Experiments

Figure 1. Protein mixture separation and ion soft-landing. (A) Photograph of four proteins soft-landed onto a gold substrate. (B) ESI-MS mass spectrum of protein mixture containing insulin (5.7 kDa), lysozyme (14.3 kDa), cytochrome c (12.4 kDa), and apomyoglobin (17.0 kDa). (C-F) ESI-MS mass spectra from rinsed soft-landed proteins (C, insulin; D, lysozyme; E, apomyoglobin; F, cytochrome c).4

 

 

Figure 2. A soft-landing instrument used for making protein arrays.4

 

Figure 3. Demonstration of spatial control of reactively soft-landed ions on a surface using a wire grid as a mask.6

 

Ion soft landing allows mass spectrometry to be used as a preparative method.

Originally implemented here at Aston Labs in 1975, ion soft-landing is a preparative mass spectrometry technique whereby hyperthermal mass-selected ions are deposited onto a surface.1 This process harnesses the resolving power of mass spectrometry in order to perform mixture separations prior to the collection of the purified ion species by low-energy deposition onto a solid or liquid surface. Following the soft-landing event, the surfaces typically undergo analysis by SIMS, chemical sputtering, or/and by spectroscopic techniques. The soft-landing experiment has been classed into three categories:2,3

  • Soft-landing: Refers in general to the deposition of hyperthermal polyatomic ions at surfaces and specifically to those depositions whereby the ion species does not react with the surface.
  • Reactive soft-landing: A sub-category of soft-landing characterized by the occurrence of an ion/molecule reaction upon deposition.
  • Dissociative soft-landing: A sub-category of soft-landing which describes the impingement upon a surface of ions such that the incident ions dissociate and a fragment species is deposited onto the surface.

Soft-landing has great potential for a diverse array of preparative mass spectrometry applications. The technique has been demonstrated to be gentle enough to ensure the intact deposition of enzymes such that they retain their bioactivity, exemplifying the method's potential for handling very fragile systems.4 One example of reactive soft-landing consists of the passivation of a hydroxyl-terminated self-assembled monolayer surface (OH-SAM) by substitution reaction between trimethylsilyl cations, Si(CH3)3+, and the hydroxyl endgroups.3 Such a technique could be valuable, for example, as a method for the fabrication of rugged surface coatings for industrial use or for the production of films used in biomedical devices.3 Soft-landed ions have also been deposited onto surfaces such that they retain their charge (both in and out of vacuum) for long periods of time, a potentially interesting preparative route for molecular synthesis or fundamental ion/surface studies.5 Many other potential applications exist for soft-landing, including the following: molecular electronics, semiconductor patterning, catalyst films, protein chips, electrochemistry, fundamental studies of ion/surface interactions, and step-wise thin film or molecular synthesis.

Current Soft Landing Research

Heterogeneous Catalyst Preparation

Industrial applications of catalyst films often depend on surface features such as defects and step edges which are created in a poorly-controlled manner. Soft-landing can be used to allow only those ions representing catalytically active structures to be purified and deposited as a film onto a surface. Instrumentation for this project is currently being constructed to include both online gas-phase catalyst bed testing by an adjacent EI-MS as well as in-situ SIMS-MS capabilities. (more information)

Improving Soft-Landing Rates

Soft-landing has potential as a viable alternative method for research and commercial mixture separations. The technique offers advantages over conventional methods when considering its separation speed and resolution; however, at the current time the technique suffers from low processing rates and inefficient sample use. One goal of Aston Labs is to improve these problem areas. This can be done by implementing new instrumentation which allows less transfer loss between ion optical components and across the atmospheric interface. Additionally, operation of the soft-landing instrumentation at higher pressures may aid by reducing losses caused by pressure differentials between vacuum regions. Also under consideration is the use of high efficiency ionization sources, high-duty-cycle mass separations, and multiplexed analysis.

Surface Patterning

There are many applications for which ion soft-landing and reactive ion soft-landing may be amenable to which require tailored surfaces patterned with ultra-pure materials. Two areas currently of interest include semiconductor synthesis and molecular electronics. A new instrument has recently been designed and built expressly for these processes, and experiments will soon be under way.

Personnel
  • Prof. R. Graham Cooks (professor)
  • Heriberto Hernandez (post-doctoral student)
  • Zongxiu Nie (post-doctoral student)
  • Michael Volny (post-doctoral student)
  • Aliah Dugas (graduate student)
  • Michael Goodwin (graduate student)
  • Scott Smith (graduate student)
  • Qingyu Song (graduate student)
References
  1. Franchetti, V.; Solka, B. H.; Baitinger, W. E.; Amy, J. W.; Cooks, R. G., Int J Mass Spectrom 1977, 23, 29-35.
  2. Luo, H.; Miller, S. A.; Cooks, R. G.; Pachuta, S. J., International Journal of Mass Spectrometry 1998, 174, 193-217.
  3. Denault, J. W.; Evans, C.; Koch, K. J.; Cooks, R. G., Anal Chem 2000, 72, 5798-5803.
  4. Ouyang, Z.; Takats, Z.; Blake, T. A.; Gologan, B.; Guymon, A. J.; Wiseman, J. M.; Oliver, J. C.; Davisson, V. J.; Cooks, R. G., Science 2003, 301, 1351-1354.
  5. Miller, S. A.; Luo, H.; Pachuta, S. J.; Cooks, R. G., Science 1997, 275, 1447-1450.
  6. Evans, C.; Wade, N.; Pepi, F.; Strossman, G.; Schuerlein, T.; Cooks, R. G., Anal Chem 2002, 74, 317-323.