A mechanism study of positive ionization processes in flowing atmospheric-pressure afterglow (FAPA) ambient ion source with controlled plasma and ambient conditions

Abstract

Although plasma based ambient desorption/ionization (ADI) sources have been widely used for direct analysis of complex samples, mass spectrometric imaging, high throughput screening etc., the ionization mechanism of plasma-based ADI remains a mystery by now. In this report, a targeted study was conducted aiming at a better understanding of the ionization processes of plasma-based ambient desorption ionization source. As a representative of ambient desorption ionization source, an FAPA source was used and modified as a test platform to control the plasma discharge parameters and ambient ionization environment such as discharge gases, environmental gases and sampling conditions. Based on the ionization results from different ambient ionization conditions, a new mechanism was proposed to reveal the nature of regent ion production of FAPA. At the same time, the effect of buffer gas was investigated. For the first time, the multi-clustered hydronium ions formed by the massive water vapor in the air were explored to clarify reasons for the occurrence of selective ionization and the factors affecting ionization efficiency in such complex events. In addition, the formation of molecular ions and relevant reagent ions was speculated based on experimental observations.