A simple procedure for creating a region of expanded diameter on a capillary column is described. This bubble-shaped oncolumn flow cell provides an extended path length for absorption detection in capillary electrophoresis (CE) or capillary chromatography. To properly probe the compressed analyte zone in this region, a laser beam with a noise-cancellation circuit was used. Up to 8X enhancement in signal was achieved. Characterization of CE flow profiles in the bubble-shaped region shows that distortions inflow lead to band broadening, especially when a rapid change in diameter over a short column length is present. The electric field lines are, however, still oriented axially throughout the bubble region. Similar disturbances are expected in any coupling schemes in CE that involve a geometry change, in addition to multipath effects and distortions in the electric field.Capillary electrophoresis (CE) as a modern separation method is well-known. The extremely high efficiency, small sample volume, and high speed of CE have given it increasing popularity.' Due to the reducedsamplevolume, it is necessary to minimize extracolumn band broadening. Therefore, sample injection and detection are usually accomplished in an oncolumr;, cross-beam configuration. A situation where a nonuniform capillary geometry exists is the special modification of the detector cell to enhance the detection limit. Xi and Yeung described a means of directing the light beam along the capillary axis. For a 50 pm 0.d. capillary and 3-mm injection plugs, a 7-fold increase in sensitivity was obtained.2 The insertion of an optical fiber into the capillary there results in a nonuniform geometry. A Z-shaped flow cell with a 3-mm path length has become commercially available. By better light coupling to reduce noise levels while maintaining a long path length, a 14-fold improvement in the signal-to-noise ratio (S/N) was ~b t a i n e d .~ This can, however, be accompanied by a significant loss of the number of theoretical plates, simply due to the long detection zone. For example, in DNA sequencing, fragment bands can be as narrow as 2-3 mm. More recently, one CE manufacturer4 introduced a bubbleshaped cell to extend the optical path length. It was made by forming an expanded region, a bubble, directly on the capillary column. In the region of the bubble, the electrical resistance is reduced and thus the electric field is decreased. When the sample zone enters the bubble region, its velocity decreases in a manner similar to field-amplified injection5 and the zone is compressed axially but spreads radially. A subtle difference (1) Kuhr, W. G.; Monnig, C. A.
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