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.

A Fourier analysis approach for capillary polarimetry

“…In the case described here it is on the order of 40 nL. As shown previously [3] when orienting the plane of polarization of the incoming light coincident with the capillary central axes an additional set of high contrast fringes with a higher spatial frequency appears ( Fig. 3).…”

“…2), or by introducing an optically active solute into the capillary. In this way, CPD can be used to quantify optically active solutes [2][3][4][5] or to determine optical rotary power of an unknown solute [5,6]. It is noteworthy that it has been previously shown that the polarimetry signal is contained within the high-frequency component of the CPD fringe pattern and the spatial frequency of this pattern does not change with concentration of nonoptically active solutes [2,5,6] (e.g., it is relatively insensitive to refractive index changes).…”

“…It has been often used in pharmaceutical industry for purity control of certain biological substances, in chemical industry for polymer identification and characterization and can also be employed in the research environment for kinetic investigations, enzymatic cleavage studies, and identification of unknowns [1]. 1 It has been recently shown that polarimetry can be performed at high sensitivities and in nanoliter detection volumes [2][3][4][5]. This capillary polarimeteric detector (CPD) is based on the micro-interferometric backscatter configuration shown in Fig.…”

Emerging methods for the rapid determination of enantiomeric excess

“…25 A quantum leap in sensitivity has been achieved by Bornhop, who has described a microinterferometry method to obtain polarimetric information from picomolar amounts of chiral compounds held in a capillary tube of the type used for GC and HPLC analysis. 26,27 While an applicable device based on this technology has not yet been invented, the fundamental method holds great promise for bringing the direct measurement of optical purity to the necessary levels of scale, speed, and generality so as to be useful in high-throughput screening.…”

Simple liquid-core waveguide polarimetry

“…1 This has lead to the development of a series of alternative approaches to enantiomeric screening which span most areas of analytical chemistry. 2 However, polarimetry has far from been abandoned and recent demonstrations of gas phase cavity ring-down polarimetry, 3,4 frequency domain polarimetry, 5 and measurements of nanoliter volumes using capillary interferometry 6 illustrate the continued appeal of the technique largely due to its conceptual simplicity and its applicability to all chiral molecules.…”