Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to characterize the desorption performance of two emerging ambient ionization mass spectrometry (MS) sources; desorption electrospray ionization (DESI) and low temperature plasma (LTP). More specifically, ToF-SIMS was used to study the effect of ambient probe incidence angle (30°, 45°, and 60° relative to the sample surface) by imaging the analyte desorption patterns on the collection substrate. For DESI, analytes were transported by secondary solvent droplets with a glancing take-off angle of less than 12°, with the desorption pattern strongly influenced by incidence angle. As the incidence angle was changed from 30° to 60°, the lateral dispersion angle of the secondary droplets changed from 79° to 62°, while the vertical take-off angle changed from 12° to 4°. More oblique incidence angles also corresponded with higher relative secondary ion yields as measured by ambient MS and ToF-SIMS. For LTP, a similar dependence on incidence angle was observed. More oblique angles corresponded with higher relative secondary ion yields, although the analyte did not display a distinct desorption pattern owing to insufficient transfer of analyte. However, other molecules such as nitrogen-containing molecules were seen on the collector, displaying impactor-like deposition patterns. For these molecules, similar take-off angles of less than 13° were observed. This proof-of-concept study tested a very small set of conditions and parameters, but showed the capabilities of ToF-SIMS as a unique tool for direct imaging of the analyte desorption process in ambient mass spectrometry (MS). Future studies will provide valuable desorption data to improve ambient MS interface designs for optimal collection of analyte molecules.
Citation: Analytical Chemistry
Pub Type: Journals
ambient ionization, low temperature plasma, desorption electrospray ionization, tof-sims, schlieren