High performance surface science methods, for instance tunneling microscopy, are able to give unprecedented insight into the atomic details of structure and function of molecules, however they require a perfectly prepared environment, typically ultrahigh vacuum(UHV) of 10-10 mbar. At this pressure, the preparation of molecular adsorbates proceeds through the sublimation of molecules and their condensation on a solid surface to ensure the chemical purity. For a long time this excluded macromolecules since they do not have significant vapour pressure.
Electrospray ion beam deposition solves this problem by bringing large molecules into the gas phase as molecular ions via electrospray ionization, a ambient, soft ionization method. The ES-IBD source forms a molecular beam and transmits it to a surface in UHV. On its way, ion optics guide the beam trough several differential pumping stages, reducing the pressure by 13 orders of magnitude. The ions are further mass-filtered, their energy is adjusted, and the ion current can be measured at any point. This allows an unprecedented level of control and precision in handling macromolecules, opening a large number of analytical and synthetic applications.
modified commercial instrument for electron microscopy
For the deposition of proteins as native gas phase protein ions we modified a commercial mass spectrometer. The Orbitrap Q-exactive II UHMR (ultrahigh mass range) is optimised for transmission and analysis of heavy ions of low charge state (m/z>10000) as well as controlling their energy along the way.
for surface modification and molecular imaging
With high intensity ion beams, the coating of macroscopic surfaces by ES-IBD is possible. We currently develop a high intensity ion source for the fabrication of surface coatings, capable of harnessing the full potential of molecular ion beam deposition, i.e. precise mass selection, chemical activation by collisions, and the use of nonvolatile molecules.
for single molecule analysis
This instrument is the basis of the ES-IBD/SPM instrumentation at the MPI-Stuttgart (former group of the PI) and was the prototype instrument of ES-IBD for surface science. It was intended for the deposition of non-volatile molecules on atomically clean surfaces in UHV to enable their investigation in a scanning tunneling microscope (STM). Due to several improvement and innovations, it became a very versatile instrument, ultimately demonstrating native protein deposition and hyperthermal surface chemistry.
Figure: Scheme of the electrospray ion beam deposition/scanning probe microscopy experiment (ES-IBD). The ion beam is generated by ESI at ambient pressure (left) and transferred to UHV via a differentially pumped vacuum system (pressures given). The SPM sample is prepared under UHV conditions and is transferred in situ to a deposition stage and finally undergoes SPM analysis. Abbreviations: ESI, electrospray ionization; ES-IBD/SPM, electrospray ion beam deposition/scanning probe microscopy; UHV, ultrahigh vacuum.
Source: online low flow or offline nanospray source
Ion Optics: Ion funnel, 6mm diam. rf-only quadrupole ion guide, 7 electrostatic lenses with beam deflectors.
Mass Filter/analysis: quadrupole mass filter, linear time-of-flight mass spec.
Samples: HV sample holder (6-fold) for AFM and TEM, UHV sample stage for in-situ STM transfer and suitcase connection.
High-Voltage rf-amplifier (HV-AMP400)
This is a highly powered rf-generator for our main mass filter quadrupole. This is able to supply 500kHz, 1MHz, and 2MHz (on the Stuttgart instrument), which allows to handle very light ions (e.g. Na+ at m/z=23) or very heavy ions (e.g. GroEL, m=815kDa, m/z=12000).
Radio frequency generator (RFG50-10)
For operating ion funnels and rf-only ion guides.
We use the rbd model 9103 picoampmeter for detection of molecular ion currents impinging on isolated, i.e. floating electrodes (apertures, detector plates, samples) throughout the instrument.
The instrument is controlled via a LabVIEW interface, made based on the library supplied by the vendor. Picoampere resolution is achieved in floating measurements in a setup outlined in the technical note on using the rbd9103.
Right: four rbd9103 picoampmeters connected to the ion optics in the lab.
An application note with in-detail wiring schemes for the rbd 9103 can be found here.
For in-situ transfer of UHV samples we developed a vacuum-suitcase system together with Ferrovac.
We deposit native protein ions on thin carbon membranes or freestanding graphene for subsequent analysis with TEM.
ES-IBD was initially constructed to produce samples of non-volatile molecules on atomically clean substrates for high resolution STM investigation. Hence, Scanning Probe Microscopy (SPM) is currently one of the main analysis methods for the investigation of molecular nanostructures. Scanning Tunneling and Force Microscopy are ultimately capable of atomic resolution, given a perfectly prepared sample. Providing such samples for macromolecules requires ES-IBD preparation, because thermal sublimation is excluded for large nonvolatile molecules.
Figure: Scheme of Scanning Tunneling Microscopy and Spectroscopy: (a) A piezo motor positions and moves an atomically sharp tip over a surface with subatomic precision. The tunneling current is used to generate a feedback loop to measure the tip-surface distance and create a topography map by (b) scanning of line profiles . (c) Energy diagram of the tip-surface interaction in tunneling microscopy. Ramping the tip-surface voltage will generate a spectrum that is related to the electronic density of states.
(in collaboration with Prof. K. Kern, MPI Stuttgart)
(in collaboration with Prof. M Duerr, Univ. of Giessen)