PNNL

Abstract

The capability, to selectively deposit ions of interest, makes ion soft-landing (ISL) a key enabling technique for creating molecular layers with high purity beneficial for many scientific research and engineering applications. While the conventional ISL method relies on ion selection and deposition based on mass-to-charge (m/z) ratio, here we demonstrate the use of Structures for Lossless Ion Manipulation (SLIM) for mobility-based ion selection and soft-landing. The ion selection and re-routing capabilities of SLIM were leveraged to enable a selected range of mobilities to be re-directed to a different SLIM path (rather than a MS) where the path terminated at a deposition surface. A mobility-selected single ionic species from a phosphazene ion mixture was deposited (by mounting suitable substrates) with a pulse of ion beam current (~150 pA) resembling its mobility peak. In addition, a mixture of tetra-alkyl ammonium (TAA) ions containing chain lengths of C5-C8 was used, and the two chain lengths of C6 and C7 were selected for deposition, and the soft-landed peaks re-introduced into solution phase and subsequently analyzed with a SLIM-qToF to obtain an IMS/MS spectrum to confirm the identity of the two selected TAA species (chain lengths C6 and C7) in conjunction with their known mobilities. The capability of selectively rerouted and collected isomers from a mixture was used to characterize negatively charged ions of phospho-tungstic acid (WPOM) having ‘Keggin’ conformations. The TEM image of a mobility-selected (isomer) species displayed a distinct structure associated with WPOM. Additionally, the identity of deposited WPOM was ascertained using x-ray diffraction (XRD) and energy-dispersive (EDS) spectroscopies. Further, theory and computations reveal the kinetic energy distributions of ions deposited under low E/N conditions (i.e., pressures in the ~ 1 torr range at the electric fields involved) can be extremely low (by modulating electric fields) to achieve ion soft-landing. In combination with orthogonal characterization methods (TEM, spectroscopy), the initial demonstration of SLIM-enabled ISL opens the door to production of molecular layers with minimal structural variability useful for characterization of structures at the single molecule level.