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inherent in all swept frequency excitation techniques is the fact that the excitation of different resonances does not occur simultaneously, but only at the time the resonant frequency is present in the excitation signal. 40 Additionally, the instrumentation required to produce chirp excitation for ICR mass spectroscopy is very sophisticated and expensive.
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High power RF plasma heating and wave propagation near the fundamental and harmonics of the ion cyclotron frequency
Another method of simultaneously exciting multiple resonances is the rf burst excitation technique. This 45 technique is commonly used in NMR. However, rf burst excitation has been found to be inadequate for broad range mass spectroscopy. It was theorized in an article entitled "Theory of Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy: Response to 50 Frequency-sweep Excitation" by Alan G. Marshall and D. Christopher Roe, published in /. Chem. Phys. Vol. 73, No. 4,1980, pp. 1581-1590, that simultaneous excitation of a broad mass range (from 15 to 500, corresponding to cyclotron frequencies from 50 kHz to 2 MHz at 55 2 Tesla) with the rf burst method would require an rf burst excitation signal having a duration of about 30 nanoseconds and an amplitude of 13,200 volts. Since it was and still is extremely impractical to create such a signal, this approach was abandoned in favor of the 60 above described frequency sweep chirp excitation.
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A further advancement in ion cyclotron resonance mass spectroscopy is disclosed in U.S. Pat. No. 4,535,235 entitled "APPARATUS AND METHOD FOR INJECTION OF IONS INTO AN ION CY- 65 CLOTRON RESONANCE CELL," issued to Robert T. Mclver, Jr. on Aug. 13, 1985. The spectrometer disclosed in this patent is more versatile than those
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previously developed because the ionizer for forming ions is outside the magnetic field and separate from the ion cyclotron resonance cell. Placing the ionizer outside of the magnetic field permits a wide variety of methods to be used to form ions from a sample. The ions are transported by a quadrupole mass filter through the fringing fields of the magnet and are injected into an ion cyclotron resonance cell that is disposed in the homogeneous region of the field. Once the ions are in the cell, they are accelerated and mass analyzed using either the methods of Fourier transform ion cyclotron resonance or ion cyclotron resonance power absorption.
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A recent development in Fourier transform mass spectroscopy is described in an article entitled "Parametric Mode Operation of a Hyperbolic Penning Trap for Fourier Transform Mass Spectrometry" by D. L. Rempel, E. B. Ledford, Jr., S. K. Huang and M. L. Gross, published in Analytical Chemistry, Vol. 59, No. 20, pp. 2527-2532 (1987). Described in this article is a system wherein the static electric and magnetic fields of a hyperbolic Penning trap form a cell having fields which are similar to those in a single region ion
cyclotron resonance cell. However, instead of six flat electrodes, as disclosed in previously discussed U.S. Pat. No. 3,742,212 issued to Robert T. Mclver, Jr., the hyperbolic Penning trap comprises three electrodes, two "end caps" and one "ring" electrode, which are hyperbolas of revolution. Usable cyclotron resonance signals were obtained with this device by applying a near critically damped sinusoidal signal between the end caps and the ring electrode. The signal used for ion excitation has a peak of approximately +80 volts and a positive voltage duration of approximately 1.55 microseconds followed by a negative voltage portion having a peak of approximately —6.4 volts. However, the authors report that the tuning behavior of the Penning trap is unexpectedly sensitive to the trap voltage and the amplitude of the excitation signal. Furthermore, they suggest that this method can excite the z-axial mode sufficiently to cause ions to be ejected from the cell.
Although there are many advantageous features of the Fourier transform ion cyclotron resonance method, a number of problems and limitations remain. One disadvantage is that the computer-controlled frequency synthesizer, which is used to generate the pulsed broadband oscillating electric field, i.e. frequency chirp, is complex and expensive. Typically, it must be capable of scanning a frequency range of several megahertz in a time period of just a few milliseconds. In addition, the synthesizer must be highly stable and reproducible from scan to scan so that repetitive scans can be summed together coherently to improve the signal-to-noise ratio of the measurement.
Another disadvantage of the above described Fourier transform ion cyclotron resonance spectroscopy techniques is that ions of different mass are accelerated at different times as the frequency of the oscillating electric field is scanned. This complicates the Fourier transform analysis because ions of different mass have different initial phase angles for their cyclotron motion. Correcting the phase angle problem is further complicated by phase shifts in the signal amplifiers. The problem is so complex that most Fourier transform ion cyclotron resonance spectrometers present only a magnitude mode spectrum, which is a composite of the real and imaginary components which result from the Fourier transform analysis. This procedure produces a significantly broader line shape and degrades the mass resolu- lyzed. The Fourier analyzer receives the analog time tion of the spectrometer by about a factor of 2, domain signal and transforms the time domain signal Many of the deficiencies found in presently used into a frequency domain signal which contains informaresonance spectrometer systems could be overcome tion about the numerical magnitude, frequency and with a system which simultaneously excites all of the 5 phase of accelerated ions of each different mass-toresonant components. At the same time, the system charge ratio trapped in the analyzer cell, should be approximately equally sensitive to all of the The improved technique of the present invention also resonant components. Such a system should not be facilitates implementation of a method for using addioverly sensitive to other system parameters. It is also tional high voltage pulses, which are precisely delayed desireable that the system be of simple construction, 10 with respect to the initial excitation pulse to accelerate adaptable to a variety of resonance spectrometer con- or decelerate ions by known amounts of energy. For figurations and cost effective. A need thus exists for a example, if a second high voltage pulse is applied to the system which excites all ions in a short time interval, plates of the analyzer cell after a time delay which corless than a microsecond so as to more closely approxi- responds to an odd number of half cycles for ions of a mate the ideal situation of a delta function acceleration IS particular cyclotron frequency, these ions will be selecof the ions. Additionally, the system should provide tively decelerated by the electric field pulse and their more stable peak heights and better isotope ratios when radius of gyration will decrease. In one embodiment, used in Fourier transform mass spectroscopy. The pres- the initial and delayed pulses are created by a signal ent invention overcomes these and other short comings generator and a delay means precisely determines the of the prior art by providing a new and improved 20 time delay of the delayed excitation signal with respect method and apparatus for impulse acceleration of ions to the initial excitation signal.which is more sensitive, provides better resolution, is The invention further comprises a method of perless complex and less expensive than other broadband forming radiofrequency spectroscopy.
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