Capillary-Scale Refractive Index Detection by Interferometric Backscatter

Abstract

We describe a novel approach for measuring fluid bulk properties such as refractive index (RI) and temperature changes in tubes of capillary dimensions based on a simple optical configuration which uses an unfocused He-Ne laser, a cylindrical tube, and a photodetector. Side illumination of a fused silica capillary produces a fan radiation in a 360°plane normal to the central axis of the capillary that is spatially well defined in the lateral direction. It is shown that, upon viewing a high-contrast interference pattern contained in the beam profile on a flat imaging plane in a direct backscattering configuration, the physical characteristics of the capillary and the bulk properties of the fluid contained within the tube can be determined. Position changes in the maxima and minima in the intensity-modulated profile (interference fringes) are directly related to the refractive index of the fluid in the tube. Measurement of the fringe shift by a slitphotodetector assembly facilitates the determination of dn/n at the level of 1.8 × 10 -7 , while temperature changes can be measured using the fringe centroid position at a level of 5.8 × 10 -5 °C (DL ) 3σ). Each determination can be made within a probe volume of 2.6 nL. Also, a wide range of tube diameters can be employed (75-775 µm i.d.) with no modification to the optical train or measurable degradation in system performance. We report results for an optical ray trace model for predicting fringe pattern energy distributions, optimization of the optical configuration, and general system performance predictions. Comparison of our four-beam interference model with experimental results illustrates the utility of the model.

Microcolumn liquid chromatography: instrumentation, detection and applications

“…With capillary LC, RI detection is even more difficult due to the fact that RI differences have to be measured in extremely small volumes [130]. With the highly collimating nature of lasers, nl to pl volumes can easily be probed.…”

“…With the highly collimating nature of lasers, nl to pl volumes can easily be probed. Laser-based RI detection has, among others, been applied by Bornhop and co-workers [130][131][132] and Bruno et al [133,134]. The capillary flow cell developed by Bruno et al [134] is based on the so-called forward scatter, off-axis technique.…”

“…A novel approach for measuring RI changes in capillary separation techniques is reported by Tarigan et al [130]. With this technique, the interference-produced fringes are detected below the plane of excitation in a direct backscatter configuration.…”

Free-Solution, Label-Free Molecular Interactions Studied by Back-Scattering Interferometry

“…We now show that BSI can be implemented in free-solution and can detect a 25 pM binding event using a simple optical train based on a PDMS microfluidic chip. Modeling of the physical mechanism behind the BSI optical phenomenon (23,(31)(32)(33) indicates that a multipass configuration leads to a long effective path-length and high sensitivity. Impinging coherent parallel rays from a fiber-coupled He-Ne laser beam onto a micrometer-dimensioned rectangular channel in the PDMS chip produces a fan of scattered light.…”

Detection in capillary electrophoresis

“…First, for most conventional RI techniques (particularly those based on refractioninduced defection) physical constraints and an optical pathlength sensitivity make detection directly on capillaries essentially problematic. Second, the change in RI with temperature (dn/dT) is large (8´10 ±4 RIU/ o C for water) [75] for most fluids, and thus small changes in temperature result in appreciable RI signals. Even so, many attempts have been made [75±87] to miniaturize these bulk property optical detectors to nanoliter volumes.…”