±15 kV Ultra-Stable Bipolar Power Supplies for Orbitrap Mass Spectrometers

The Orbitrap analyzer achieves resolving powers exceeding 1 000 000 and mass accuracy below 0.5 ppm only when the central electrode is held at an extraordinarily stable ±15 kV potential with ripple below 400 µV p-p and long-term drift under 1.2 ppm over 24 hours. Bipolar supplies must therefore deliver perfectly matched but opposite polarities with independent fine adjustment while rejecting common-mode disturbances from ion injection transients and vacuum pump vibration.

The dual supply uses two identical but anti-phase resonant flyback converters operating at 135 kHz from a common 48 V rail, each driving a ten-stage air-insulated Cockcroft-Walton multiplier potted in low-dielectric-absorption silicone. Positive and negative outputs are referenced to a common floating midpoint that is actively servoed to within ±60 mV of instrument ground through a separate low-voltage regulator, eliminating common-mode voltage that would otherwise degrade orbital frequency stability.

Absolute voltage is set by a precision 15.0000 V buried-zener reference followed by a chopper-stabilized instrumentation amplifier and 20-bit DAC. Feedback from each multiplier is taken via temperature-compensated 2000:1 capacitive dividers immersed in circulating fluorinert maintained at 34.000 ± 0.003 °C. The digital control loop updates every 180 µs with adaptive integral action that nulls drift from capacitor SOA (soakage) and resistor VCR effects, achieving 0.8 ppm hour-to-hour stability after 4-hour warm-up.

Ripple suppression combines passive and active stages: inherent low ripple from the ten-stage multiplier is further reduced by distributed pi-filters using polypropylene capacitors with near-zero voltage coefficient, followed by a linear post-regulator on each polarity capable of 110 dB rejection up to 8 kHz. Residual ripple remains below 280 µV p-p across the full 0–800 µA load range created by C-trap RF switching.

Transient response during ion injection events (up to 120 µA steps in <2 µs) is handled by local 4.7 µF low-inductance film capacitors mounted directly at the Orbitrap electrode feedthroughs, backed by active replenishment from the main supply within 45 µs. Settling to 1 mV occurs in under 120 µs, preventing detectable frequency modulation of the trapped ion cloud.

Temperature coefficient matching between positive and negative channels is performed at the factory by laser-trimming thin-film resistors in the feedback dividers to <0.07 ppm/°C differential drift. Quarterly in-situ calibration injects precision currents through calibrated 10 GΩ resistors at the electrode terminals and updates digital compensation tables automatically.

Vacuum compatibility is absolute: all high-voltage components are UHV-cleaned, and the multiplier assemblies are baked at 180 °C under vacuum before sealing. Outgassing remains below 2 × 10⁻¹² Torr·L/s after 5000 hours. These bipolar supplies routinely support resolving power >1 200 000 at m/z 200 and sub-ppm mass accuracy over 18-hour proteomics runs with zero manual readjustment.