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.

The general ion chemistry of the explosive molecule cyclotrimethylenetrinitramine (RDX) was studied with an atmospheric pressure ionization mass spectrometer (API-MS) fitted with a desorption electrospray ionization (DESI) source. Explosive molecule chemistry within trace detection techniques such as ion mobility spectrometry (IMS) is an area of intense interest because of the widespread deployment of IMS-based explosive detectors for counterterrorism efforts. As in IMS, the DESI-MS experiments analyze material that starts in the solid phase and is detected in the gas phase. Using the unique chemical characterization inherent in mass spectrometry, information pertinent to the atmospheric ionization of RDX is obtained in order to help explain the behavior of explosive molecule signatures observed within IMS experiments. Qualitative and quantitative information was obtained over 3 orders of magnitude of deposited mass (nanograms to greater than micrograms). A method was developed to use the relative integrated mass spectral peak intensities of RDX monomer and dimer chloride adducts to determine the amount of explosive present on a surface. The ratio of RDX dimer chloride adduct to monomer chloride adduct ranged from 0.1 for 15 ng to 1.0 for 1.5 microg of deposited explosive. The results are explained in terms of mechanisms reported in the literature for electrospray ionization (ESI), as well as by simple solution dynamics and the interaction chemistry between RDX molecules. On the basis of all available data, the RDX dimer chloride adduct becomes disproportionately favored over the monomer chloride adduct at larger amounts of explosive because of effects related to desorbed droplet charge, solvent declustering, and the strong intermolecular forces between RDX molecules in the solid, liquid, and gas phases. Additionally, considerations for optimization of the DESI-MS process are described in order to increase the practicality for this technique as an explosives detection tool in the public domain.

Presented here is a method for the quantitative determination of iron-containing metalloproteins. Four iron-containing metalloproteins (transferrin, myoglobin, hemoglobin, and cytochrome c) were separated by high-performance liquid chromatography (HPLC) and determined through particle beam/hollow cathode-optical emission spectroscopy (PB/HC-OES) by the Fe (I) 371.9 nm optical emission. Parametric optimization of sample introduction, nebulization, and hollow cathode source conditions is performed for the suite of Fe-metalloproteins. Response curves for the Fe (I) emission were obtained under optimized conditions with detection limits for triplicate injections occurring on the nanogram level for iron ( approximately 24 ng) with variability of <7% RSD over the concentration range of 0.1-100 microg/mL iron in the metalloproteins. Response curves for S (I) emission yielded similar analytical characteristics. Optical emission detection of the liquid chromatography separations of the iron-containing metalloproteins demonstrates the feasibility of the PB/HC-OES system as a simple element-specific detector for liquid chromatography. The retention times of the four analytes are similar to those determined by UV absorbance (216 nm), demonstrating the ability of the PB interface to preserve the chromatographic integrity of the separation. Additionally, empirical formula calculations based on Fe (I) and S (I) emission response ratios provide a much higher level of specificity than single-element protein determination.

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