Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Christiansen Effect Detector > Page 26

The Christiansen Effect Detector

This procedure for measuring refractive index arose from the work of Christiansen on crystal filters (14,15). Consider a cell packed with particulate material having the same refractive index as the mobile phase passing through. If a beam of light passes through the cell there will be little of no refraction or scattering. However, if the refractive index of the mobile phase changes, there will now be a refractive index difference between the mobile phase and that of the packing. As a consequence some light will be refracted away from the incident beam and the intensity of the transmitted light will be attenuated. Thus, if the transmitted light is focused onto an appropriate photocell, then any change in refractive index caused by the elution of a solute will produce scattering and a consequent change in electrical output.

In practice, he optical dispersions of the media are likely to differ, and consequently the refractive index will only match at one particular wavelength. As a result the fully transmitted light will be largely monochromatic. Light of different wavelengths will be proportionally dispersed depending on the wavelength at which the two media have the same optical dispersion. Thus, a change in mobile phase refractive will change both the intensity of the transmitted light and its wavelength.

This device was made by GOW-MAC Inc., who claimed it had a sensitivity of 1 x 10-6 refractive index units (the maximum that cold be expected). This would be equivalent to a sensitivity of 9 x 10-6 g/ml of benzene (refractive index 1.501) eluted in n-heptane (refractive index 1.388). The cell volume was kept to 8 ml (a little large for modern sensors) which was small enough to work satisfactorily with 4.6 mm I.D. LC columns. Different cells packed with appropriate materials were necessary to cover the refractive index range of 1.31 to 1.60. A diagram of the Christiansen detector is shown in figure 5.