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

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.