Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
1999-01-28
2001-02-27
Berman, Jack (Department: 2878)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S282000
Reexamination Certificate
active
06194717
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to quadrupole mass analyzers, and more particularly is concerned with quadrupole mass analyzers operated in an RF only mode which provides mass resolution, with a reduced background signal.
BACKGROUND OF THE INVENTION
Quadrupole mass spectrometers have proven to be useful as general purpose mass analyzers. These devices are four rod structures and when designed for operation in a resolving mode are usually about 20 cm in length and require extreme mechanical precision in terms of fabrication and alignment. When operated in a resolving mode quadrupole mass spectrometers conventionally have both RF and DC voltages applied to them. Values of these voltages vary with the frequency and mass range of operation, but can be on the order of 1600 volts (peak-to-peak) RF for operation at 1 MHz and ±272 volts DC for a rod array inscribed radius r
0
of 0.418 cm and a mass range of 600 Daltons. The high degrees of mechanical and electrical sophistication required means that the costs of these mass spectrometers are high.
The most common mode of operation teaches that the operating line q should cross just below the tip of the first stability region. The stability region is plotted as a function of the well-known Mathieu parameters a and q. Operation at the tip of the first stability region means that only ions with a narrow range of m/z will be transmitted, giving the potential for high mass resolution.
In theory operation with no applied DC voltage equates to operation along the horizontal axis of the Mathieu stability diagram and this should give transmission of a wide range of ions up to an m/z given by the limit of the first stability region.
However, as taught in U.S. Pat. No. 4,090,075, a quadrupole rod array can provide mass resolution in the absence of applied resolving DC voltages. This so called RF-only mode of operation has several advantages over conventional RF/DC operational modes. Conventional RF/DC quadrupole rod mass spectrometers supply mass resolution based on the intrinsic stability or instability of given ions within the rod structure in the combination of the time varying RF and the time independent DC fields. In contrast to the more common RF/DC quadrupole mass analyzers, mass resolution for an RF-only instrument is thought to occur when ions that are only marginally stable with a particular applied RF voltage gain excess axial kinetic energy in the exit fringing field of the rod structure. Since a large part of the phenomena leading to mass resolution of an RF-only mass analyzer occurs at the exit of the rod array the length limitations characteristic of RF/DC resolving quadrupoles no longer apply and mechanical tolerances for rod roundness and straightness are considerably relaxed. Finally, there is no need for a high precision high voltage DC power supply in the RF-only mode of operation. Taken together the inherent advantages of RF-only operation suggest the opportunity for a much smaller and less costly mass analyzer than conventional RF/DC quadrupoles. Although the potential of such a device is significant problems such as sample dependent background from high velocity ions and clusters have considerably limited the commercial use of RF-only mass analyzers, especially when coupled with electrospray and other atmospheric pressure ion sources.
In the RF-only mode, to separate out ions with higher energies, energy filtering is accomplished, typically by placement of a retarding grid either at the exit of the quadrupole or further downstream. This has the effect of separating particles having higher energy, i.e. those ions with a q near 0.907 which have acquired a higher kinetic energy, from other ions with lower kinetic energy.
A drawback associated with this energy filtering technique is that there can be a significant high energy tail in the energy distribution of ions entering the quadrupole rods, i.e. ions with a q substantially less than 0.907. These high energy ions can originate from a variety of sources, but the net effect is overlap of the energy distribution of these ions, and of the curve representing ions with a q of near 0.907. This in turn results in the appearance of a continuum background signal upon which overlaps the peaks of the ions with a q near 0.907.
One technique proposed for reducing this background is in U.S. Pat. No. 4,189,640. This teaches providing a centrally located attractively biased disk of appropriate size, located after the analyzing quadrupole. The intention is to reduce high velocity and higher mass species. However, in practice this also reduces analyte ion intensity off-setting much of the expected gains in signal-to-noise ratio.
SUMMARY OF THE PRESENT INVENTION
In accordance with a first aspect of the present invention, there is provided a method of mass analysis utilizing a quadrupole rod set located in a vacuum chamber, the method comprising:
A method of mass analysis utilizing a quadrupole rod set located in a vacuum chamber, the method comprising:
(1) providing a stream of ions and supplying the stream of ions to one end of a first multipole rod set;
(2) supply an RF-only voltage to the multipole rod set, whereby the first multipole rod set acts as an ion guide and transmits desired ions therethrough;
(3) passing the ions into an analyzing quadrupole rod set;
(4) supplying an RF-only voltage to the analyzing quadrupole rod set, whereby desired ions having a mass-to-charge ratio giving a q value of substantially 0.907 gain additional axial kinetic energy upon leaving the analyzing quadrupole rod set;
(5) detecting ions leaving the analyzing quadrupole rod set having the increased axial kinetic energy; and
(6) selecting the pressure within the first multipole rod set to enhance separation between ions gaining the additional axial kinetic energy and other ions.
Preferably, step (5) comprises:
(a) passing ions leaving the analyzing quadrupole rod set through an energy filter, whereby only ions with additional axial kinetic energy have sufficient energy to pass through the energy filter; and
(b) detecting ions passing through the energy filter at a detector.
The multipole rod set can comprise a first quadrupole rod set. Then, the gas pressure provided in the first quadrupole or other multipole rod set can be such that the multiple of the pressure in the rod set times the length of the rod set is at least 4×10
−2
torr-cm, preferably at least 10
−1
torr-cm, and more preferably equal to or greater than 1.6×10
−1
torr-cm. The first quadrupole rod set can then be 10 cm long.
In accordance with another aspect of the present invention, there is provided an apparatus for mass analysis of an ion stream, the apparatus comprising:
a generation means for generating a stream of ions;
a multipole rod set connected to the generation means for receiving ions and for collimating and cooling ions;
an analyzing quadrupole rod set having an inlet for the ion stream from the multipole rod set and an outlet;
a means for maintaining a variable pressure in the multipole rod set and adjusting the pressure to improve separation, in the analyzing quadrupole rod set, between ions with a q of substantially 0.907 and other ions, the ions having a q of substantially 0.907 gaining axial kinetic energy on leaving the outlet of the analyzing quadrupole rod set; and
a means for detecting the ions having the additional axial kinetic energy, whereby ions having the higher axial kinetic energy are resolved with respect to other ions.
REFERENCES:
patent: 4090075 (1978-05-01), Brinkmann
patent: 4137750 (1979-02-01), French et al.
patent: 4189640 (1980-02-01), Dawson
patent: 4963736 (1990-10-01), Douglas et al.
patent: 5248875 (1993-09-01), Douglas et al.
patent: 5652427 (1997-07-01), Whitehouse et al.
patent: 5942752 (1999-08-01), Wang
patent: 6028308 (2000-02-01), Hager
patent: 6093929 (2000-07-01), Javahery et al.
patent: 0 373 835 (1990-06-01), None
patent: WO 98/22972 (1998-05-01), None
patent: WO 98/52209 (1998-11-01), None
patent: WO 98/56030 (1998-12-01), None
Bereskin & Parr
Berman Jack
MDS Inc.
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