Continuously Tuning the Air Plasma Desorption Ambient Ionization Source for Broad Polarity Compound Detection

Ambient ionization sources directly analyze different polarity analytes in the samples, but they struggle to simultaneously analyze polar/nonpolar analytes and minimize fragmentation caused by the weakness of controlling the internal energy distribution of the sources. Herein, a plasma desorption ionization (PDI) mass spectrometry (MS) was developed for a continuously controllable internal energy distribution of the ion source, which enables tuning the ionization modes of either proton transfer or charge exchange. By controlling the duty ratio, the PDI was tuned to ionize molecules of different polarity, to minimize fragmentation, and to reduce the matrix effect. Lower duty ratios (1−5%) yield lower Emax with narrow internal energy distributions characteristic of soft ionization, while higher duty ratios (40−100%) produce higher Emax with broad distributions characteristic of hard ionization. Above all, it showed an excellent performance in improving the signal-to-noise ratio by 40-fold. In addition, by coupling this PDI to a portable MS, real-world samples, like 2 polycyclic aromatic hydrocarbons and 25 pharmaceuticals (20 tablets, two liquid medicines, and three salves), were detected within 30 s, and the limit of detection of medicines was as low as 0.15 ng·mL−1. The PDI−MS has promise for applications in high-throughput screening of samples on site.