Study on the factors of Ionization Efficiency of Paper Spray Ionization

Abstract

Paper spray ionization (PSI) has become one of the most popular ambient ionization techniques for mass spectrometry. However, its ionization mechanism has not been fully understood. Herein, my study focused on spray mechanism of PSI in the comparison with the conventional electrospray ionization source. The paper spray was classified into three modes: single cone-jet, multi-jet, and rim-jet mode. The rim-jet mode showed the lowest standard deviation and high ionization efficiency. The spray mode was a result of the droplet charged density, which is influenced by electric field and solvent flow rate. Paper thickness was proved to influence the electric field and the formation of jets. The thicker paper required a lower applied voltage to reach the rim-jet mode and yielded a higher signal. The spraying rate increased with the growth of the electric field. Lowering the flow rate caused the mode transition from the single cone-jet to the rim-jet mode, possibly due to the increased droplet charge density. Paper surface property directly affected the ionization mechanism in PSI. The ionization efficiency was significantly enhanced when the hydrophobicity of the paper substrate matches that of the spray solvent. The hydrophobic paper was supposed to facilitate the ion evaporation model of ionization. This systematic study proposes a plausible mechanism on PSI with respect to its contributing factors, which provides a fundamental comprehension for the development of new PSI applications as well as other substrate spray techniques.