Microplasma-Based Flowing Atmospheric-Pressure Afterglow Source for Ambient Desorption-Ionization Mass Spectrometry

Offer Zeiri, Analytical Chemistry, NRCN, Beer Sheva, Israel
Andrew Storey, Chemistry, Indiana University, Bloomington, United States
Steve Ray, Chemistry, Indiana University, Bloomington, United States
Gary Hieftje, Chemistry, Indiana University, Bloomington, United States

Ambient desorption/ionization mass spectrometry (ADIMS) has become a rapidly growing field in analytical chemistry.  The technique generally requires no sample preparation, desorbing ions directly from samples next to the Mass Spectrometer (MS) inlet.  Sources for ADIMS can be divided into two main groups – spray and plasma based.  Plasma-based sources vary with respect to different discharge conditions and geometries.  One such source is the FAPA, which utilizes the afterglow from a DC helium discharge to thermally desorb and ionize samples.  Applications for the FAPA-MS already include detection of illicit drugs and drug metabolites, imaging and polymer fingerprinting, among others.

Microplasmas are plasmas confined to a size below 1 mm in at least one dimension, and are used in a variety applications such as detectors, biomedical uses and lab-on-a-chip technology.  Microplasmas possess several properties that make them of interest for ADIMS such as high stability to arcing, and high electron density.  Such qualities would contribute to a source’s sensitivity and robustness for ADIMS.  Despite these attractive properties, there have only been a few examples of microplasmas used as sources for ADIMS in the literature.  These studies took advantage of the microplasmas’ low gas temperature and power consumption, by using either a pulsed power supply, or low operating currents.

We now present the micro-FAPA (μ-FAPA), a new FAPA design that incorporates a direct current, concentric electrodes microplasma as the afterglow source.  This new system was characterized by current-voltage behavior, gas plume temperature at different conditions, and mass spectrometry.  Solid, liquid and gas samples were analyzed to provide an understanding of the source’s operating principle and mass range, as well as provide limits of detection for several substances.  Finally, the effect of competitive ionization in the sample matrix was examined, to evaluate the systems robustness.