Microdroplet impact-induced
spray ionization (MISI) is demonstrated
involving the impact of microdroplets produced from a paper and their
impact on another, leading to the ionization of analytes deposited
on the latter. This cascaded process is more advantageous in comparison
to standard spray ionization as it performs reactions and ionization
simultaneously in the absence of high voltage directly applied on
the sample. In MISI, we apply direct current (DC) potential only to
the terminal paper, used as the primary ion source. Charge transfer
due to microdroplet/ion deposition on the flowing analyte solution
on the second surface generates secondary charged microdroplets from
it carrying the analytes, which ionize and get detected by a mass
spectrometer. In this way, up to three cascaded spray sources could
be assembled in series. We show the detection of small molecules and
proteins in such ionization events. MISI provides a method to understand
chemical reactions by droplet impact. The C–C bond formation
reactions catalyzed by palladium and alkali metal ion encapsulation
using crown ether were studied as our model reactions. To demonstrate
the application of our ion source in a bioanalytical context, we studied
the noninvasive in situ discrimination of bacteria samples under ambient
conditions.