Reversible-Polarity High-Voltage Power Supplies for Capillary Zone Electrophoresis (CZE)
Capillary zone electrophoresis separates charged biomolecules with theoretical plate counts exceeding one million, but only when the applied electric field remains perfectly stable and instantly reversible without introducing baseline disturbances or bubble-generating electrolysis products. Modern reversible-polarity high-voltage systems for CZE therefore operate in the ±30 kV range with transition times under 50 ms and residual current offset below 50 nA across the full reversal cycle.
The core topology uses a bipolar flyback converter feeding a compact air-insulated Cockcroft-Walton multiplier pair—one for positive and one for negative polarity—mounted directly on the separation compartment lid to minimize cable capacitance. Polarity switching occurs through simultaneous deactivation of the active multiplier and activation of the opposite multiplier using optically triggered solid-state relays with greater than 40 kV isolation. This avoids mechanical relays that previously contributed charge injection spikes visible as ghost peaks in low-attomole DNA sequencing runs.
Voltage regulation employs a dual-loop strategy: a fast inner current loop operating at 250 kHz bandwidth that limits electrolysis current to safe levels even during buffer mismatches, and a slower outer voltage loop with 80 dB DC gain that maintains setpoint within ±1 V over hours. A precision 40-bit sigma-delta ADC samples the actual capillary voltage via a 1000:1 compensated resistive divider immersed in circulating fluorocarbon fluid for thermal stability better than 3 ppm/°C.
Reversal transients are suppressed by active damping networks that inject a small counter-pulse during the dead-time between multiplier hand-off. The damping waveform is pre-computed from measured capillary and electrode capacitance and adjusted daily via an automated self-calibration routine that applies a 100 Hz square wave and optimizes coefficients to null the observed overshoot. Resulting voltage settling to 0.01 % occurs within 35 ms, fast enough to support bidirectional sequencing protocols that reverse polarity every 90 seconds without peak broadening.
Bubble management relies on controlled current pre-bias before each reversal. For 200 ms prior to switching, the supply reduces voltage to 2 kV while maintaining anodic current flow to evolve oxygen rather than hydrogen at the newly cathodic electrode, preventing the nucleation of gas bubbles that disrupt electroosmotic flow. This pre-bias current is automatically adjusted based on real-time buffer conductivity measured via a four-electrode AC bridge integrated into the electrode reservoirs.
Long-term stability against buffer depletion is maintained through active leakage compensation. Platinum-black electrodes exhibit microampere-level faradaic currents that gradually shift effective field strength; the supply continuously monitors total integrated charge in each direction and injects sub-nanoampere offset currents to rebalance net electrolysis, preserving migration time reproducibility below 0.3 % RSD over 48-hour unattended runs of peptide mapping separations.
Safety interlocks incorporate capillary current signature analysis. Sudden increases in current slope indicative of capillary rupture trigger immediate voltage collapse to zero in under 8 ms followed by isolation contactor opening, preventing buffer spillage onto 30 kV terminals. All high-voltage surfaces are fully shrouded with interlocked polycarbonate shields that maintain greater than 50 mm creepage in 95 % relative humidity environments typical of clinical proteomics laboratories.
These reversible-polarity systems routinely deliver greater than 1 200 000 plates for dsDNA fragments and sub-1 % migration time variability across 1000 consecutive injections, enabling high-throughput glycan analysis and exon-level sequencing previously restricted to unidirectional instruments.
