Polarimetry in Capillary Dimensions

Abstract

Microinterferometeric backscatter is employed to detect optically active molecules in capillary tubes. The capillary polarimetric detector (CPD) is based on the interaction between a polarized laser beam and a capillary tube. The simple optical configuration employs a He−Ne laser, a glass polarizing plate, a fused silica tube, and a CCD-based laser beam analyzer. Side illumination of the capillary produces a 360° fan of scattered light that contains two sets of high-contrast interference fringes. These light and dark spots are viewed on a flat plane in the direct backscatter configuration. It is shown that the modulation depth for the high-frequency component is sensitive to the polarization plane for the illumination source and facilitates the quantitative determination of optical activity for a fluid contained in a capillary. A 90° rotation of exciting plane-polarized light results in a depth of modulation change of ∼80% when unmodified capillaries ranging from 75 to 530 μm i.d. are employed. Signal interrogation in the CPD is based on quantifying the relative intensities (depth of modulation) of adjacent high-frequency interference fringes. Mandelic acid, [α]23 of −153°, is used as the optically active molecule and can be detected at a 2σ detection limit of 1.49 × 10-3 M within a probe volume of about 30 nL. This limit of detection corresponds to 44 pmol or 6.7 ng of mandelic acid. The folded optical arrangement is simple and can be miniaturized with low-cost components. Initial investigations suggest that the CPD can be employed for detection in capillary electrophoresis or for studying the onset and level of solute aggregation.