Universal detection for capillary electrophoresis-using micro-interferometric backscatter detection

We describe a method for determining subpicomole quantities of analyte in c apillary electrophoresis by probing fluid bulk properties with laser interf erometric backscatter. In micro-interferometric backscatter detection (MIBD ), an unfocused beam from a He-Ne laser impinges onto unmodified fused sili ca capillary tube and subsequent backscattered laser light is analyzed. Pos itional changes of interference pattern extrema (backscattered light) are d irectly related to refractive index (RI), n, of fluid in the tube, and allo w for determination of Delta n at the 10(-7) level. MIBD was interfaced to commercial CE instrument allowing for simultaneous RI/ UV-Vis detection. Ut ility of MIBD detector for capillary electrophoresis was shown by detecting caffeine in beverages. Analysis was performed in 100-mu m capillary, and d irect comparison of RI detector's performance with UV-Vis absorbance detect or (211 nm) was possible. In system without active thermal control, RI dete ction limit (3 sigma) for caffeine was 2.33*10(-6) M, whereas detection lim it of the commercial UV-Vis detector was 1.16 *10(-6) M. At detection limit of 2.33 mu M, MIBD is capable of performing universal detection on just 2. 02*10(- 14) or 20 fmole of solute in probe volume of 4.7 nL. It is shown th at a solute like caffeine, which has large molar absorptivity (epsilon) at probe wavelength, MIBD has comparable sensitivity. Therefore, it is useful in other CE applications, particularly those involving solutes with low eps ilon values. (C) 1999 John Wiley Sc Sons, Inc.