Development of a Hexapole Traveling-Wave Ion Guide for Enhanced Ion Utilization Efficiency in Electrospray Ionization Time-of-Flight Mass Spectrometry

Electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) is a powerful analytical technique widely used for characterizing biomacromolecules and synthetic polymers, offering advantages such as high analysis speed, broad detection range, and high resolution. However, a fundamental limitation of this technique arises from the inherent mismatch between the continuous ion beam produced by electrospray ionization and the pulsed sampling requirements of time-of-flight mass analyzers. This duty cycle mismatch results in substantial ion losses and limits the achievable sensitivity of the instrument. This work presents the design and development of a single-channel planar hexapole traveling-wave ion guide (PHTWIG) intended to enhance ion utilization efficiency in ESI-TOF MS systems. The PHTWIG functions as an ion accumulation and packaging device that captures the continuous ion stream from the ESI source, temporarily stores it, and releases discrete ion packets in synchronization with TOF pusher pulses. This synchronous operation enables efficient coupling between the continuous ionization source and the pulsed mass analyzer, thereby significantly enhancing the overall duty cycle and ion utilization efficiency. The ion transport dynamics within the PHTWIG under traveling-wave electric fields in rough vacuum conditions were investigated. A simulation model was developed using Simion 8.1 to characterize ion trajectories and optimize the ion guiding performance. Critical parameters, including background gas pressure and traveling-wave voltage parameters (amplitude, frequency, and waveform), were systematically optimized through simulations combined with experimental validation. An ESI-TOF MS experimental platform integrated with the PHTWIG was constructed to validate its performance. Under optimized synchronous operation between low-speed traveling waves and TOF extraction pulses, the instrument achieved substantial improvements in detection sensitivity. The detection sensitivities for caffeine, reserpine, and their fragment ions were improved by factors ranging from 2.5 to 8.8 compared with the conventional operation without the PHTWIG. This work provides a novel strategy for improving ion utilization and enhancing detection sensitivity in ESI-TOF MS instruments. The PHTWIG design presents an effective solution for bridging the gap between continuous ionization sources and pulsed mass analyzers, with promising applications in high sensitivity analysis of complex samples. Future studies will focus on extending the mass range coverage and integrating the device with advanced ion mobility separation techniques.