Desorption Electrospray Ionization of Explosives on Surfaces:  Sensitivity and Selectivity Enhancement by Reactive Desorption Electrospray Ionization

Abstract

Desorption electrospray ionization (DESI), an ambient mass spectrometry technique, is used for trace detection of the explosives trinitrohexahydro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6-trinitrotoluene (TNT), Pentaerythritol tetranitrate (PETN), and their plastic compositions (Composition C-4, Semtex-H, Detasheet) directly from a wide variety of surfaces (metal, plastic, paper, polymer) without sample preparation or pretreatment. Analysis of the explosives is performed under ambient conditions from virtually any surface in very short times (<5 s) including confirmatory tandem mass spectrometry (MS/MS) experiments, while retaining the sensitivity and specificity that mass spectrometry offers. Increased selectivity is obtained both by MS/MS and by performing additional experiments in which additives are included in the spray solvent. These reactive DESI experiments (reactions accompanying desorption) produce such ions as the chloride and trifluoroacetate adducts of RDX and HMX or the Meisenheimer complex of TNT. Desorption atmospheric pressure chemical ionization, a variant of DESI that uses gas-phase ions generated by atmospheric pressure corona discharges of toluene or other organic compounds, provides evidence for a heterogeneous-phase (gaseous ion/absorbed analyte) charge-transfer mechanism of DESI ionization in the case of explosives. Plastic explosives on surfaces were analyzed directly as fingerprints, without sample preparation, to test DESI as a possible method for in situ detection of explosives-contaminated surfaces. DESI also allowed detection of explosives in complex matrixes, including lubricants, household cleaners, vinegar, and diesel fuel. Absolute limits of detection for the neat explosives were subnanogram in all cases and subpicogram in the case of TNT. The DESI response was linear over 3 orders of magnitude for TNT. Quantification of RDX on paper gave a precision (RSD) of 2.3%. Pure water could be used as the spray solution for DESI, and it showed ionization efficiencies for RDX in the negative ion mode similar to that given by methanol/water. DESI represents a simple and rapid way to detect explosives in situ with high sensitivity and specificity and is especially useful when they are present in complex mixtures or in trace amounts on ordinary environmental surfaces.

Native Desorption Electrospray Ionization Liberates Soluble and Membrane Protein Complexes from Surfaces

“…Comparing our approach with ambient ionization methods described previously we note the addition of chemicals in the spray solution in reactive DESI applications for small molecule analyses 17, 28. Protein ligand binding experiments have also been achieved with reactions taking place by mixing in solution in a liquid sample DESI approach6 rather than by interaction following protein deposition on the planar target as shown here.…”

“…Accordingly with N ‐acetyl‐glucosamine (NAG) substrate added to the desorption spray and directed at lysozyme deposited on the stage we observed additional peaks assigned to binding of intact NAG‐5 to lysozyme (Figure 1 c). The rapid turnover of this substrate precludes its observation in solution‐based ES16 but since substrate binding takes place during rapid desorption, analogous to reactive DESI experiments reported for small molecules,17 the transient bound state can be captured using this native DESI approach. …”

Salt tolerance of desorption electrospray ionization (DESI)

“…The successes of DESI in a large variety of applications [4,6,[15][16][17][18][19] are partly due to the availability of various surfaces from which the sample can be analyzed and, even more important, due to the ability to optimize the spray solvent for the analyte(s) of interest. Addition of a selective reagent to the solvent spray can be used to enhance the analyte signal by an appropriate chemical reaction; an experiment that is termed reactive DESI [20 -22].…”

“…In highly concentrated samples (Ͼ10 Ϫ5 M), there is competition among analyte ions during droplet formation for outer surface space and this is believed to contribute to ion suppression [11]. Ion suppression is also thought to occur due to the masking of analytes by matrix ions of greater polarity or even by high mass compounds masking lower mass analytes [10,12].The successes of DESI in a large variety of applications [4,6,[15][16][17][18][19] are partly due to the availability of various surfaces from which the sample can be analyzed and, even more important, due to the ability to optimize the spray solvent for the analyte(s) of interest. Addition of a selective reagent to the solvent spray can be used to enhance the analyte signal by an appropriate chemical reaction; an experiment that is termed reactive DESI [20 -22].…”

Internal energy distributions in desorption electrospray ionization (DESI)

“…These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters. [1, 24, 29 -32], and explosives detection [4,8,[33][34][35][36][37]. Investigations into the fundamentals of the droplet dynamics and ionization mechanism have also been conducted through systematic measurements of the droplet size and velocity [38], computer simulations [39], surface charging effects [40], or more simply, by evaluating the mass spectra of various types of analytes in conjunction with variations in the experimental parameters [41].…”