Ion profiling in an ambient drift tube-ion mobility spectrometer using a high pixel density linear array detector IonCCD.

A linear pixel-based detector array, the IonCCD, is characterized for use under ambient conditions with thermal (<1 eV) positive ions derived from purified air and a 10 mCi (63)Ni foil. The IonCCD combined with a drift tube-ion mobility spectrometer permitted the direct detection of gas phase ions at atmospheric pressure and confirmed a limit of detection of 3000 ions/pixel/frame established previously in both the keV (1-2 keV) and the hyper-thermal (10-40 eV) regimes. Results demonstrate the "broad-band" application of the IonCCD over 10(5) orders in ion energy and over 10(10) in operating pressure. The Faraday detector of a drift tube for an ion mobility spectrometer was replaced with the IonCCD providing images of ion profiles over the cross-section of the drift tube. Patterns in the ion profiles were developed in the drift tube cross-section by control of electric fields between wires of Bradbury Nielson and Tyndall Powell shutter designs at distances of 1-8 cm from the detector. Results showed that ion beams formed in wire sets, retained their shape with limited mixing by diffusion and Coulombic repulsion. Beam broadening determined as 95 µm/cm for hydrated protons in air with moisture of ~10 ppmv. These findings suggest a value of the IonCCD in further studies of ion motion and diffusion of thermalized ions, enhancing computational results from simulation programs, and in the design or operation of ion mobility spectrometers.

IonCCD detector for miniature sector-field mass spectrometer: investigation of peak shape and detector surface artifacts induced by keV ion detection.

A recently described ion charge coupled device detector IonCCD (Sinha and Wadsworth, Rev. Sci. Instrum. 76(2), 2005; Hadjar, J. Am. Soc. Mass Spectrom. 22(4), 612-624, 2011) is implemented in a miniature mass spectrometer of sector-field instrument type and Mattauch-Herzog (MH)-geometry (Rev. Sci. Instrum. 62(11), 2618-2620, 1991; Burgoyne, Hieftje and Hites J. Am. Soc. Mass Spectrom. 8(4), 307-318, 1997; Nishiguchi, Eur. J. Mass Spectrom. 14(1), 7-15, 2008) for simultaneous ion detection. In this article, we present first experimental evidence for the signature of energy loss the detected ion experiences in the detector material. The two energy loss processes involved at keV ion kinetic energies are electronic and nuclear stopping. Nuclear stopping is related to surface modification and thus damage of the IonCCD detector material. By application of the surface characterization techniques atomic force microscopy (AFM) and X-ray photoelectrons spectroscopy (XPS), we could show that the detector performance remains unaffected by ion impact for the parameter range observed in this study. Secondary electron emission from the (detector) surface is a feature typically related to electronic stopping. We show experimentally that the properties of the MH-mass spectrometer used in the experiments, in combination with the IonCCD, are ideally suited for observation of these stopping related secondary electrons, which manifest in reproducible artifacts in the mass spectra. The magnitude of the artifacts is found to increase linearly as a function of detected ion velocity. The experimental findings are in agreement with detailed modeling of the ion trajectories in the mass spectrometer. By comparison of experiment and simulation, we show that a detector bias retarding the ions or an increase of the B-field of the IonCCD can efficiently suppress the artifact, which is necessary for quantitative mass spectrometry.

Drift tube soft-landing for the production and characterization of materials: applied to Cu clusters.

We have recently developed a soft-landing (SL) instrument that is capable of depositing ions onto substrates for preparative and developmental research of new materials using a laser ablation source. This instrument was designed with a custom drift tube and a split-ring ion optic for the isolation of selected ions. The drift tube allows for the separation and thermalization of ions formed after laser ablation through collisions with an inert bath gas. These collisions allow the ions to be landed at energies below 1 eV onto substrates. The split-ring ion optic is capable of directing ions toward the detector or a landing substrate for selected components. Experiments will be shown ablating Cu using an Nd:YAG (1064 and 532 nm) for cluster formation and landing onto a muscovite (mica) surface. The laser ablation of Cu in 8 Torr of He gas gives a spectrum that contains multiple peaks corresponding to Cu(n), Cu(n)O(m) clusters, and their corresponding isomers. Atomic force microscopy and drift tube measurements were performed to characterize the performance characteristics of the instrument.