Sensitivity “Hot Spots” in the Direct Analysis in Real Time Mass Spectrometry of Nerve Agent Simulants

Abstract

Presented here are findings describing the spatial-dependence of sensitivity and ion suppression effects observed with direct analysis in real time (DART). Continuous liquid infusion of dimethyl methyl phosphonate (DMMP) revealed that ion yield "hot spots" did not always correspond with the highest temperature regions within the ionization space. For instance, at lower concentrations (50 and 100 μM), the highest sensitivities were in the middle of the ionization region at 200°C where there was a shorter ion transport distance, and the heat available to thermally desorb neutrals was moderate. Conversely, at higher DMMP concentrations (500 μM), the highest ion yield was directly in front of the DART source at 200°C where it was exposed to the highest temperature for thermal desorption. In matching experiments, differential analyte volatility was observed to play a smaller role in relative ion suppression than differences in proton affinity and the relative sampling positions of analytes. At equimolar concentrations sampled at the same position, suppression was as high as 26× between isoquinoline (proton affinity 952 kJ mol -1 , boiling point 242°C) and p-anisidine (proton affinity 900 kJ mol -1 , boiling point 243°C). This effect was exacerbated when sampling positions of the two analytes differed, reaching levels of relative suppression as high as 4543.0×±1406.0. To mitigate this level of relative ion suppression, sampling positions and molar ratios of the analytes were modified to create conditions in which ion suppression was negligible.

Rapid analysis of trace drugs and metabolites using a thermal desorption DART-MS configuration

“…Direct analysis in real time mass spectrometry (DART-MS) has been shown to be able to detect a large array of drugs and metabolites 3–10 however traditional DART-MS analysis, commonly completed by dissolving the sample and analyzing an aliquot with a glass capillary, suffers from poor reproducibility due to inconsistent sample introduction resulting from the small spatial footprint of the DART sampling jet and variations in sampling geometry as highlighted by Harris et al . 11 …”

Trace detection and competitive ionization of erythritol tetranitrate in mixtures using direct analysis in real time mass spectrometry

“…The combination of mass spectrometry's (MS) superior selectivity and sensitivity as well as recent advancements in ambient ionization techniques such as desorption electrospray ionization (DESI) 8 and direct analysis in real time (DART), 9 have delivered appealing alternatives. 4,[17][18][19] Prior investigations have also demonstrated the use of dopants for enhanced ionization and specic adduct formation with DART. Trace detection of a range of explosive compounds directly from paper, metals, plastics, and human skin has been demonstrated with DESI-MS. [11][12][13] DESI and other liquid dropletbased ambient ionization sources 14 ionize explosive compounds through the traditional mechanisms studied for electrospray ionization (ESI), 15,16 forming adducts with available nitrite (NO 2 À ), nitrate (NO 3 À ), or reactive species added to the spray solvent (Cl À ).…”

Direct analysis in real time—Mass spectrometry (DART‐MS) in forensic and security applications

“…Harris, Falcone, & Fernández () used DMMP to study the spatial and temperature dependence on the ion abundance in DART‐MS. They determined that for organophosphates, and presumably for other compounds, “hot spots” exist within the ionization zone.…”