Desorption electro-flow focusing ionization of explosives and narcotics for ambient pressure mass spectrometry†‡ Thomas P. Forbes, * Tim M. Brewer and Greg Gillen Desorption electro-flow focusing ionization (DEFFI), a desorption-based ambient ion source, was developed, characterized, and evaluated as a possible source for field deployable ambient pressure mass spectrometry (APMS). DEFFI, based on an electro-flow focusing system, provides a unique configuration for the generation of highly charged energetic droplets for sample analysis and ionization. A concentrically flowing carrier gas focuses the liquid emanating from a capillary through a small orifice, generating a steady fluid jet. An electric field is applied across this jet formation region, producing high velocity charged droplets that impinge on an analyte laden surface. This configuration separates the jet charging region from the external environment, eliminating detrimental effects from droplet space charge or target surface charging. The sample desorption and ionization processes operate similar to desorption electrospray ionization (DESI). DEFFI demonstrated strong signal intensities and improved signal-to-noise ratios in both positive and negative mode mass spectrometry for narcotics, i.e., cocaine, and explosives, i.e., cyclotrimethylenetrinitramine (RDX), respectively. A characterization of DEFFI ionization mechanisms identified operation regimes of both electrospray and corona discharge based analyte ionization, as well as limitations in overall signal. In addition, the DEFFI response was directly compared to DESI-MS under similar operating conditions. This comparison established a wider and more stable optimal operating range, while requiring an order of magnitude lower applied gas pressure and applied potential for DEFFI than DESI. These reductions are due to the physical mode of jet formation and geometric configuration differences between DEFFI and DESI, pointing to a potential benefit of DEFFI-MS for field implementation. Introduction The accurate identication and sensitive trace detection of analytes on solid surfaces has broad signicance to many applications in analytical chemistry. A large number of tech- niques have been developed and successfully implemented for trace detection from a variety of environments, including immunoassays, 1,2 gold nanoparticles, 3 Raman spectroscopy, 4 ion mobility spectrometry (IMS), 5,6 and atmospheric pressure ionization mass spectrometry (API-MS). 7–10 Ambient pressure mass spectrometry is a powerful detection and mass analysis tool that has resulted in the development of a wide range of ambient ionization techniques, most notably, desorption electrospray ionization (DESI) 8–12 and direct analysis in real time (DART). 13,14 Recent reviews have been published covering the large number of ambient ionization techniques that have become available for ambient mass spectrometry. 15,16 There has also been a growing need for sensitive, fast, and eld deployable trace detectors to accommodate in situ forensic analyses and threat detection, i.e., chemical, explosive, and biological. The need for deployable instrumentation and reduced consumable load has led to the development of a number of miniature 17–19 and portable 20,21 mass spectrometers. To facilitate efficient coupling, ion source evolution must also focus on eld compatibility. In addition, as centralized labora- tories become burdened with extensive analysis loads, ion sources capable of high-throughput screening that reduce necessary consumables and operate under a exible range of conditions are imperative. Here, we implement the recently established electro-ow focusing (EFF) technique 22,23 in the development of a desorption-based ion source for mass spec- trometric analysis. Similar to DESI, charged solvent droplets are sprayed onto a surface containing analyte molecules. The interaction of these high velocity micro drops provides solvation of the analyte molecules and subsequent desorption of analyte National Institute of Standards and Technology, Materials Measurement Science Division, Gaithersburg, MD, USA. E-mail: thomas.forbes@nist.gov; Tel: +1 (301) 975-2111 † Electronic supplementary information (ESI) available: Additional data and gures, including secondary droplet current, additional mass spectra for RDX, PETN, and TNT, and mass spectrometric signal decay as a function of time. See DOI: 10.1039/c3an01164j ‡ Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States. Cite this: Analyst, 2013, 138, 5665 Received 11th June 2013 Accepted 20th July 2013 DOI: 10.1039/c3an01164j www.rsc.org/analyst This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 5665–5673 | 5665 Analyst PAPER