Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Interferometer Detector > Page 28
The Interferometer Detector
The interferometer detector was first developed by Bakken and Stenberg (16) in 1971. The response of the detector depends on the change in the effective path length of a beam of light passing through a cell when the refractive index of its contents changes due to the presence of an eluted solute. Light that has passed through the cell is focused on a photocell. Coincidentally a reference beam of light from the same source is focused on the photocell, interference fringes are produced. The fringes change as the path length of one light beam changes with reference to the other, thus, as the concentration of solute increases in the sensor cell during an elution of a peak, a series of electrical pulses will be generated as each fringe passes the photocell.
The optical path length (d) of light through the cell depends on the change in refractive index (Dn), and the path length (l), thus,
d = Dnl
In addition, the number of fringes (N) which move past a given point on the photo cell (or the number of cyclic changes of the central portion of the fringe pattern) is given by,
where (l) is the wavelength of the light employed.
The larger the value of (N) for a given (Dn), the more sensitive the detector will be. It follows that, (l) should be made as large as possible. However, this procedure for increasing the sensitivity is limited by the dead volume of the column and the dispersion that can be tolerated before chromatographic resolution is impaired. A diagram of the simple optical system originally employed by the authors is shown in figure 17.
Figure 17 The Original Optical System Used by Bakken and Stenberg in Their Interferometer Detector