Capillary Electrophoresis High Voltage Power Supply with Zeeman Effect Compensation

High-performance capillary electrophoresis (CE) is a mainstay technique for separating ionic analytes, from small inorganic ions to large biomolecules. Its operation relies on the application of a high electric field (typically 10-30 kV) across a fused silica capillary to drive electroosmotic and electrophoretic migration. The primary high-voltage power supply is the engine of this separation. However, for the most demanding applications, particularly those employing on-capillary detection methods like absorbance or laser-induced fluorescence (LIF), a subtle physical phenomenon—the Zeeman effect—can introduce baseline noise and drift, limiting sensitivity. Advanced power supply designs now incorporate specific features to mitigate this interference, elevating the stability of the entire analytical system.

The Zeeman effect describes the splitting of spectral lines in the presence of a magnetic field. In a typical CE instrument, the most significant magnetic field present is the Earth's magnetic field, which is relatively weak but non-negligible. The high current flowing through the capillary (microamperes to tens of microamperes) generates a circular magnetic field around the capillary axis (according to Ampère's law). When combined with the Earth's field, this creates a complex, low-strength, but dynamically shifting local magnetic field in the vicinity of the capillary. For detection systems that rely on the polarization of light, such as those using lasers or certain lamp-based designs with polarizing filters, this fluctuating field can slightly rotate the plane of polarization of the excitation or emission light. Since optical components like beam splitters, dichroic mirrors, and emission filters often have polarization-dependent efficiencies, this rotation translates into a modulation of the detected light intensity. This manifests as low-frequency baseline noise (at the AC line frequency harmonics related to current ripple) and longer-term drift as the instrument warms up and current stabilizes.

The primary strategy for compensation is to attack the source: the magnetic field generated by the separation current. This is achieved through meticulous control of the current itself, mandated by the capabilities of the high-voltage power supply. The supply must operate in a highly stable, constant current mode. The inherent current ripple of the switching or linear regulation circuits must be minimized to an extraordinary degree, often specified in the nanoampere range for a 10-50 µA operating current. This directly reduces the AC component of the magnetic field. Furthermore, the supply's feedback loop for current regulation must have high gain at low frequencies to suppress any slow drift in current output due to temperature changes in the capillary or buffer reservoirs. A drift of even 0.1% of the nominal current can cause a measurable change in the local magnetic field over time.

A more active compensation technique involves the physical layout and, implicitly, the power supply's architecture. Some advanced designs employ a coaxial return path for the high voltage current. Instead of a single electrode at the outlet end, a configuration with a concentric guard or shield conductor around the capillary (or its conceptual equivalent in the vial arrangement) is used. The separation current flows down the capillary and returns via this outer shield. Since the current in the shield is equal in magnitude but opposite in direction to the current in the capillary, their magnetic fields outside the coaxial structure ideally cancel. Implementing this requires a power supply with a dedicated, low-impedance return terminal designed for such a configuration and a deep understanding of the field geometry to ensure effective cancellation at the precise location of the detection window.

The power supply must also exhibit exceptional voltage stability in constant voltage mode, as this is the other common operational modality. Voltage ripple or noise translates into field strength variations, which can cause migration velocity jitter, broadening peaks and reducing resolution. For Zeeman-sensitive detection, even in constant voltage mode, the current is not perfectly steady (it changes as the buffer composition heats and evolves), so the principles of low-noise design still apply. Additionally, the physical placement of the high-voltage supply within the instrument is critical. It must be magnetically "quiet"; large transformers or inductors should be shielded or positioned away from the capillary and optical detection path. The use of modern, high-frequency switching topologies with smaller magnetic components is beneficial.

Integration with the detector can also be part of the solution. In some systems, a reference signal from the power supply's current monitor can be fed into the data system. Sophisticated software algorithms can then use this current trace to perform post-acquisition baseline correction, modeling and subtracting the component of noise correlated with current fluctuations. This requires the power supply's current monitor to have high bandwidth and linearity. In essence, a CE high-voltage supply designed with Zeeman effect compensation in mind is an instrument of precision metrology. It prioritizes the cleanliness and stability of its electrical output—both voltage and current—to an extent that minimizes its own physical interaction with the sensitive optical detection system, thereby unlocking the ultimate sensitivity and baseline stability required for analyzing trace components in complex matrices.