Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2000-06-09
2003-02-11
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S3960ML
Reexamination Certificate
active
06518569
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to ion mirrors for mass spectrometry.
2. Background Art
In the earliest time-of-flight (TOF) mass spectrometers, ions were extracted from a source by a single linear extraction field to a field-free region. The arrival times of ions that traversed this region varied as a function of their m/z (mass/charge) ratios.
Two articles by Wiley and McLaren (Wiley, W. C.; McLaren, I. H.
Rev. Sci. Instrum
., 26, 1150 (1955) and Wiley, W. C.
Science
, 124, 817 (1956)) disclose that the space focus plane could be moved to the detector plane with a two-field extraction. Wiley and McLaren also combined this with time-lag extraction. Time-lag extraction transformed the ion thermal energy distribution into a spatial distribution that was subsequently corrected by space focusing at the detector. The disadvantage of the time-lag extraction is its mass dependence, which prevents simultaneous focusing over the whole m/z range.
An ion mirror introduced by Karataev et al. (Karaev, V. I; Mamyrin, B. A.; Shmikk, D. V.; A.
Sov. Phys. Tech. Phys
., 16, 1173 (1972)) solved the focusing problem reported by Wiley and McLaren. To solve the problem, a potential hill in the ion mirror was introduced, which produced a longer flight path for more energetic ions. Thus, due to the potential hill, two ions with the same m/z value but different kinetic energies spend different amount of time in the ion mirror. For example, an ion with higher kinetic energy spends less time in the field free region but penetrates deeper into the ion mirror, while an ion with lower kinetic energies spends more time in the field free region but penetrates the ion mirror less deeply. Thus, the ion mirror compensates for much of the difference in ion kinetic energies.
However, the ion mirror of Karaev et al. could not correct for initial kinetic energy distribution and/or spatial distribution of ions in the ion source at the same time. Essentially, the turn-around time of ions with random thermal motion in the source cannot be eliminated at the time of extraction; therefore, the turnaround time eventually limits the achievable resolving power unless random ion motion is avoided.
To effectively minimize the initial kinetic energy distribution along the time-of-flight (TOF) axis of an ion, orthogonal acceleration was introduced, referred to herein as “TOF-oa.” Theoretically, when TOF-oa is combined with a mirror that has an optimum field shape, a high-resolution mass spectrometer should be achieved.
In 1989, Dawson and Guilhaus built the first TOF-oa instrument for improving resolving power and duty cycle with an electron impact (El) ion source (Dawson, J. H. J.; Guilhaus, M.,
Rapid Commun. Mass Spectrom
., 3, 155 (1989) and Dawson, J. H. J.; Guihaus, M. Australian Provisional Patent P16079, 1987; Int. Patent Appl. PCT/AU88/00498, 1988) and U.S. Pat. No. 5,117,107. According to the Dawson and Guilhaus instrument, ions are collimated by an electrostatic lens system and injected into an orthogonal extraction region. As a result, in a linear TOF instrument, the ion extraction and acceleration fields provide space focusing at the detector. The Dawson and Guilhaus instrumented reportedly achieved a resolution of 2000 at full width at half maximum (FWHM) of a spectral peak.
Dodonov et al. (Dodonov, A. F.; Chernushevich, I. V.; Laiko, V. V., International Mass Spectrometry Conference, Amsterdam, August 1991; Extended Abstracts, p153 and Dodonov, A. F.; Chernushevich, I. V.; Laiko, V. V. in Time-of-Flight Mass Spectrometry; Cotter, R. J. Ed.; ACS Symposium Series 549; American Chemical Society, Washington, DC, 1994. pp108-23) developed an orthogonal acceleration instrument that coupled electrospray ionization (ESI) and a dual-stage ion mirror mass analyzer with a resolution of about 1000 (FWHM).
Verentchikov et al. (Verentchikov, A. N.; Ens, W.; Standing, K. G.,
Anal. Chem
., 66, 126 (1994)). reported an orthogonal acceleration instrument with a resolution of about 5000 (FWHM) by using a single-stage ion mirror. An improvement of this instrument reportedly achieved a resolution between 7000 and 10000 (FWHM) (Krutchinsky, A. N.; Chernushevich, I. V; Spicer, V. L.; Ens W.; Standing, K. G.,
J. Amer. Soc. Mass Spectrom
., 9, 569 (1998)).
To date, ion mirrors have been a key element in providing improved resolution over the entire m/z range. In general, ion mirrors can be divided into two groups, linear and non-linear, according to the distribution of the electric field within the mirror. Linear ion mirrors are referred to as staged ion mirrors. Staged ion mirrors may have one or more stages, each stage having a linear electric field. In contrast, a non-linear ion mirror has an electric field contour that is curved along the mirror axis, particularly, in an ion turn-around region. Researchers have demonstrated that non-linear ion mirrors can achieve higher resolution than can linear ion mirrors (Cornish, T. J. and Cotter, R. J.,
J. Rapid Commun. Mass Spectrom
., 8, 781-785 (1994)). Depending on the system, an “ideal” non-linear ion mirror should exist. An ideal non-linear ion mirror preferably has an electric field with the theoretically optimum contour along the mirror axis and an absolutely homogeneous field in the off-axis directions. Inhomogeneity in the off-axis, or radial, directions results in ion dispersion away from the beam center and inequity in ion flight time across the useful beam diameter. Therefore, an ion mirror with a large off-axis homogeneous region near the beam center is desirable, in turn, an enlarged, useable beam center region results.
An “ideal” ion mirror should achieve infinite order focusing of kinetic energy as reported by Rockwood, A L., Proceedings of the 34
th
ASMS Conference on Mass Spectrometry and Allied Topics; Cincinnati, Ohio, June 8-13, P173 (1986). The voltage in the electric field of an “ideal” ion mirror follows the parabolic equation U=ax
2
where a is a constant and x is the depth in the ion mirror along the axial direction. Unfortunately, such a parabolic field ion mirror is difficult to implement and has the disadvantage of having no field-free flight path.
To date, ion mirrors have primarily used two different configurations to create a non-linear electric field. One reported configuration uses stacks of many ring-like diaphragm elements (U.S. Pat. No. 4,625,112, entitled “Time of flight mass spectrometer,” to Yoshida, issued Nov. 25, 1986; U.S. Pat. No. 5,464,985, entitled “Non-linear field reflection,” to Cornish and Cotter, issued Nov. 7, 1995; U.S. Pat. No. 5,017,780, entitled “Ion reflector,” to Kutscher et al., issued May 21, 1991) while the other configuration uses simple geometric shapes (Cornish, T. J; Cotter. R. J.,
J. Anal. Chem
., 69, 4615 (1997); U.S. Pat. No. 5,814,813 entitled “End cap reflection for a time-of-flight mass spectrometer and method of using the same,” to Cotter et al, issued Sep. 29, 1998; U.S. Pat. No. 5,077,472, entitled “Ion mirror for a time-of-flight mass spectrometer,” to Davis, issued Dec. 31, 1991).
Disadvantages of the stacks of ring-like diaphragm configuration are the non-homogeneity of the electric field in the off-axis directions and the number of conductive elements required. Each additional element adds critical spatial and voltage control requirements. Although the reported configurations that use simple geometric shapes are easier to implement for non-linear electric fields, off-axis homogeneity has, to date, limited the achievable resolution. Therefore, a need exists for an ion mirror that is not as limited by off-axis inhomogeneity or the requirements inherent in the use of a large number of elements.
U.S. Pat. No. 5,017,780, entitled “Ion reflector,” to Kutscher et al., issued May 21, 1991, discloses an ion mirror with at least one special element of conical construction and many ring-like diaphragms. The implementation is difficult, in part, because all the conductive elements require distinct voltages and tight focusing of the ion beam c
Enke Christie G.
Gardner Benjamin D.
Zhang Jun
Myers Jeffrey D.
Nguyen Kiet T.
Pangrle Brian J.
Science & Technology Corporation @ UNM
LandOfFree
Ion mirror does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ion mirror, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ion mirror will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3138020