Very fast electrophoretic separation on commercial instruments using a combination of two capillaries with different internal diameters

A capillary formed by connecting a 9.7 cm‐long separation capillary with id 25 μm with an auxiliary 22.9 cm‐long capillary with id 100 μm (coupled capillary) was tested for electrophoretic separation at high electric field intensities. The coupled capillary was placed in the cassette of a standard electrophoresis apparatus. It was used in the short‐end injection mode for separation of a mixture of dopamine, noradrenaline, and adrenaline in a BGE of 20 mM citric acid/NaOH, pH 3.2. An intensity of 2.7 kV/cm was attained in the separation part of the capillary at a separation voltage of 30 kV, which is 2.9 times more than maximum intensity value attainable in a capillary with the same length with uniform id. At these high electric field intensities, the migration times of the tested neurotransmitters had values of 12.3–13.3 s and the attained separation efficiency was between 2350 and 2760 plates/s. It is thus demonstrated that an effective separation instrument ‐ a coupled capillary ‐ can be used for very rapid separation in combination with standard, commercially available instrumentation.