Rapid detection and isotopic measurement of discrete inorganic samples using acoustically actuated droplet ejection and extractive electrospray ionization mass spectrometry.

RATIONALE The rapid detection, screening, and isotopic signature analysis of inorganics provide invaluable information for a variety of applications including explosive device detection, nuclear forensics, and environmental monitoring. The coupling of ultrasonic nebulization and extractive electrospray ionization (EESI) enabled the mass spectrometric (MS) detection and analysis of inorganics from microliter sample solution aliquots. METHODS Ultrasonic nebulization and acoustic pressure wave focusing within an array of exponential horn structures were utilized for the efficient atomization of discrete liquid samples ranging in volume from 3 μL to 10 μL pipetted aliquots. In conjunction with an electro-flow focusing source for extractive electrospray ionization (EESI), in-source collision-induced dissociation (CID) was utilized to enhance inorganic detection through fragmentation of adducts and reduction in chemical noise from organic compounds. RESULTS The investigated system enhanced detection of the singly charged elemental cation species and provided accurate measurements of isotopic distributions for a number of metal ions. The extent of CID demonstrated the competition between ligand loss from hydrate clusters and charge reduction from the doubly charged to singly charged cations for the alkaline earth metal ions of strontium and barium. Inorganics were also detected from complex matrices, including synthetic fingerprint material and sediment, without detriment to device operation. CONCLUSIONS The described system provides a versatile tool for the rapid detection, speciation, and isotopic identification of inorganic compounds at nanogram to sub-nanogram levels from microliter aliquots. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.