Qingyu Song

Ion trap mass analysis at high pressure: a theoretical view.

The mass-selective manipulation of ions at elevated pressure, including mass analysis, ion isolation, or excitation, is of great interest for the development of mass spectrometry instrumentation, particularly for systems in which ion traps are employed as mass analyzers or storage devices. While experimental exploration of high-pressure mass analysis is limited by various difficulties, such as ion detection or electrical discharge at high-pressure, theoretical methods have been developed in this work to study ion/neutral collision effects within quadrupole ion traps and to explore their performance at pressures up to 1 Torr. Ion trapping, isolation, excitation, and resonance ejection were investigated over a wide pressure range. The theoretically calculated data were compared with available experimental data for pressures up to 50 mTorr, allowing the prediction of ion trap performance at pressures more than 10 times higher.

Ion trap mass analysis at high pressure: an experimental characterization

In recent years, it has become increasingly interesting to understand the performance of mass spectrometers at pressures much higher than those employed with conventional operating conditions. This interest has been driven by several influences, including demand for the development of reduced-power miniature mass spectrometers, desire for improved ion transfer into and through mass spectrometers, enhanced-yield preparative mass separations, and mass filtering at the atmospheric pressure interface. In this study, an instrument was configured to allow for the performance characterization of a rectilinear ion trap (RIT) at pressures up to 50 mtorr with air used as the buffer gas. The mass analysis efficiency, mass resolution, isolation efficiency, and collision-induced dissociation (CID) efficiency were evaluated at pressures ranging from 1 to 50 mtorr. The extent of degradation of mass resolution, isolation efficiency and ion stability as functions of pressure were characterized. Also, the optimal resonance ejection conditions were obtained at various pressures. Operations at 50 mtorr demonstrated improved CID efficiency in addition to peak widths of 2 and 5 m/z units (full width at half-maximum, FWHM) for protonated caffeine (m/z 195) and Ultramark (m/z 1521) respectively.

Novel linear ion trap mass analyzer composed of four planar electrodes.

A novel linear ion trap mass analyzer was developed using just four elongated planar electrodes, mounted in parallel, and employing an RF potential for ion trapping in the radial and axial directions. Mass analysis was achieved using the mass-selective instability scan with ion ejection in the radial direction. The performance of this new device was characterized in comparison with the 6-electrode rectilinear ion trap (RIT) from which it is derived. The 4-electrode trap gives optimum performance in an asymmetric geometry, just like the original optimized 6-electrode RIT. The strong RF fringing fields at the ends of the RF rods account for axial ion trapping without use of extra electrodes or an axial DC voltage. Field calculations and simulations have been carried out to study the trapping potential inside RITs with various configurations. Demonstrated capabilities include analysis of externally injected ions with mass resolution in excess of 1000 and a mass/charge range of 650 Th as well as tandem mass spectrometry capabilities. The geometric simplicity and performance characteristics of the 4-electrode RIT make it particularly attractive in the development of next generation miniaturized mass spectrometers.

Mass selection of ions from beams using waveform isolation in radiofrequency quadrupoles.

A waveform isolation method is described for the mass-selective transmission of ions through quadrupole mass filters, and it is implemented on a new tandem mass analyzer instrument. The method features the application of broad-band waveforms comprising appropriate frequencies to cause mass-selective instability in ions of particular mass-to-charge (m/z) and to transmit all others. The experiment is implemented in a tandem quadrupole system in which the first mass filter is a rectilinear ion trap (RIT) operated in a continuous mass-selective mode to transmit ions of ions of one or more arbitrarily selected m/z value(s). The second analyzer was used to verify the quality of the mass selection achieved using the first analyzer via conventional quadrupole ion trap mass-selective instability scanning. A new subtype of product ion tandem mass spectrometry (MS/MS) scan, termed the summed product ion scan, is demonstrated with a mixture of biological compounds. It is used to characterize product ions arising after simultaneous isolation and collisional activation of multiple precursor species, in this case ions of the same analyte generated in different charge states. The summed product ion scan can be useful for enhancing sensitivity for the analyte of interest or for providing more comprehensive information about an analyte than is available by monitoring a single ionized form of the analyte. The analytical performance of the waveform isolation method is tested using simple drug mixtures, and its potential for increasing overall yields in preparative mass spectrometry is explored briefly. It is shown that efficiencies of ca. 70% of ion transfer to a surface for ion soft landing surface can be achieved. The upper mass range is limited by axial acceleration arising from the stretched geometry, and one solution to this problem is provided.