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.
© American Society for Mass Spectrometry, 2011