Rapid Detection and Isotopic Measurement of Discrete Inorganic Samples using Acoustically Actuated Droplet Ejection and Extractive Electrospray Ionization Mass Spectrometry
Thomas P. Forbes
The coupling of ultrasonic nebulization and extractive electrospray ionization (EESI) for the mass spectrometric (MS) detection and analysis of inorganic compounds from discrete liquid samples was demonstrated. The rapid detection, high throughput screening, and isotopic signature analysis of inorganics provides invaluable information for a variety of applications including explosive device detection, nuclear forensics, and environmental monitoring. Ultrasonic nebulization and acoustic pressure wave focusing within an array of exponential horn structures was utilized for the efficient atomization of discrete liquid samples, i.e., 3 μL to 10 μL pipetted aliquots. In conjunction with an electro-flow focusing source for extractive electrospray ionization (EESI) and in-source collision induced dissociation (CID), enhanced detection of the singly charged elemental cation species and accurate measurements of isotopic distributions for a number of metals were achieved. The extent of in-source CID also identified the hydration equilibria for the alkaline earth metals strontium and barium, and demonstrated the competition between ligand loss from hydrate clusters and charge reduction from the doubly charged to singly charged cations. The described system provides a versatile tool for the rapid detection, speciation, and isotopic identification of inorganic compounds at nanogram to sub-nanogram levels.