Instrumentation and characterization of surface desorption atmospheric pressure chemical ionization mass Spectrometry

Combining the advantages of desorption electrospray ionization( DESI) and atmospheric pressure chemical ionization ( APCI), a novel ambient surface desorption atmospheric pressure chemical ionization (SDAPCI) source was developed for direct detection of trace amounts of analytes in various complex matrices without any sample pretreatment. In the SDAPCI source, a self-assembled corona discharge was employed to generate the primary ions such as H3O+ in positive ion detection mode and H2O center dot OH- in negative ion detection mode, which were directed to impact the sample surface to yield positive/ negative ions of interest for mass analysis. Using ambient air about 45% - 60% relative water moisture as a reagent for chemical ionization, SDAPCI allows mass spectrometric analysis of ambient samples without toxic contamination and provides simplicity for in-situ analysis in a miniature mass spectrometer. The principle and construction of the SDAPCI source are presented in this contribution. A commercially available Claritin tablet, which was used to characterize a DESI source previously, was used to characterize the SDAPCI source. Under the optimal experimental conditions, loratadine ( MW 382), an active ingredient of Claritin tablet, yields a high signal as protonated molecule ( m/z 383 which loses ethanol to give a major fragment ( m/z 337) upon collision-induced dissociation. Compared to DESI, a higher intensity was achieved in SDAPCI at 4 kV with ambient air as reagent gas for the Claritin tablet. Using pure argon gas instead of the ambient air, radical cations of acetaminophenol and peptide I-7 were successfully observed and structurally characterized by MS/MS experiments, which provided the first experimental evidence for production of radical cation of peptides at atmospheric pressure ambient. An electron stripping process caused by argon cation was thereby proposed to be the mechanism of radical cation formation in the SDAPCI source. The data show promising applications for direct analysis of ambient surfaces without sample contamination by toxic chemicals.