Transmission-mode direct analysis in real time (TM-DART) is presented as an alternative sampling strategy to traditional methods of sample introduction for DART MS analysis. A custom-designed sample holder was fabricated to rapidly and reproducibly position insecticide-treated nets normal to the ionizing metastable gas stream, enabling transmission of desorbed analyte ions through the holder cavity and into the MS. Introduction of the sample at this fixed geometry eliminates the need for optimizing sample position and allows spectra based on factors such as metastable gas temperature and flow to be systematically evaluated. The results presented here, supported by computational fluid dynamic simulations, demonstrate the effects of these factors on the resulting mass spectra and the potential of this sampling strategy to be used for qualitative and quantitative analyses. Transmission-mode desorption electrospray ionization (TM-DESI) experiments on similar insecticide-treated nets were performed for comparison purposes.

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Adaptation of desorption electrospray ionization to a transmission mode (TM-DESI) entails passing an electrospray plume through a sample that has been deposited onto a mesh substrate. A combination of mass spectrometry and fluorescence microscopy studies is used to illustrate the critical role material composition, mesh open space, and mesh fiber diameter play on the transmission, desorption, and ionization process. Substrates with open spaces less than 150 m and accompanying minimal strand diameters produce less scattering of the plume and therefore favor transmission. Larger strand diameters typically encompass larger open spaces, but the increase in the surface area of the strand increases plume scattering as well as solvent and analyte spreading on the mesh. Polypropylene (PP), ethylene tetrafluoroethylene (ETFE), and polyetheretherketone (PEEK) materials afford much better desorption than similarly sized polyethylene terephthalate (PETE) or nylon-6,6 (PA66) substrates. Ultimately, the manner in which the electrospray plume interacts with the mesh as it is transmitted through the substrate is shown to be critical to performing and optimizing TM-DESI analyses. In addition, evidence is presented for analyte dependent variations in the desorption mechanisms of dry and solvated samples. . In DESI, ions are produced by directing charged solvent droplets from an electrospray source toward a sample that is either a bulk material in its native state (e.g., pharmaceutical tablet) or one that has been deposited from solution onto a sampling surface. Analytes present at the surface are desorbed and ionized by the incoming plume and subsequently transferred to the mass spectrometer inlet by the influence of the applied potential and the pressure differential between atmospheric pressure and the low-pressure region of the mass analyzer. To date, DESI has found many applications including forensic analysis [2][3][4][5] Recent adaptations of DESI, including geometry independent DESI in gas tight enclosures [35] and transmission mode desorption electrospray ionization (TM-DESI), have been developed to reduce the geometry dependence of DESI experiments [36]. In the transmission DESI mode the sample is not deposited onto a continuous solid surface but rather onto a sampling mesh. In this adaptation, the incident spray angle and collection angle are fixed at 0°and the spray is transmitted through the sample (Figure 1). Along with the simplification of the experimental geometry, the transmission mode also allows convenient analysis of both dry (i.e., following evaporation of the deposition solvent) and wet (i.e., solvated) samples with similar performance characteristics to those achieved using traditional DESI [36].Surface variables including the chemical composition, porosity, texture, and electrical conductivity of the substrate have been reported to affect DESI analyses [2,24,[37][38][39]. Dramatic reductions in response have been noted for high conductivity surfaces due to neutralization of the incoming ion plume at the su...

Fabrication and utilization of mesh materials specifically designed to capture analytes from solution facilitates the direct coupling of affinity capture and ambient ionization mass spectrometry via surface-enhanced transmission mode desorption ionization (TM-DESI). Incorporation of photolabile groups within the linkage between the mesh surface and the covalently modified reactive probe affords facile release of mass tagged analytes directly to mesh surfaces that have been rinsed free of matrix interferences. The approach introduces increased specificity to the already rapid TM-DESI analysis technique, resulting in a powerful tool for high throughput screening of targeted analytes. Specific capture of thiols is discussed herein, but the surface-enhanced TM-DESI technique can be readily extended to other functional groups by alteration of the capture agent.A new era of high-throughput mass spectrometry emerged with the nearly simultaneous introduction of two ambient ionization techniques: desorption electrospray ionization (DESI) 1 and direct analysis in real time (DART).2 Recognition of the enormous potential of these ionization methods has resulted in a growing number of related techniques, including ones that integrate laser desorption, the use of plasmas, and extraction methods.3 The most compelling motivations for ambient ionization mass spectrometry are the ability to analyze surfaces directly, the speed of the analysis and the elimination of difficult or time consuming sample preparation steps. Most ambient ionization methods require only seconds per sample, which is a substantial improvement in throughput compared to the multiple minutes required to separate and analyze components in GC-MS and LC-MS analyses. Moreover most of the cumbersome sample preparation steps such as derivatization reactions and extensive sample cleanup protocols are alleviated. Although one of the major benefits touted for ambient ionization mass spectrometric methods is the direct analysis of complex samples, the elimination of chromatographic separation generally results in reduced specificity and ion suppression for low concentration species. One set of performance metrics (i.e., specificity and/or sensitivity) has been compromised for another (i.e., analytical speed) in desorptionbased ambient ionization mass spectrometry. A newer variation of DESI, termed reactive DESI, has been previously reported to address this issue and exploits the addition of specific reagents to the spray solution that undergo ion/molecule reactions with the analytes of interest, thus offering improved selectivity and sensitivity for the detection of targeted molecules. [4][5][6] In the present report, a surface-enhanced DESI-MS strategy is described which allows selective Corresponding author: jbrodbelt@mail.utexas.edu. NIH Public Access Author ManuscriptAnal Chem. Author manuscript; available in PMC 2011 January 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript capture, release, and analysis of targeted analytes fro...