Search

Although Fourier transform ion cyclotron resonance mass spectrometry is a powerful tool in the qualitative observation of gas phase reactions, ion detection is on the millisecond time scale, orders of magnitude longer than typically found when using a sector instrument. Observations of short-lived species such as chemically activated adduct ions can be accomplished using selective ion excitation as a probe of intermediate lifetime. Whereas ion elimination has been shown to be effective in monitoring ion lifetimes on the microsecond time scale, problems associated with detecting ions produced with high kinetic energies limits the technique. Use of a kinetic energy orifice as an ion skimmer effectively eliminates ions near the center of the ion cell at relatively low kinetic energies. By modifying a single section cell to include a kinetic energy orifice, the lifetimes of chemically activated adduct ions have been investigated.

Rapid bipolar pulsing of an electrostatic particle guide (EPG) provides a method of both ion deflection and enhanced ion transmission for TOF mass spectrometry. This mode of operation effectively eliminates the intense low-mass component of the spectrum which normally saturates microchannel plate (MCP) detectors, while at the same time transporting the higher mass component with high efficiency. The ability to selectively reduce the effects of unwanted background peaks is achieved using a tailored pulse sequence for specific elimination of ions of a given m/z ratio. The mass selectivity of this technique and improvements in signal-to-noise ratio are examined

This study addresses a problem inherent in the use of microchannel plates as detectors for matrix-assisted laser desorption/ionization. We have developed a unique new method of ion transport using fast bipolar pulsing of an electrostatic particle guide to provide ion deflection and enhanced ion transmission. This mode of operation effectively eliminates the intense low-mass component of the spectrum which normally saturates microchannel -plate detectors, while at the same time transporting the higher-mass component with high efficiency. Enhancement of the resulting signal-to-noise ratio has been achieved.

(24) Wan , M ; Ledford, E. B., Jr.; Marshall, A. G. FACSS XIV Annual Me&, betrolt, M I October 1987; Abstract No. 43. (25) Comisarow, M. B. J. Chem. Fhys. 1978, 69, 4097-4104. (26) Comisarow, M. B. In Ion Cyclotron Resonance Specfrometfy 11; Springer-Verlag: Berlin, 1982; pp 484-513. (27) Grosshans, P. B.; Marshall, A. G. Cdlected Abstracts; American Society f w Mass Spectrometry 37th Annual Conference on Mass Spectrometry & Allied Topics, 1989: American Society for Mass Spectrometry: East Lansing, MI, 1989; pp 1232-1233. (28) Rempei, D. L.: Huang, S. K.; Gross, M. L. Int. J . Mass Spectrom. Ion Processes 1988, 70, 183-184. (29) Huang, S. K.; Rempel, D. L.; Gross, M. L. Int. J . Mass Specfrom. Ion Processes 1988, 72, 15-31. (30) Allemann, M.; Kofel, P.; Kellerhals, Hp.; Wanczek, K.-P. Int. J . Mass Spectrom. Ion Processes 1987, 75, 47-54. (31) Van der Hart, W. J.; Van De Guchte, W. J. Int. J . Mass Spectrom. Ion Processes 1988, 62, 17-31. (32) Dunbar, R. C. Int. J . Mass Spectrom. Ion Processes 1984, 56, 1-9. (33) Chen, L.: Cottrell, C. E.; Marshall, A. G. Chemom. Intell. Lab. Syst.

A continuous zero angle reflecting time-of-flight mass spectrometer capable of tandem mass spectrometry measurements with high resolution and high sensitivity has been developed. The instrument design features two pulsed-ion mirrors in a coaxial geometry. Ions can be reflected back and forth with the mirrors, which increases the net flight length and permits kinetic energy focusing for enhanced resolution. The instrument also contains an electrostatic particle guide which increases ion transmission efficiency and can be used in a bipolar pulsed mode to isolate ions of interest for structural study.

Ion detection by Fourier transform ion cyclotron resonance (FT-ICR) is accomplished by observing a coherent ion packet produced from an initially random ensemble of ions. The coherent packet is formed by excitation with a resonant oscillating electric field. Ions that are out of phase with the applied radio frequency (rf) electric field experience a continuous misalignment of the electric field vector. The misalignment creates a net force of the electric field perpendicular to ion motion. The perpendicular component of the rf electric field creates a frequency shift resulting in phase synchronization of the ion ensemble. The phase coherence of the ion packet affects both the sensitivity and the resolution of FT-ICR.

Fast atom bombardment mass spectrometry (FAB-MS) has been used to measure positional isotope exchange rates in enzyme-catalyzed reactions. The technique has been applied to the reactions catalyzed by acetyl-CoA synthetase and argininosuccinate synthetase. The FAB technique is also able to quantitatively determine the oxygen-18 or oxygen-17 content of nucleotides on as little as 10 nmol of material with no prior derivatization. Acetyl-CoA synthetase has been shown by FAB-MS to catalyze the positional exchange of an oxygen-18 of ATP from the beta-nonbridge position to the alpha beta-bridge position in the presence of acetate. These results are consistent with acetyl adenylate as a reactive intermediate in this reaction. Argininosuccinate synthetase was shown not to catalyze a positional isotope exchange reaction designed to test for the formation of citrulline adenylate as a reactive intermediate. Argininosuccinate synthetase was also found not to catalyze the transfer of oxygen-18 from [ureido-18O]citrulline to the alpha-phosphorus of ATP in the absence of added aspartate. This experiment was designed to test for the transient formation of carbodiimide as a reactive intermediate. These results suggest that either argininosuccinate synthetase does not catalyze the formation of citrulline adenylate or the enzyme is able to completely suppress the rotation of the phosphoryl groups of PPi.

The Fourier transform ion cyclotron resonance (FT-ICR) signal is produced by the coherent motion of a population of ions. The ability to produce a well-defined ion packet by excitation of an initially random ion ensemble is a major limiting factor of high mass FT-ICR. Ions must be both resonant and in phase with the applied radio frequency excitation field to be accelerated to radii suitable for detection by FT-ICR. Synchronization of the phase angles of an ensemble of ions occurs by off-resonant acceleration during frequency swept excitation. Results from computer-simulated ion trajectories suggest that phase synchronization of the ion packet prior to resonant excitation results in better spatial definition of the ion ensemble.

The intrinsic versatility of Fourier transform ion cyclotron resonance (FT-ICR) for analytical studies has been widely demonstrated. The utility of ion traps arises from the ability to manipulate ions that are stored in the trap. Ion manipulation is accomplished in FT-ICR by ion ejection e.g., ion selection for chemical studies and ejection of high abundance ions for enhancement of dynamic range. Ion ejection is achieved by accelerating unwanted ions (by resonant radio frequency (rf) excitation) until their radii exceed the confines of the ICR cell. Although the excitation magnitude is substantially by accelerating unwanted ions to radii which need only exceed the dimensions of the conductance limit, tailing of an excitation sweep can still result in inadvertent excitation of the ion of interest. A technique is described for ion selection that combines phase-specific excitation and ion discrimination on partitioning in a two-section FT-ICR cell. This method greatly enhances ion isolation by applying the relationship of the excitation phase angle to ion acceleration. The radial velocity of ions is modulated by phase-specific excitation. By control of the radial velocity (i.e., cyclotron radius), selected ions can be manipulated to produce high ion selectivity upon partitioning.