Light-scattering studies of packed stationary phases for capillary electrochromatography

Multiply scattered light through turbid media, packed particles, or compressed powders will inherently have a significantly longer optical path length than that of light which is not scattered. The concept of using the multiply scattered light potentially generated in the packed stationary phase of a capillary electrochromatography (CEC) column for enhanced detection as a result of its increased optical path length was examined. Ultraviolet (UV) light at 365 nm or laser light at 635 nm was focused to a small spot onto the packed section of a 3 microns spherisorb ODS1 CEC column (100 microns i.d.). The light was transported inside the capillary, and an image of the multiply scattered light several millimeters along the capillary was collected using a charged-couple device detector. Even if the spot size was less than 100 microns in diameter, evidence of light scattering was observed at a detection spatial off-set distance of 1-2 mm from the illumination point. When the calcium channel blocking drug felodipine was flushed through the column, the light intensity value dropped (increase in absorbance) to a greater degree at a spatial off-set (1.5 mm) than at the illumination point. The greater absorbance values at the spatial off-set were examined experimentally when felodipine was eluted from the column in the CEC mode in 6 min using MeCN/50 mM TRIS (pH 8.0) (80:20, v/v) at an applied voltage of 300 V/cm and an injection time of 2 s at 10 kV. A factor of 8.5 increase in absorbance was observed at a spatial off-set of 1 mm compared to the value obtained at the illumination point. An efficiency value of approximately 234,000 plates m-1 was obtained for this higher felodipine peak. Higher noise values, however, were also observed with this increase in absorbance. Using a spectrophotometer or an open capillary to obtain reference values for optical length, it was possible to estimate the average optical path length of light traveled through the packed stationary phase when transmitted at a spatial off-set. It was concluded that, although an increase in absorbance of 8.5 was observed at a spatial off-set, this most likely arises from the light being "redirected" and scattered in a straightforward fashion along the capillary. It was expected that if substantial multiple scattering did occur inside the packed stationary phase, a significantly larger absorbance increase would be attained. A number of proposals are thus given to explain the relatively low degree of multiple scattering in this stationary phase and suggestions offered on means to attain even higher absorbance increases at a spatial off-set. Additional potential applications are also discussed.