The present invention relates to mass analyzers and, more specifically to a mass analyzer that employs a quadrupole ion trap.
Mass spectrometers allow for the determination of the chemical constituents in a sample, and as such they have wide ranging applications in the modern world. For example, mass spectrometers can be used to detect specific types of gasses and other chemical compounds. Effective portable mass spectrometers could be used in a wide variety of applications, including chemical weapons detection, pollutant detection, environmental applications and quality assurance. Unfortunately, many portable mass spectrometers, because of their bulk and power requirements, cannot practically be mounted on a small vehicle or worn by a person. A chip-scale mass spectrometer would enable that, as well as networked arrays of fieldable mass spectrometers.
A typical mass spectrometer consists of several subsystems, including: an ion source, a mass analyzer, an ion detector, vacuum system/pumps, and various electronic circuits). Each of these items would need to be miniaturized to produce a working chip-scale mass spectrometer. However, the mass analyzer is the heart of a mass spectrometer, and ultimately determines the instrument's overall ability to discern the constituents of a material under analysis. While attempts have been made at fabricating chip scale mass analyzers, their performance characteristics are not yet at a level where they can provide useful mass analysis in many portable applications.
Existing portable systems require a large power draw (e.g., 50 W), are expensive, require complicated maintenance and must be operated by expert users.
An important requirement for working portable mass spectrometers is that they include effective vacuum pumps. Microfabrication of such devices can be quite challenging. However, chip-scale ion trap mass analyzers would be able to operate effectively up to about 1 Torr, which would reduce the requirements of the currently large and power hungry vacuum pumps associated with larger mass analyzers.
Therefore, there is a need for a chip scale mass analyzer having a geometry that allows for microfabrication.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is an ion trap that includes a first electrode pair and a second electrode pair. Each electrode pair includes a first conductive member and a second conductive member. The first electrode pair faces the second electrode pair so that the first conductive member of the first electrode pair is on a first common plane with the second conductive member of the second electrode pair, and so that the second conductive member of the first electrode pair is on a second common plane with the first conductive member of the second electrode pair. The first common plane is spaced apart from and parallel to the second common plane. The first electrode pair is spaced apart from the second electrode pair so as to define a gap therebetween. A signal generator is configured to generate a first periodic signal and to apply the first periodic signal to the first conductive member of the first electrode pair and the first conductive member of the second electrode pair. A phase shifter is electrically coupled to the signal generator and is configured to generate a second periodic signal that is out of phase by a predetermined phase shift with the first periodic signal and to apply the second periodic signal to the second conductive member of the first electrode pair and the second conductive member of the second electrode pair, wherein ions of a predetermined type that are introduced into the gap are trapped by a resulting electric field. The ion trap has relative dimensions including: a space in a range of 6.0 units to 8.2 units defined between the first conductive member of the first electrode pair and the second conductive member of the first electrode pair; a space in a range of 6.0 units to 8.2 units defined between the first conductive member of the second electrode pair and the second conductive member of the second electrode pair; and the first electrode pair is spaced apart from the second electrode pair so as to define a space therebetween of about 10 units. Ions escaping through the gap have an ion mass and an ion charge and the signal generator is configured to generate the first periodic signal so as to have an RF drive frequency and an RF voltage amplitude determined by:
wherein:
In another aspect, the invention is a quadrupole ion trap mass analyzer that includes an ion source and an ion trap. The ion trap that includes: a first electrode pair and a second electrode pair. Each electrode pair includes: a first conductive member including a first conductive surface; a second conductive member including a second conductive surface. The first electrode pair faces the second electrode pair so that the first conductive surface of the first electrode pair is on a first common plane with the second conductive surface of the second electrode pair and so that the second conductive surface of the first electrode pair is on a second common plane with the first conductive surface of the second electrode pair. The first common plane is spaced apart from and parallel with the second common plane. The first electrode pair is spaced apart from the second electrode pair so as to define a gap therebetween. A signal generator is configured to generate a first periodic radio frequency signal that is biased by an alterable bias signal and is configured to apply the first periodic signal to the first conductive surface of the first electrode pair and the first conductive surface of the second electrode pair. A phase shifter is electrically coupled to the signal generator and is configured to generate a second periodic signal that is out of phase by a 180° phase shift with the first periodic signal and that is configured to apply the second periodic signal to the second conductive surface of the first electrode pair and the second conductive surface of the second electrode pair. Ions that are introduced into the gap are trapped by a resulting electric field. An ion detector is disposed relative to the gap so that ions exiting the ion trap will intersect a surface thereof. The ion detector is configured to generate a signal indicating detected ions.
In yet another aspect, the invention is a method of trapping ions, in which a first periodic signal is applied to a first conductive member of a first electrode pair and the first periodic signal is applied to a first conductive member of a second electrode pair. The first conductive member of the first electrode pair and the second conductive member of the second electrode pair are disposed along a common first plane. The first periodic signal is phase shifted by a predetermined phase shift so as to generate a second periodic signal. The second periodic signal is applied a second conductive member of the first electrode pair and the second periodic signal is applied to a second conductive member of the second electrode pair. The second conductive member of the first electrode pair and the first conductive member of the second electrode pair are disposed along a common second plane that is parallel to and spaced apart from the first plane. The first electrode pair is spaced apart at a predetermined distance from the second electrode pair so as to form a gap therebetween. Ions are introduced into, or produced in, the gap. A selected type of ions is trapped by an electric field resulting from application of the first periodic signal to the first conductive members and application of the second periodic signal to the second conductive members. The selected type of ions is determined as a function of the predetermined distance between the first electrode pair and the second electrode pair.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. Unless otherwise specifically indicated in the disclosure that follows, the drawings are not necessarily drawn to scale. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described below. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.”
As shown in
The first electrode pair 110 faces the second electrode pair 130 so that the first conductive member 112 of the first electrode pair 110 is on a first common plane 150 with the second conductive member 120 of the second electrode pair 130. Also, the second conductive member 120 of the first electrode pair 110 is on a second common plane 152 with the first conductive member 112 of the second electrode pair 130. (The first common plane 150 and the second common plane 152 are shown in broken lines to demonstrate the dispositions of the conductive members and are not tangible physical objects.) The first common plane 150 is spaced apart from and parallel to the second common plane 152. The first electrode pair 110 is separated from the second electrode pair 130 by a gap 132. The width of the gap 132 and the height of the spacer 118 determines the type of electric field within the ion trap 100 and, thus, the mass-to-charge ratio of the ions that will be trapped therein. The length of the gap 132 determines the overall ion capacity of the ion trap 100.
As shown in
It should be noted that other modes of mass analysis are possible. For example, instead of ramping the voltage up, the frequency can be ramped down (also, the ramps don't need to be linear in time). One alternate embodiment can employ additional RF signals to resonantly excite the ions to the point where they are ejected from the ion trap. While there are many methods of mass analysis that can be employed with the structure disclosed here, with proper selection of the vertical and horizontal spacing, the resulting fields can obtain better than unit mass resolution.
In one alternate embodiment, this structure can function as an ion trap by grounding the second conductive members of each electrode pair and putting RF on only the first members. The equation we included then just changes by adding a factor of two and, thus, the voltage in this configuration would be two times higher. However, such a trap might not be used in a mass analyzer then due to poorer resolution and poorer ejection efficiency.
As shown in
One macro-scale experimental embodiment, as shown in
If the rectangular rods are surrounded by a grounded box 430 (which occurs in many embodiments because they are in a metal vacuum chamber), a DC bias can be added to the rods in additional to their RF voltages. A negative bias will lead to axial confinement of positive ions, and vice versa for negative ions. The trap can also be operated with square wave voltages to act as a digital ion trap. By changing the duty cycle of the square waves an average DC bias that also leads to axial confinement can be obtained. Other applied RF voltage waveforms (e.g. triangle wave) can also be used to trap and analyze ions.
The ion trap can be scaled so long as the relative dimensions of the elements are maintained. Such dimensions for one experimental embodiment are shown in
Where:
In one experimental embodiment (in which the units were mm), each rod was 2.5 mm thick, 10 mm wide and about 50 mm long. The rods of each electrode pair were spaced apart from each other at a distance of 7.1 mm and the gap between the electrode pairs was 10.0 mm. In this experimental embodiment, the RF drive frequency was about 1 MHz, and the ramped-up biasing voltage amplitude went from about 30 V to about 300 V. Employing these parameters, the experimental embodiment was able trap and analyze ions from about 6 amu up to 60 amu. Thus, in this experimental embodiment, dropping the frequency a factor of 2 would make the mass range 26-260 amu. And if one used 50V-500V at that frequency the mass range would be about 43-430 amu.
The present ion trap can be used in a linear quadrupole ion trap mass analyzer, which simulations and experiments so far performed have indicated can provide better than unit mass resolution and high detection efficiency, thus enabling useful mass analysis with a micron-scale device that can be microfabricated with well-established techniques. The ion trap produces a highly quadrupolar potential despite the use of flat, rectangular shaped electrodes. By putting the linear ion trap on a chip, it is possible to have separate regions of the chip where the ions are created, analyzed and detected as the ions can be moved through different regions with the use of additional DC voltages.
Simulations to determine specific dimensions and other operating parameters can be performed using software to calculate electric fields and the trajectories of charged particles in those fields when given a configuration of electrodes with applied time-varying voltages and particle initial conditions. One example of such a software package is SIMION® (available through https://simion.com/). If the thickness of the rods changes sufficiently, the optimal vertical to horizontal spacing of the electrodes can change.
A chip scale linear quadrupole ion trap of the type disclosed herein simultaneously confines a large range of particles with varying m/z (mass-to-charge ratio) values and then ejects them from the trap in a m/z specific way so that they can be detected. An advantage of this device is that its geometry enables sub-unit mass resolution even though the electrode shapes are non-hyperbolic and flat. While other flat electrode devices often produce major electric field deviations from the desired quadrupolar potential that enables mass analysis, the geometry of the present ion trap produces a sufficiently quadrupolar potential that allows sub-unit resolution in mass analyzing systems.
It is estimated that the present ion trap can result in about three times the mass resolution of similar systems employing flat electrode surfaces (e.g. an existing rectilinear ion trap. Additionally, it can produce a three times higher ion ejection/detection efficiency as that of similar systems. If manufactured at the chip scale, it can be made small and low weight at a low per-unit cost. These features would enable them to be deployed while mounted on small vehicles and war fighters, etc., and in large networked arrays.
The applications for mass analyzers employing linear quadrupole ion trap as disclosed herein can include:
In certain embodiments, as shown in
As shown in
Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description. It is understood that, although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. The operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. It is intended that the claims and claim elements recited below do not invoke 35 U.S.C. § 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim. The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/828,171, filed Apr. 2, 2019, the entirety of which is hereby incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2020/026190 | 4/1/2020 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
62828171 | Apr 2019 | US |