Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Angle of Deviation Method > Page 21

The Angle of Deviation Method

When a monochromatic ray of light passes from one isotropic medium, (A), to another, (B), it changes its wave velocity and direction. The change in direction is called the refraction and the relationship between the angle of incidence and the angle of refraction is given by Snell's law, namely,

 

where (i) is the angle of incident light in medium (A),
(r) is the angle of refractive light in medium (B),
(nA) is the refractive index of medium (A),
(nB) is the refractive index of medium (B),
and (n'B) is the refractive index of medium (B) relative to that of medium (A).

Refractive index is a dimensionless constant that normally decreases with increasing temperature. The reported values are usually taken at 20o or 25oC and are mean values measured for the two sodium lines. If the mobile phase is allowed to flow through a hollow prism and a ray of light passes through the prism it will be diverged from its original path and can be focused onto a photocell. If the refractive index of the mobile phase changes, due to the presence of a solute, the angle of deviation of the transmitted light will also alter and the amount of light falling on the photocell will change. A number of manufacturers have employed the angle of deviation method in refractive index detector design. A diagram of a simple refractive index detector that is based on the angle of deviation method of measurement is shown  in figure 11. The differential refractometer monitors the deflection of a light beam caused by the difference in refractive index between the contents of the sample cell and those of the reference cell. A light beam from an incandescent lamp is confined to the region of the cell by  an optical mask. A lens collimates the light beam through both the sample and reference cells to a plane mirror. The mirror reflects the beam back through the sample and reference cells to a lens which focuses it onto a photocell.