On‐line sample preconcentration by polarity switching in floating electrode‐integrated microchannel

: In this study, we found that the polarity switching was effective to enrich and separate fluorescent analytes which have weakly-dissociated groups in a floating platinum electrode (width, 50 µm; thickness, 2.5 µm)-integrated straight-channel in microchip electrophoresis (MCE). In the straight channel filled with an Alexa Flour 488 (AF488) solution, a sharp peak was observed after the polarity inversion with a 530-fold enhancement of the sensitivity relative to the conventional MCE analysis. By using a fluorescent pH indicator, we verified that a sharp high-pH zone was generated nearby the floating electrode and moved toward the anode with maintaining the high pH, which induced the sample enrichment like a dynamic pH junction mechanism. In the floating electrode-embedded channel, the mixture of AF488-labeled proteins was also well concentrated and separated within 100 s.