Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2000-05-25
2004-07-13
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S291000
Reexamination Certificate
active
06762406
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to ion trap mass spectrometers, and more particularly to mass spectrometers employing an array of miniature ion traps of the same or different sizes, or a combination thereof.
BACKGROUND OF THE INVENTION
An area of increasing interest in mass spectrometry is that of miniature instrumentation. Recent progress has been made toward the total miniaturization (sample introduction, ion source, mass analyzer, ion detection, data acquisition, and vacuum systems) of all the common types of mass spectrometers. The mass analyzers which are currently the main focus of miniaturization efforts are the linear quadrupole and time-of-flight (TOF) mass analyzers. A number of groups have developed single miniature linear quadrupole analyzers (Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S.
Electron. Lett.
1996, 32, 2094-2095) (Taylor, S.; Tunstall, J. J.; Syms, R. R. A.; Tate, T.; Ahmad, M. M.
Electron. Lett.
1998, 34, 546-547) (Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S.
IEEE Trans. Electron Devices,
1998, 45, 2304-2311) (Holkeboer, D. H.; Karandy, T. L.; Currier, F. C.; Frees, L. C.; Ellefson, R. E.,
J Vac. Sci. Technol. A,
1998, 16, 1157-1162) (Taylor, S.; Tunstall, J. J.; Leck, J. H., Tindall, R. F.; Jullien, J. P.; Batey, J; Syms, R. R. A.; Tate, T; Ahmad, M. M.,
Vacuum
1999, 53, 203-206) (Freidhoff, C. B.; Young, R. M.; Sriram, S.; Braggins, T. T.; O'Keefe, T. W.; Adam, J. D.; Nathanson, H. C.; Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S.; Tunstall, J.,
J. Vac. Sci. Technol. A
1999, 17, 2300-2307).
Arrays of mass analyzers have been used previously, starting with the commercial double-beam Kratos MS30 sector instrument of a generation ago, and, more recently, including multiple linear quadrupoles each of identical size (Ferran, R. J.; Boumsellek, S.,
J. Vac. Sci. Technol. A
1996, 14, 1258-1265) (Orient, O. J.; Chutjian, A.; Garkanian, V.,
Rev. Sci. Instrum.
1997, 68, 1393-1397). In the latter cases, multiple analyzers are specifically used in order to provide higher ion currents while maintaining the favorable operating conditions of physically smaller devices, including higher pressure tolerance and lower working voltages. As an example of this approach, Kirchner (Kirchner, N. J.: U.S. Pat. No. 5,206,506, 1993) proposed a parallel electrostatic ion processing device composed of a parallel series of channels. Each channel was designed to store, process, and then detect ions. Due to the parallel architecture, high ion throughput and high capacity were expected.
Miniature mass spectrometers that can be operated in non-laboratory and harsh environments are of interest for continuous on-line and other monitoring tasks. Simplicity of operation and small size are the premier qualities sought in these devices. Only modest performance in terms of resolution and dynamic range is needed to address many of the problems to which these small instruments might be applied. Miniaturization of the mass analyzer must be accompanied by miniaturization of the entire system, including the vacuum system and control electronics. The ion trap mass analyzer is physically small. Nearly a decade ago a miniature version (2.5 mm internal radius) was described by Kaiser et al. (Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H.,
Int. J. Mass Spectrom. Ion Processes
1991, 106, 79-115).
One major issue with miniaturized mass spectrometers is the pressure tolerance of the device. Currently, pumping systems are size, power and weight prohibitive, and the miniature devices available do not provide the pumping speeds or base pressures associated with full-size pumps. Offsetting this is the fact that the pressure tolerance of small analyzers is greater than that of larger analyzers, since the shorter path lengths decrease the probability of ion
eutral atom or molecule collisions. Even though ion traps have relatively long path lengths, collisions with gases of lower mass and higher ionization potential have beneficial effects on resolution since they cool ions to near the center of the device (Stafford G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F.,
Int. J. Mass Spectrom. Ion Processes
1984, 60, 85-98). The result is that quadrupole ion traps are the most pressure-tolerant of all the major types of mass analyzers, and small ion traps should be even more so. A pressure tolerant analyzer like the quadrupole ion trap, therefore, is of special interest as a miniature mass spectrometer, since base pressure can be higher and pumping capacity lower, allowing use of a simpler pumping system.
In the search for a robust mass analyzer for miniaturization, the quadrupole ion trap is a prime candidate due to its overall performance characteristics and operating conditions that are beneficial for the miniaturization process. Operation of the trap using simplified-applied voltages simplifies the control electronics needed to operate the ion trap as a mass analyzer. Also, given that a reduction in size causes a reduction in ion trapping capacity, a method to gain back total ion trapping capacity is needed when miniaturized ion traps are used, and the use of multiple individual traps is suggested for this purpose.
The conventional method of operating a hyperbolic quadrupole ion trap as a mass spectrometer is to perform a mass-selective instability scan. In this experiment the amplitude of the applied rf voltage is scanned so as to force ions of increasing m/z ratios into unstable trajectories, causing them to leave the trap and allowing them to impinge on an external detector such as an electron multiplier (Stafford G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F.,
Int. J. Mass Spectrom. Ion Processes
1984, 60, 85-98). The relationship between the parameters involved is given by the Mathieu equations. The solution for ion motion in the z (axial) direction can be expressed in terms of the Mathieu parameter q
z
where:
q
z
=
8
z
V
m
Ω
2
(
r
0
2
+
2
z
0
2
)
.
In this equation, V is the amplitude of the trapping rf voltage, m is the mass of the ion of interest, r
0
and z
0
are the inscribed dimensions of the ion trap, and &OHgr; is the angular frequency of the rf voltage. It has been previously noted that, in principle, at a fixed value of q
z
, variation in V, &OHgr; or r will correspond to selection of ions of different m/z values (Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H.
Int. J. Mass Spectrom. Ion Processes
1991, 106, 79-115) (Kaiser, R. E.; Cooks, R. G.; Moss, J.; Hemberger, P. H.
Rapid Comm. Mass Spectrom.
1989, 3, 50-53). Indeed, scans of V have been used to record mass spectra, the value of q
z
being fixed by the boundary for ion stability or some other operating point in the stability diagram (Stafford, G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J.
Int. J. Mass Spectrom. Ion Processes
1984, 60, 85-98).
A cylindrical ion trap (CIT) was first described by Langmuir for use as an ion containment device, but not as a mass spectrometer. Subsequently, the use of CITs has focused mainly on ion storage, although recent experiments by Badman (Badman, E. R.; Johnson, R. C.; Plass, W. R.; Cooks, R. G.
Anal. Chem.
1998, 70, 4896-4901) and Kornienko (Kornienko, O.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M.
Rapid Commun. Mass Spectrom.
1999, 13, 50-53) (Kornienko, O.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M.
Rev. Sci. Instrum.
1999, 70, 3907-3909) have shown them to perform well as mass spectrometers. CITs are also simpler to machine than standard hyperbolic quadrupole ion traps, especially on the millimeter scale. A cylindrical ion trap (CIT) consists of a barrel-shaped central ring electrode with two flat endcap electrodes, and as such, it is extremely simple to machine compared to the hyperboloid shapes of the electrodes in the standard quadrupole ion trap.
OBJECTS AND SUMMARY OF THE INVENTION
It is
Badman Ethan R.
Cooks Robert G.
Ouyang Zheng
Wells James M.
Dorsey & Whitney LLP
Lee John R.
Purdue Research Foundation
Smith II Johnnie L
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