The Evolution of Secondary/Extractive Electrospray Ionization: From Ionization Mechanism to Instrumental Advances

Secondary electrospray ionization (SESI) and extractive electrospray ionization (EESI), as derivative technologies of electrospray ionization (ESI), have empowered the real-time analysis of trace compounds residing in gases and aerosols. Over the past three decades, SESI and EESI have demonstrated remarkable potential in a wide spectrum of applications, spanning disease diagnosis, drug detection, food safety, and environmental surveillance. Concurrently, the strides made in deciphering the ionization mechanisms of SESI and EESI have spurred the creation of diverse ion source configurations that are characterized by enhanced sensitivity and diminished background noise. This comprehensive review encapsulates the ionization mechanisms inherent in SESI and EESI processes, with particular emphasis on the impact of analyte characteristics (such as proton affinity, dipole moment, polarizability, and solubility) and ion source operational parameters (encompassing temperature, humidity, voltage, flow rate and electrospray composition) on ionization efficiency. Additionally, it delves into the progression of SESI and EESI sources, highlights recent breakthroughs, and probes into future trajectories, furnishing novel perspectives for the development of both technologies and the associated instruments.