Real-Time Charge Monitoring of Electrosprayed Methanol and Micrometer-Sized Polystyrene Droplets at Ambient Temperature

Shao-Yu Liang^1*, Wei-Jyun, Ciou¹, Shiu-Yao Yeh¹, Avinash A. Patil¹, Wen-Ping Peng¹

¹Physics, National Dong Hwa University, Hualien, Taiwan

* Presenter:Shao-Yu Liang, email:jyjo49482k7@gmail.com

Electrospray ionization (ESI) produces charged droplets through a high-voltage electric field, followed by solvent evaporation and Coulombic fission, enabling gentle ionization of non-volatile analytes. The efficiency of this process depends strongly on droplet desolvation. In this study, the evaporation and fission behaviors of electrosprayed methanol and micron-sized polystyrene (PS) droplets were investigated under elevated ambient temperatures using a charge-sensing particle detector (CSPD) for real-time charge monitoring. For methanol droplets, the average charge decreased from 12,776 ± 5,899 at 23 °C to 9,461 ± 5,545 at 43 °C, indicating droplet shrinkage due to evaporation. At 62 ± 2 °C, three charge populations (2,207 ± 1,293; 6,892 ± 1,964; and 11,749 ± 5,057) emerged, signifying the onset of Coulombic fission. Most droplets completely evaporated above 90 °C, a temperature much lower than that required for capillary heating (~200 °C) in vacuum. Similar trends were observed for the ESI ionization of 2 μm PS beads. As the ambient temperature was set to 31 ± 1 °C, 47 ± 2 °C, 55 ± 2 °C, and 63 ± 3 °C, the inlet capillary temperature required to induce evaporation and fission gradually decreased. When the ambient temperature exceeded 63 ± 3 °C, no significant additional evaporation was observed, indicating that desolvation was nearly complete under these conditions. In contrast, when only inlet capillary heating was applied, temperatures of ≧ 80 ± 2 °C were required, demonstrating that elevated ambient temperature provides higher desolvation efficiency and reduces the thermal energy needed for 2 μm PS ions desolvation.

Keywords: Electrospray ionization (ESI), Charge-Sensing Particle Detector (CSPD), ambient heating of ESI droplets