhole and on to the photocell is reduced. The cell can be made a few microlitres in volume and would thus be suitable for use with microbore columns. A sensitivity of 10~6 AU (the expected limiting sensitivity of a bulk property detector) was claimed for the detector and a linear dynamic range of about three orders of magnitude.

The use of two lasers adds significantly to the cost of the device. Basically, the thermal lens detector is a special form of the refractive index detector and as a consequence can be considered as a type of universal detector. However, it cannot be used with gradient elution or flow programming and its sensitivity is no better than other refractive index detectors.

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Solar Panel Basics

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