Ambient desorption/ionization mass spectrometry using a liquid sampling-atmospheric glow discharge (LS-APGD) ionization source

A novel approach to ambient desorption/ionization mass spectrometry (ADI-MS) is described, based on a recently developed liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source. The device is essentially unmodified relative to its implementation in elemental mass spectrometry, where the operational space is characterized by low operation power (<10 W) and low solution delivery rates (<50 μL min^-1). In this implementation, the plasma is produced between a Ni anode and an electrolytic liquid (1 M HNO₃) cathode flowing through a glass capillary that is angled towards the sample surface, at a distance of ∼2 mm away. Analyte species can be desorbed/ionized from neat solution residues and complex solid samples. The ADI-LS-APGD source is mounted onto the source interface of a Thermo Finnigan LCQ Advantage Max quadrupole ion trap mass spectrometer without modifications to the instrument faceplate or ion optics. Described here is the initial evaluation of the roles of source geometry and working parameters, including electrolytic solution composition and plasma current, on the response of caffeine residues, with preliminary limits of detection based on the relative standard deviation of the spectral background suggested to be on the 10-pg level. Demonstrative spectra are presented for green tea extracts and raw leaves, coffee beans, a dried (raw) tobacco leaf, an analgesic tablet, and paper currency. Versatility is further revealed through the determination of components in common cigarette smoke. In each case, the spectra are characterized by (M + H)⁺ species of the expected constituents. The capacity for a single source to perform both in solution and particulate elemental analysis (as shown previously) and ADI of molecular species is unique in the realm of mass spectrometry.