Laser-based refractive-index detection for capillary electrophoresis: ray-tracing interference theory

Abstract

The fringe pattern observed in a far field after a laser beam illuminates a fused silica capillary immersed in a refractive-index matching material and filled with an analyte fluid is exploited to develop a sensitive optical detector for capillary chemical analysis. The inner capillary interface splits the laser beam into a reflected beam fan and a refracted beam fan, which, on overlapping in the far field, lead to interferences. The intensity and the position of the fringes for capillaries with 250 microm >/= i.d. (inner diameter) >/= 25 microm are well reproduced by the presented model. The calculation predicts the fringe pattern for various beam/i.d. geometric configurations and is used to optimize the performance of the nanoliter-picoliter refractive-index on-column detection studied. It is found that the best contrast corresponds to a capillary that is illuminated with a beam waist of omega(0) ~ i.d./12, which is off-center focused with an offset of s ~ i.d./2. For a given interference pattern, the fringes that are found to be more sensitive to Deltan are those that appear near the optical axis but still retain high intensity and contrast. The sensitivity increases approximately linearly with the fringe number, and the maximal fringe number increases proportionally with the i.d.

Scattering of a Gaussian beam by an elliptical cylinder using the vectorial complex ray model

“…However, there is much less work on the geometrical optics for the scattering of shaped beams by cylinders. We find only that Krattiger et al used the interference of reflected and transmitted rays in the far field for measurement of the refractive index of capillary eletrophoresis [23].…”

Sensitive photothermal interferometric detection method for characterization of transparent plate samples

“…In recent years, due to the rapid developments of HPLC and CE separation techniques, a number of interference methods based on transmitted-beam interference [33][34][35] have been developed and successfully used as universal refractive index ͑RI͒ detection for HPLC and CE separations. Keeping other conditions the same, the retroreflected beam interference method is potentially more sensitive than the transmitted beam interference approach because the probe beam passes twice through the detection region in the retroreflected beam configuration.…”

Tapered fiber sensor in the near infrared wavelength

“…There are many fiber optical sensors used in bio-chemical detection [1,[7][8][9][10][11][16][17][18], environment monitoring [12][13][14][15] and physical tests [2][3][4][5]. The fiber optical sensors are used in transmission or reflection measurement setups.…”

“…Such modification may be performed by modification of fiber material parameters, for example long period gratings [14], or fiber shape modification by taper [16][17][18]20] or D-shape [29] forming. The fiber sensor may be developed using experimental results [12][13][14][15] or mathematical modeling [16][17][18] approximation. The fiber optics may be made of polymer or silica.…”