Anal. Chem. 2008, 80, 6640–6649
Ion Soft Landing Using a Rectilinear Ion Trap Mass Spectrometer Wen-Ping Peng, Michael P. Goodwin, Zongxiu Nie, Michael Volny´, Zheng Ouyang, and R. Graham Cooks* Department of Chemistry, 560 Oval Drive, Purdue University, West Lafayette, Indiana 47907 A new ion soft landing instrument has been built for the controlled deposition of mass selected polyatomic ions. The instrument has been operated with an electrospray ionization source; its major components are an electrodynamic ion funnel to reduce ion loss, a 90-degree bent square quadrupole that prevents deposition of fast neutral molecules onto the landing surface, and a novel rectilinear ion trap (RIT) mass analyzer. The ion trap is elongated (inner dimensions: 8 mm × 10 mm × 10 cm). Three methods of mass analysis have been implemented. (i) A conventional mass-selective instability scan with radial resonance ejection can provide a complete mass spectrum. (ii) The RIT can also be operated as a continuous rf/dc mass filter for isolation and subsequent soft landing of ions of the desired m/z value. (iii) The 90-degree bent square quadrupole can also be used as a continuous rf/ dc mass filter. The mass resolution (50% definition) of the RIT in the trapping mode (radial ion ejection) is ∼550. Ions from various test mixtures have been mass selected and collected on fluorinated self-assembled monolayers on gold substrates, as verified by analysis of the surface rinses. Desorption electrospray ionization (DESI) has been used to confirm intact deposition of [Val5]-Angiotensin I on a surface. Nonmass selective currents up to 1.1 nA and mass-selected currents of up to 500 pA have been collected at the landing surface using continuous rf/ dc filtering with the RIT. A quantitative analysis of rinsed surfaces showed that the overall solution-to-solution soft landing yields are between 0.2 and 0.4%. Similar experiments were performed with rf/dc isolation of both arginine and lysine from a mixture using the bent square quadrupole in the rf/dc mode. The unconventional continuous mass selection methods maximize soft landing yields, while still allowing the simple acquisition of full mass spectra. Ion soft landing (SL) uses mass spectrometric techniques to select gas-phase ions of a desired mass-to-charge ratio and then deposit those ions onto a surface via low energy collisions. Relevance of low energy ion surface collisions extends to a wide range of areas, including surface science, spectroscopy, and mass * To whom correspondence should be addressed. R. G. Cooks, Department of Chemistry, Purdue University, West Lafayette, IN 47907. E-mail: cooks@ purdue.edu.
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spectrometry.1–3 The basic concept was first introduced by this laboratory in 19774 and reintroduced two decades later.5 Softlanding on surfaces is a dominant process during interaction of low-energy (
