Patent Grant
7928375
The present invention relates to mass spectrometry and, in particular, to microfabricated linear Paul-Straubel ion trap that can be used to trap and filter gaseous ions.
A mass spectrometer (MS) is a device that filters gaseous ions according to their mass-to-charge (m/z) ratio and measures the relative abundance of each ionic species. Mass spectrometry is particularly attractive for in-situ analysis, due to its inherent speed, excellent sensitivity, molecular selectivity, and capability for continuous real-time measurements. A typical mass spectrometer comprises an ion source, wherein the ions are generated; a mass filter, wherein the ions are separated in space or in time; an ion detector, wherein the filtered ions are collected and their relative ion abundance measured; a vacuum system; and means to power the spectrometer. Depending on the type of sample and the method of introducing the sample into the mass spectrometer, ions can be generated in the ion source by electron impact ionization, photoionization, thermal ionization, chemical ionization, desorption ionization, spray ionization, or other processes. Mass spectrometers are generally classified according to the method on which mass filtering is accomplished using electric and/or magnetic fields. Mass filter types include magnetic-sector, time-of-flight, linear quadrupole, ion cyclotron resonance, and ion traps. Detection of ions is typically accomplished by a single-point ion collector, such as a Faraday cup or electronic multiplier, or a multipoint collector, such as an array or microchannel plate collector, whereby all of the ions arrive at the collector simultaneously.
Mass spectrometer performance is generally given in terms of mass range, resolution (i.e., resolving power), and sensitivity of the instrument. Mass range is the lowest and highest masses that can be measured. A large mass range is desired for the analysis of high molecular weight organic and biological analytes. Resolution measures the ability of the instrument to separate and identify ions of slightly different masses. Typically, the resolution for singly charged ions is given by
where m is the mass of an ion peak in atomic mass units and Δm is the width of the peak at some peak height level (e.g., half peak height). In many cases, the minimum resolution required is such that a molecular ion can be resolved from an adjacent peak having a unit mass difference. According to this requirement, the resolution R should be at least 100 for a chemical species having a nominal mass of 100. High-resolution instruments, required for organic mass spectrometry, can detect peaks separated by fractions of a mass unit. Sensitivity is a measure of the instrument's response to ions of an arbitrary m/z ratio for a particular sample. Sensitivity is typically a function of the efficiency of the ion source and ion detector, as well as the analyzer method used. The sensitivity limit, or detection limit, is the minimum amount of a sample that can be detected under a given set of experimental conditions and distinguished from the instrument noise level and background. Resolution and sensitivity are approximately inversely related to each other. Other important characteristics of a spectrometer instrument include overall size, operating pressure, voltage, and power consumption.
Mass spectrometers can be used for chemical sensing. A pulse of sample gas can be injected directly into the ion source of the mass spectrometer. However, analyzing mixtures may be difficult when the mass spectrometer is used alone, since the resulting mass spectrum would be a complex summation of the spectra of the individual components. Therefore, analytical techniques combining the separation methods of gas chromatography and mass spectrometry are often used for chemical sensing. A gas chromatograph (GC) separates volatile mixtures into their component chemical species, which are eluted from a long capillary. The eluents can then be transferred into a mass spectrometer to obtain a mass spectrum of each of the separated components, from which the molecular structure of the individual component species can be inferred. The GC/MS is therefore capable of separating highly complex mixtures, identifying the components, and quantifying their amounts. Alternatively, tandem (MS/MS) or multistage (MSn) mass spectrometers can be combined, wherein one of the mass spectrometers is used to isolate individual ions according to their m/z ratio, and the other is used to examine the fragmentation products of the individual ions. Thus, multiple stages of mass analysis can be obtained in a single analyzer.
Recently, there has been a growing interest in miniature mass spectrometers that enable reduced size, power requirements, vacuum system demands, cost, and complexity. In particular, a microfabricated mass spectrometer combined with a microfabricated GC would enable a very attractive portable, handheld microanalytical system. Such a “chemical laboratory on a chip” would enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents in the field.
Because magnetic separators do not scale well into the microstructure size range, most microfabricated mass filters have performed separation using time-varying electric fields (i.e., Paul-type traps). The 3D quadrupole ion trap (QIT) consists of two hyperbolic endcap electrodes with their foci facing each other and a hyperbolic ring electrode halfway between the two endcap electrodes. Ions are trapped in the space between these three electrodes by RF and DC electric fields. The QIT is well suited to miniaturization due to pressure intolerance and low voltage and power requirements. Cylindrical ion traps (CIT), comprising flat endcap electrodes and a cylindrical ring electrode, have much simpler geometries than the QIT. Massive arrays of such CITs have been fabricated on a micro-scale. See U.S. Pat. Nos. 6,870,158 and 7,154,088. However, such CITs are difficult to fabricate on the micro-scale, require high RF driving powers, and require unconventional ion detection schemes due to their relatively closed geometry.
The linear Paul ion trap is a two-dimensional version of the 3D QIT. A quadruple mass spectrometers based on the conventional linear Paul trap filters ions by passing them through tuned radiofrequency (RF) and direct current (DC) electrical fields defined by four symmetrically-spaced parallel rods within which a 2D quadrupole field is established. The QMS permits only those ions with a stable trajectory, determined by their m/z ratio, to travel along the entire length of the central axis of the rod assembly without being deflected out of the intra-rod space. Ions with different m/z ratios can be scanned through the QMS by continuously varying the field between the quadrupole rods. Therefore, the QMS is a variable bandpass filtering ion optic.
The QMS is relatively pressure tolerant and can operate effectively at relatively high pressures (e.g., 10−4 Torr). Therefore, they are amenable to miniaturization due to the avoidance of bulky vacuum pumping systems. Miniature linear quadrupoles require lower drive voltages and higher RF drive frequencies to filter heavier ions and maintain resolution as the electrode dimensions decrease. Further, the linear ion trap has much greater efficiency for trapping externally generated ions and much greater ion trapping capacity, due to reduced space charge effects, than 3D QITs of the same volume. However, miniaturized linear ion traps require precise fabrication to maintain acceptable performance. See Z. Ouyang et al., Eur. J. Mass Spectrom. 13, 13 (2007).
Recently, there has been interest in microfabricated versions of the linear ion trap for quantum computing (QC). To increase the speed of QC operations, there has been an effort to reduce the size of trap dimensions. This has lead to the development of a microfabricated surface-electrode ion trap analog of the conventional four-rod Paul ion trap. With the surface-electrode ion trap, all of the trap electrodes reside on a single surface, with alternating flat RF and DC control electrodes, and the ions are trapped above the surface plane. Ions are confined transversely to the null ion trap axis of the RF quadrupole electric field and axial confinement and ion transport operations are controlled by the DC electric field. For QC applications, the control electrodes can be segmented along their length and appropriate static potentials can be applied to the different segments to enable longitudinal confinement of ions in a single trapping location and transportation of the ions between trapping locations. However, ion trap depths of the surface-electrode trap are very shallow, not much above the mean kinetic energy of the ions. Therefore, ion injection and trapping with a thermal ion source can be problematic. However, shallow pseudopotential depths are tolerable for quantum computing applications which use laser cooling to initialize the internal state of the ions. See J. Chiaverini et al., Quantum Inf. Comput. 5, 419 (2005); S. Seidelin et al., Phys. Rev. Lett. 96, 253003 (2006); and C. E. Pearson et al., Phys. Rev. A 73, 032307 (2006).
Many of the requirements for micro-scale ion traps for mass spectrometry differ from those for quantum computing. The basic requirements for a useful micro-scale ion trap for mass spectrometry are: an ionization method that enables ionizing many different atomic and molecular species such that nearly all component species in the sample can be ionized for analysis; an ability to trap many target species to enable species identification and the determination of relative concentrations of all of the components in the sample; and an ability to trap a large quantity of ions to enable direct detection of the ions with adequate sensitivity for analysis.
The present invention is directed to a microfabricated linear Paul-Straubel ion trap array that satisfies these requirements for mass spectrometry in a small package.
The present invention is directed to a microfabricated linear Paul-Straubel ion trap, comprising a substrate having an opening therethrough; a dielectric layer on the substrate, and two parallel inner RF electrodes disposed on the dielectric layer and suspended over the opening in the substrate, wherein an RF quadrupole electric field potential well is established on a trap axis in the plane between the RF electrodes when a radiofrequency potential is applied to the RF electrodes. Each RF electrode has a first end and a second end and the first ends can be electrically connected and the second ends can be electrically connected to form a rectangular structure. The ion trap can further comprise two parallel outer DC control electrodes deposed on opposite sides of the two parallel inner RF electrodes and symmetrically about the trap axis in the plane of the RF electrodes. At least one additional ion trap, comprising two additional parallel inner RF electrodes and two parallel outer DC control electrodes deposed on opposite sides of the additional parallel inner RF electrodes and symmetrically about the additional trap axis, wherein one of the outer DC control electrodes is a common DC outer electrode with a first trap, can form an array of ion traps.
The planar micro ion trap array is specifically designed for mass spectrometry applications. Microfabrication techniques can be used to fabricate the planar rectangular Paul-Straubel-type ion trap over an opening in a substrate, enabling ionization, trapping, ion accumulation, and ion ejection that can be used for detection of atomic and/or molecular ions. The substrate can comprise silicon, the dielectric layer can comprise silicon nitride, and the electrodes can comprise tungsten.
The accompanying drawings, which are incorporated in and form part of the specification, illustrate the present invention and, together with the description, describe the invention. In the drawings, like elements are referred to by like numbers.
Any system that produces a point with a vanishing electrode field can potentially trap ions. The Paul-Straubel ring trap, consisting of a hollow ring and a pair of distant compensation electrodes, has been demonstrated to trap ions with a more open structure than the conventional 3D QIT. A linear version of the 3D Paul-Straubel ring trap consists of a pair of rods driven by the same RF applied voltage. This electrode configuration has a zero field line midway between the two rods. See N. Yu and W. Nagourney, J. Appl. Phys. 77(8), 3623 (1995); G. R. Janik et al., J. Appl. Phys. 67(10), 6050 (1990); and C. A. Schrama et al., Optics Comm. 101, 32 (1993).
The present invention is directed to a microfabricated planar version of the two-rod Paul-Straubel ion trap comprising two inner parallel electrodes, forming a rectangular electrode structure, driven by an RF applied potential. Two outer parallel compensation electrodes on a common plane are driven by a DC control potential. A zero field line forms between the RF electrodes to provide an ion trapping zone in the plane. In a typical mass spectrometry application, ions are created in situ via electron bombardment and the resulting ions are ejected towards an ion detector. Alternatively, the ion trap can comprise three- or five-electrodes, similar to the symmetric surface-electrode traps used for quantum computing. However, ions are trapped above the center DC electrode in such ion traps, which can make ion ejection from the traps difficult when used for mass spectrometry.
Planar arrays of such ion traps can be fabricated on a substrate by micromachining techniques. The ion trap array provides a reasonably large trapping volume in a very small package. In particular, the relatively simple planar geometry is easy to fabricate and the common DC electrodes enables a scalable array comprising a plurality of ion traps. Scalability is important for mass spectrometry applications, since sensitivity is proportional to the number of ions trapped and detected.
The microfabricated ion trap array can be used in mass spectrometry applications. The ions can be created, accumulated and stored in the ion trap array. The ions can then be ejected as a function of their mass. In a single-stage mass spectrometer, the resulting ions can be ejected to an ion detector. In a multistage mass spectrometer, the ejected ions can be fed to a second-stage mass spectrometer, such as a quadrupole, time-of-flight (TOF) or magnetic sector instrument. A microfabricated array-based mass spectrometer preferably has reasonable sensitivity, mass range and mass resolution for the desired application while maintaining a small volume and low power requirement. When used with a chemical laboratory on a chip, a preconcentrator can be used to collect the sample of interest. The preconcentrator can release the collected sample as a pulse directly into the ion trap array or into an intervening GC column. The sample gas can be ionized in the trapping volume of the trap array. Ionization can occur for a period of time, in which the ions can accumulate in the traps of the array. After accumulation, the ions can be ejected from the trap as a function of the mass-to-charge ratio of the ions and detected directly by an ion detector, such as a microchannel plate (MCP).
A linear Paul-Straubel ion trap array, comprising a plurality of micro-scale linear Paul-Straubel ion traps, can be monolithically fabricated on a substrate by surface micromachining techniques generally known to the IC manufacturing and MEMS industries. Preferably, the ion trap array can be fabricated using a metal damascene process. Metal damascene is an inlaid process in which a trench is formed in a surface layer and a metal overfill is deposited into the trench. Preferably, the metal damascene process comprises a molded tungsten process. A chemical-mechanical-polishing (CMP) step is used to re-planarize the surface and isolate the metal in the trench. In
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Proof-of-concept experiments were conducted to demonstrate the ability of the ion trap array to trap room temperature ions. The ion trap array comprised seven parallel linear Paul-Straubel ion traps suspended over an opening in a silicon chip that was fabricated according to the method described above. The opening in the chip was 2.2 mm by 3.8 mm. The electrodes were suspended over the opening to allow for ejection of the ions from the trapping zones for ion detection. Along the axial direction of the trap, each RF and DC electrode was approximately 2.2 mm in length, 10 microns in width, and 2 microns in thickness. Each of the seven trapping zones had an RF electrode to RF electrode separation of 100 microns and DC electrodes spaced 75 microns outside of the RF electrodes. Alternatively, the array can comprise asymmetric traps (arrays in which the RF electrode separations differ) to alter the m/z trapping properties of the traps on a chip with a common RF applied voltage. The seven trapping zones share DC electrodes, with the exception of the DC electrodes of the outer most trapping zones which are not shared. The applied RF voltage was 100 V amplitude (RF ground to maximum) and the resonance frequency was 27 MHz. The DC electrodes were held at RF ground.
The target gases used in these experiments were krypton and nitrogen. Krypton has an atomic mass of approximately 84 and contains several nuclear isotopes. Nitrogen is a diatomic molecule of mass 28. The target gas was introduced to the ion trap array from a research grade lecture bottle via a gas handling manifold and an adjustable leak valve. Before introduction of the target gas, the vacuum chamber had a base pressure of approximately 2×10−7 Torr consisting primarily of residual hydrogen, water, nitrogen, oxygen and carbon dioxide, as measured using a residual gas analyzer (RGA). A typical RGA scan of the chamber without a target gas present is shown in
Electron impact ionization was used to produce ions from the target gas. Electron impact ionization is a reasonably efficient method of ionization that is general enough for ionizing most atoms and molecules. Tabulations of the electron impact ionization cross-sections and fragmentation species exist for many atoms and molecules.
Since the ion trap depth is very shallow, the ions must be produced inside the trapping volume of the ion traps to be successfully trapped. The trapping volume is determined by the geometry of the traps and the applied RF and DC voltages. In these experiments, ions were produce with the RF voltage being applied to the trap. The presence of the RF alters the effective energy of the ionizing electrons, thereby affecting the ion production rate. A simple model was developed to illustrate the effect of RF voltage on the ion production rate. In this model, the energy of the electron beam was 150 eV, the RF amplitude was 50 V and frequency was 50 MHz, the target gas partial pressure was 1×10−6 Torr, and ionization length was 1 mm.
A series of steps were taken to ensure electrons can ionize the target gas inside of the trapping volume. First, the electron beam was focused such that the focal point of the beam was at the trap chip opening. Second, the electron beam was rastered and the count rate of the CEM was monitored to determine the best x and y deflections to center the electron beam in the trap. Finally, the rastering of the electron beam was repeated under differing beam focus conditions to determine a focus condition that flooded the majority of the trap with electrons. Shown in
A 150 eV electron beam was used in these experiments that was focused in a manner such that a large portion of the trap was in the electron beam path. Additionally, deflection voltages were determined to center the beam on the trap. For these experiments the electron beam was ‘on’ for 500 microseconds. Ions were produced during the ‘on’ period and some fraction of the ions produced were trapped. After the 10 microsecond ‘on’ period, the electron beam was turned ‘off.’
A helical resonator circuit was built to effectively couple RF power to the ion trap array. For these experiments, an arbitrary waveform generator supplied a sine wave to a RF amplifier that fed the amplified RF to a helical resonator. The arbitrary waveform generator was configured to deliver a 27 MHz sine wave with amplitude of 0.3 V peak-to-peak. The RF power amplifier was set to deliver a RF signal to the helical resonator with 1 watt of power and an extraction pulse of +500 V was applied across a pair of grids surrounding the trap to extract ions for detection. The relationship between the voltage applied to the ion trap electrodes, the Q of the trap system, and the RF power applied to the resonator is:
V0-peak=α√{square root over (PQ)}
12≦α≦25
where V is the amplitude of the applied RF, P is the applied power in watts, Q is the quality factor, and α is a value that ranges from 12 to 25, as found in the literature. The voltage applied to the RF electrodes of the trap is an important consideration for both the trap depth, which determines the trapping stability for the various m/z ions species, and for the ion production rate.
After a period of time in which the ions were created and accumulated in the trap, a high voltage pulse was applied to a grid above the trap forcing the ejection of ions toward the CEM detector. The time-of-flight of the charged particles from the trap to the detector is a function of the mass-to-charge ratio of the ions and the voltage of the extraction pulse applied to the grid. The pulse width of the extraction pulse was 10 microsecond with an extraction voltage of +500 V having an approximately 100 ns rise time. The transmission efficiency of the extraction grid was approximately 77.4%.
The ejected ions were detected with the CEM operating in a single particle detection mode. Each particle that impinged on the detector resulted in one count in a timing bin collected on a multi-channel scalar. The various timing bins of the multi-channel scalar are a discrete representation of the time-of-flight of the ions from the time the extraction pulse is applied to the time of ion detection. In these experiments, each bin was 5 ns wide and 1024 bins were scanned sequentially for each repetition of the experiments.
The sequences of events for a given experimental run was as follows. Electrons were passed through the trap for 500 microseconds. During this time, the target gas was ionized and the trap RF power was on, causing the ions to accumulate in the trap. After the electron beam was turned off, the RF remained on for a period of time greater than 100 microseconds. During this period, the trapped ions were held in the trap and the non-trapped ions were allowed to neutralize. After the hold time, the RF was turned off and a 500 V extraction pulse was applied, sweeping the trapped ions from the trap towards the charged particle detector. Each ion that impinged on the detector resulted in a pulse that was counted as a function of the time-of-arrival at the detector using the CEM. This procedure resulted in a time-of-flight spectrum that demonstrates the ability of this device to trap thermal ions.
Time-of-flight spectra for krypton and nitrogen target gases were obtained using the ion trap array chip. The spectra were obtained from a typical trapping experiment as described above, whereby data was collected with trap off and trap on, whilst holding the other experimental parameters essentially constant. Background was collected with krypton in the system, with the electron gun on, and with the extraction pulses operating, but with no RF potential applied to the trap. This procedure allowed the background signal from the small number of non-trapped residual ions to be removed from the trapped ion signal that was obtained under the same conditions but with RF applied to the trap. The background signal was primarily random noise. The statistical significance of the ion peak in the spectra remained after the background subtraction.
The present invention has been described as a microfabricated linear Paul-Straubel ion trap. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.
This application claims the benefit of U.S. Provisional Application No. 60/982,160, filed Oct. 24, 2007, which is incorporated herein by reference.
This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.
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| Number | Date | Country | |
|---|---|---|---|
| 60982160 | Oct 2007 | US |
