Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source for Elemental Mass Spectrometry

A new, low power ionization source for elemental MS analysis of aqueous solutions is described. The liquid sampling-atmospheric pressure grow discharge (LS-APGD) operates by a process wherein the surface of the liquid emanating from a 75 mu m i.d. glass capillary acts as the cathode of the direct current glow discharge. Analyte-containing solutions at a flow rate of 100 mu L min(-1) are vaporized by the passage of current, yielding gas phase solutes that are subsequently ionized in the <5 W (maximum of 60 mA and 500 V), similar to 1 mm(3) volume, plasma. The LS-APGD is mounted in place of the normal electrospray ionization source of a Thermo Scientific Exactive Orbitrap mass spectrometer system without any other modifications. Basic operating characteristics are described, including the role of discharge power on mass spectral composition, the ability to obtain ultrahigh resolution elemental isotopic patterns, and demonstration of potential limits of detection based on the injection of aliquots of multielement standards (S/N > 1000 for 5 ng mL(-1) Cs). While much optimization remains, it is believed that the LS-APGD ion source may present a practical alternative to high-powered (>1 kW) plasma sources typically employed in elemental mass spectrometry, particularly for those cases where costs, operational overhead, simplicity, or integrated elemental/molecular analysis considerations are important.