Sensitivity Enhancement by Dual Deflection Electrodes in an Electrospray Ionization Source

In electrospray ionization-mass spectrometry (ESI-MS), ESI configurations such as orthogonal spray arrangements are often used to suppress droplet introduction into the vacuum chamber, but they can also reduce the ion flux into the vacuum region. Previous methods that electrostatically deflected ions toward the vacuum inlet using a single electrode achieved signal enhancement but could not adequately control deflection position, limiting performance. Therefore, we investigated whether adding a second deflection electrode to create a more localized electric field could further increase ion flux into the vacuum region and enhance sensitivity.

Electrostatic analysis was performed using a two-dimensional cross-sectional model for three ion-source configurations: no electrode, single-plate electrode, and dual electrodes (plate and cylindrical rod). For experimental evaluation, direct-infusion experiments (100 μL min^-1) were performed using reserpine (m/z = 609.3) and a CsI/diethylamine mixture spanning m/z values of 74-912, and electrode voltages were stepped (E1, 0-5 kV; E2, 1-3 kV). Flow-injection analysis (FIA) of reserpine (0.5-20 ng mL^-1) was conducted to evaluate quantitative performance.

Electrostatic analysis showed that the dual-electrode configuration concentrated the inlet-directed field near the inlet and suppressed the unwanted field above it. Experimentally, the dual-electrode configuration increased the reserpine signal by 3.3-fold compared with the configuration without electrodes. Across five chemically diverse ions, signal enhancements were obtained. For singly charged species, the gain scaled inversely with collision cross-section, indicating a specific benefit for low-ion-mobility analytes. FIA yielded a 4.0-fold increase in the calibration curve slope and halved the lower limit of quantification without degrading linearity (R² = 0.998).

Localizing the electric field using two deflection electrodes enhances ion signal intensity, sensitivity, and quantitative limits in the ESI source configuration examined here. These results indicate that spatial control of the deflection field is an effective strategy for improving ion transmission in an orthogonal ESI source.