Novel spectropolarimeter employing fixed polarizers for the determination of optically active samples

Measuring optical activity with the internal reflection in a glass prism

A novel internal reflection-based polarimeter and measurement method to characterize optically active substances is presented. The working principle relies on the phase difference acquired by s- and p-polarized light undergoing internal reflection: they interfere after passing through an analyzer and produce a minimum near the critical angle, whose position depends on the rotation angle imposed by the optically active sample. Since the location of the minimum does not depend on the laser power, the measurement is nearly insensitive to any power fluctuation. Furthermore, this low-cost device is rugged, very compact and stable, and practically immune to mechanical vibrations. It was used to measure concentrations of sucrose and fructose dissolved in distilled water and presented a resolution better than 0.04°.

A new approach to polarimetric measurements based on birefringent crystals and diode lasers

A new polarimetric instrument and measurement method is described based on the use of diode lasers as radiation source (532, 650 and 1064nm) and birefringent prisms, such as Glan-Laser and Wollaston, as analyzers. The laser radiation is passed through a dichroic polarizer film for further orientation of its polarization plane at 45° in relation to the polarization plane of the analyzer. The polarized beam, oriented in that way, passes the sample cell, impinges the prism surface, and the intensities of the two emerged beams are detected by two twin silicon detectors. Ideally, in the absence of any optically active substances, the crystals produces two orthogonally polarized refracted beams of equal intensity. In the presence of an optically active substance, the arctangent of the square root of the beam intensities ratio is equal to the new polarization angle (β) of the laser beam. The rotation angle imposed for any optically active substance present in the sample cell is then given by: α=(45-β)°. Because the rotation is obtained by the ratio of the intensities of two beams, it is independent of the laser intensity, which can vary up to ±15% with no significant effect on the accuracy of the polarimetric measurement. The instrument has been evaluated for measurement of optically active substances such as sucrose and fructose. The instrument employs low cost components, is capable of attaining a repeatability of ±0.003° and can measure the rotation angle, over a ±45° range, in less than 2s. Because it does not present any moving parts it can be easily adapted for in/on-line process monitoring of optically active substances.