Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2002-07-22
2003-11-18
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S281000, C250S282000, C250S283000, C250S288000, C250S290000, C250S292000, C250S297000, C250S299000, C250S300000
Reexamination Certificate
active
06649911
ABSTRACT:
The present invention relates to a method of selecting ions in an ion storage device with high resolution in a short time period while suppressing amplitude of ion oscillation immediately after the selection.
BACKGROUND OF THE INVENTION
In an ion storage device, e.g. a Fourier transformation ion cyclotron resonance system or an ion trap mass spectrometer, ions are selected according to their mass-to-charge (m/e) ratio. While the ions are held within an ion storage space, a special electric field is applied to the ion storage space to selectively eject a part of the ions having specified m/e values. This method, including the storage and selection of ions, is characteristically applied to a type of mass spectrometry called an MS/MS. In an MS/MS mass spectrometry, first, ions with various m/e values are introduced from an ion generator into the ion storage space, and an ion-selecting electric field is applied to the ion storage space to hold within the space only such ions having a particular m/e value while ejecting other ions from the space. Then, another special electric field is applied to the ion storage space to dissociate the selected ions, called precursor ions, into dissociated ions, called fragment ions. After that, by changing the system parameters, the fragment ions created in the ion storage space are ejected toward an ion detector to build a mass spectrum. The spectrum of the fragment ions contains information about the structure of the precursor ions. This information makes it possible to determine the structure of the precursor ions, which cannot be derived from a simple analysis of the m/e ratio. For ions with complex structures, more detailed information about the ion structure can be obtained by a repetition of selection and dissociation of the ions within the ion storage device (MS
n
analysis).
The special electric field for selecting ions is usually produced by applying voltages having waveforms with opposite polarities to a pair of opposite electrodes which define the ion storage space. The special electric field is produced without changing the ion storage condition. In an ion trap mass spectrometer, voltages having waveforms of opposite polarities are applied to a pair of end cap electrodes, while a radio frequency (RF) voltage is applied to a ring electrode placed between the end cap electrodes. The RF voltage independently determines the ion storage condition.
Each of the ions stored in the ion storage device oscillates at the secular frequency which depends on the m/e value of the ion. When an appropriate electric field for selecting particular ions is applied, the ions oscillate according to the electric field. If the electric field includes a frequency component close to the secular frequency of the ion, the oscillation of the ion resonates to that frequency component of the electric field, and the amplitude gradually increases. After a period of time, the ions collide with the electrodes of the ion storage device or are ejected through an opening of the electrodes to the outside, so that they are evacuated from the ion storage space. In the case of an ion trap mass spectrometer, the secular frequency of an ion in the radial direction differs from that in the axial direction. Usually, the secular frequency in the axial direction is used to remove ions along the axial direction.
Waveforms available for selecting ions include the Stored Waveform Inverse Fourier Transformation (SWIFT; U.S. Pat. No. 4,761,545), Filtered Noise Field (FNF; U.S. Pat. No. 5,134,826), etc. Each of these waveforms is composed of a number of sinusoidal waves with different frequencies superimposed on each other, wherein a frequency component of interest is excluded (this part is called a “notch”). The strength of the ion-selecting electric field produced by the waveform is determined so that ions having such secular frequencies that resonate to the frequency component of the waveform are all ejected from the ion storage space. Ions having secular frequencies equal or close to the notch frequency, which is not contained in the waveform, do not resonate to the electric field. Though these ions might oscillate with a small amplitude, the amplitude does not increase with time, so that the ions are not ejected from the ion storage space. As a result, only such ions that have particular secular frequencies are selectively held in the ion storage space. Thus, the selection of ions is achieved.
However, even if the frequency of the excitation field slightly differs from the secular frequency of the ions, the ions can be excited and the amplitude of the oscillation of the ions increases. This means that the ion selection does not depend solely on whether the waveform contains a frequency component equal to the secular frequency of the ion. Therefore, the notch frequency is determined to have a certain width. However, the ions having a secular frequency at the boundary of the notch frequency are still unstable in oscillation.
As regards the conventional ion-selecting waveforms represented by SWIFT and FNF, past significance has primarily focused on whether the frequency components of the ion-selecting wave include the secular frequency of the ions to be held in the ion storage space.
In a practical mass spectrometry, various processes are performed after the ions are selected. An example of the process is the excitation of precursor ions with an electric field to produce fragment ions, called “fragmentation”. In this process, the strength of the excitation field needs to be properly adjusted so as not to eject the precursor ions from the ion storage space. Excessive decrease in the strength of the electric field, however, results in an inefficient fragmentation. Accordingly, the strength of the electric field needs to be controlled precisely. When the initial amplitude of the ion oscillation is large before the excitation field is applied, the ions may be ejected even with a weak electric field. In an ion trap mass spectrometer, the RF voltage needs to be lowered before fragmentation to establish a condition for the fragment ions to be stored. In this process, if the initial amplitude of the oscillation of the precursor ions is large, the motion of the precursor ions becomes unstable, and the ions are ejected from the ion storage space. It is therefore necessary to place a “cooling process” for waiting for the oscillation of the precursor ions to subside before fragmentation. Placing such a process consequently leads to a longer time for completing the entire processes, and deteriorates the throughput of the system.
In theory, in an ion trap mass spectrometer, the strength of the RF electric field within the ion storage space determines the secular frequencies of the ions according to their m/e values. In practice, however, the RF electric field deviates slightly from the theoretically designed quadrupole electric field, so that the secular frequency is not a constant value but changes according to the amplitude of the ion oscillation. The deviation of the electric field is particularly observable around a center of the end cap electrodes because they have openings for introducing and ejecting ions. Around the opening, the secular frequency of the ion is lower than that at the center of the ion storage space. In the case of an ion whose secular frequency is slightly higher than the notch frequency, its amplitude increases due to the excitation field when it is at the center of the ion storage space. As the amplitude becomes larger, however, the secular frequency becomes lower, and approaches the notch frequency. This makes the excitation effect on the ion poorer. Ultimately, the amplitude stops increasing at a certain amplitude and begins to decrease.
In the case of an ion whose secular frequency is slightly lower than the notch frequency when it is at the center of the ion storage space, on the other hand, its amplitude increases due to the excited oscillation, and the secular frequency gradually departs from the notch frequency. This increases the efficiency of excitation, and the io
Lee John R.
Shimadzu Corporation
Souw Bernard
LandOfFree
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