Edward R. Sisco, Matthew E. Staymates, Thomas P. Forbes
The use of direct analysis in real time mass spectrometry (DART-MS) is seeing increased use in a number of fields, including forensic science, environmental monitoring, food safety, and healthcare. With the increased use, novel configurations have been created to either aid in analysis of difficult to detect compounds, provide a more repeatable analysis, or even chemically image surfaces. This work focusses on increasing the fundamental understanding of one type of configuration, where DART ionization gas is confined, such as with thermal desorption (TD) DART-MS. Using five representative compounds and a suite of visualization tools, the role of the DART ionization gas, Vapur flow rate, gas back pressure, and exit grid voltage were examined to better understand the chemical and physical processes occurring inside the confined configuration. The use of nitrogen as a DART ionization gas was found to be more beneficial than helium because of enhanced mixing with analyte vapors, providing a more reproducible response. Lower Vapur flow rates were also advantageous as they increased the residence time in the junction, increasing the probability of ionization. Operation at lower Vapur flow rates was achieved by modifying the junction to restrict the DART gas flow. The DART exit grid voltage and gas back pressure had little impact on analyte response. These results provide a foundation for a better understanding of the considerations that must be made when using a confined DART-MS configuration.