The Interferometer Detector

The interferometer detector was first developed by Bakken and Stenberg in 1971. The detector responds to 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. If the light transmitted through the cell is focused on a photocell coincident with a reference beam of light from the same source, interference fringes will be produced. These fringes will change, as the pathlength of one light beam changes with reference to the other. Consequently, as the concentration of solute increases in the sensor cell, a series of electrical pulses will be generated as each fringe passes the photocell.

The effective optical path length (d) depends on the change in refractive index (An), and the path length (l) of the sensor cell:

It is possible to calculate the number of fringes (N: sensitivity) which move past a given point (or the number of cyclic changes of the central portion of the fringe pattern) in relation to the change in refractive index by the equation:

2Anl

where X is the wavelength of the light employed.

As N increases for a given refractive index change, An, so will the detector sensitivity. Therefore l should be made as large as possible commensurate with the chromatographic properties of the system. The simple optical system originally employed by Bakken and Stenberg is shown in Figure 6. Light from a source strikes a half-silvered mirror and is divided into two paths. Part of the beam is reflected by a plane mirror back along the same path and on to a photocell. The other part of the beam passes through the sensor cell to a plane mirror, where it is reflected back again through the sensor cell to the half-silvered mirror that reflects it on to the photocell where interference takes place with the other half of the light beam.

The number of fringes that pass the sensor will be directly proportional to the total change in refractive index, which will be proportional to the total amount of solute present. Although it establishes the technical viability of the system, the apparatus has limited use, but it has been developed into a practical instrument and an example is that developed by Wyatt Technology. The optical system of the Optilab interference detector is shown in Figure 7.

Light from an appropriate source is linearly polarized at — 45° to the horizontal plane. Horizontal and vertical polarized light beams are produced and, on passing through the Wollaston prism, one passes through the sample cell and the other through the reference cell. The beam passing through the sample cell is horizontally polarized and that through the reference cell is vertically polarized. After passing through the cells, the beams are focused on a second Wollaston prism and then through a quarter-wave plate which has its fast axis set — 45° to the horizontal plane.

A beam that is linearly polarized in the fast-axis plane will, after passing through the plate, lead another linearly polarized, but orthogonal, beam by a quarter of a wavelength. The phase difference results in a circularly polarized beam. It can be assumed that each of the beams focused on the Wollaston prism consists of two such perpendicular beams which, after the quarter-wave plate, result in two circularly polarized beams of opposite rotation. These beams will interfere with each other to yield the original linearly polarized beam. A second polarizer is placed at an angle (90 — ft) to the first (for the significance of ft, see below), allowing about 35% of the signal to reach the photocell. A filter-transmitting light at 546 nm precedes the photocell.

If the sample cell contains a higher concentration of solute than the reference cell, in general the refractive index will be higher and the interfering beams will be out of phase. The refractive index difference (An) and

Photocell

Figure 6 The original optical system used by Bakken and Stenberg in their interferometer detector.

Photocell

Figure 6 The original optical system used by Bakken and Stenberg in their interferometer detector.

the phase difference (Ap) are related by:

Ap/2 rad, and the amplitude of the light striking the photocell (Ap) will be given by:

Ap"

2nL An

where L is the length of the cell and X is the wavelength of the light.

The circularly polarized beams will interfere to The smallest cell (1.4 ||L: a cell volume that would yield a linearly polarized beam which is rotated be suitable for use with microbore columns) is re-

Figure 7 The Optilab interference refractometer detector.

Figure 8 The layout of a thermal lens detector.

ported to give a sensitivity of about 2x 10 ~7 RI units at a signal-to-noise ratio of 2. Consequently, for benzene (RI = 1.501) sensed as a solute in «-heptane (RI = 1.388), this sensitivity represents a minimum detectable concentration of 5.6 x 10_5gmL_1. The alternative 7 | L cell would decrease the minimum detectable concentration to about 1x10_6gmL_1, similar to that obtained for other refractive index detectors. However, this cell volume is slightly large for modern high efficiency columns.

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