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| Non-Destructive Ion Trap Mass Analysis at High Pressure |
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 A method for performing non-destructive ion trap mass analysis at high pressure (> 1 mTorr) has been developed using image current measurement with constant dipolar excitation. Instead of monitoring the ion secular motion, a harmonic ion motion was used for the narrow band image current measurement followed by Fourier Transform. The capability of this technique has been demonstrated with mass analysis using a single measurement at pressures of 10 mTorr or higher. Methods for mixture analysis and tandem mass spectrometry have also been developed with non-destructive mass analysis.
Figure: (a) The linear ion trap with line electrodes in the ejection slits, fabricated by SLA. (b) Schematics of the instrumental setup.
- W. Xu, J. B. Maas, F. J. Boudreau, W. J. Chappell and Z. Ouyang, "Nondestructive Ion Trap Mass Analysis at High Pressure", Analytical Chemistry, 2011, 83, 685-689, doi:10.1021/ac1027808.
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| Modeling of Ion Transient Motions |
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 A theoretical modeling method was explored for characterizing the ion transient response to a dipolar AC excitation in quadrupole ion trap. The ion motion equation was established by applying the pseudo-potential approximation to Mathieu Equation with an ion-molecule collision term included. A step function was introduced to model the transient state of applying a dipolar excitation The Laplace transformation was performed to solve the ion motion equation. The high-frequency ion motion components, typically ignored by the pseudo-potential approximation, have also been included in the modeling. Characterization of ion motions at the excitation frequency, secular frequency and their corresponding high-order harmonic frequencies was achieved using this method. The capability of this theoretical modeling method was validated by applying it for interesting phenomena previously observed, including ion beat motions, frequency broadening at higher pressures, and ion bunching. Numerical simulations were also performed to confirm the results obtained with the theoretical modeling.
Figure: (a) Ion motion frequency components under off- resonance excitation. Calculated (b) and simulated (c) ion motion under off-resonance excitation.
- W. Xu, W. J. Chappell and Z. Ouyang, "Modeling of ion transient response to dipolar AC excitation in a quadrupole ion trap", International Journal of Mass Spectrometry, doi: 10.1016/j.ijms.2011.07.022
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| Gas Dynamics and Ion Transfer |
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 Methods and devices that use gas flows to collect ions and transfer them over long distances for mass spectrometric analysis have been developed. Gas flows derived from the ionization source itself or provided by means of additional pumping, were used to generate a laminar flow inside cylindrical tubing. Hydrodynamic simulations and experimental tests demonstrate that laminar flow protects ions during long distance transfer. The typical angular discrimination effects encountered when sampling ions from ambient ionization sources are minimized and the sampling of relatively large surface areas is demonstrated with desorption electrospray ionization (DESI). Ion transfer over 6 meters has been achieved and its application to multiplexed chemical analysis is demonstrated with simulations and experiments on samples at locations remote from the mass spectrometer.
Figure: a) Schematic of a device for transferring ions from a DESI source. b) CFD simulations of gas flow through a capillary of 550 µm ID (top) and a tube of 4.3mm ID. c) Simulated flow velocity contour maps at 2.3 cm and 5 cm from the sample surface (cross section view of transfer tube). d) Simulated relative ion concentration after transfer through a 50 cm tube as function of gas velocity and tube diameter. e) Signal intensity as function of the transfer distance, experimentally measured for 1 µg cocaine on surface using a device shown in a) with stainless steel transfer tubes of 4.3mm ID. f) MS and MS/MS spectra recorded for detection of 5ug cocaine on a glass slide using DESI with ion transfer over 2 m with a flexible 4.3 mm ID Tygon tubing. The gas flow rate for DESI is 1.1 L/min.
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