WO2005022096A2

WO2005022096A2 - Mass spectrometer with light source and/or direct charge measuring

Info

Publication number: WO2005022096A2
Application number: PCT/US2004/027615
Authority: WO
Inventors: Adi A. Scheidemann
Original assignee: Oi Corporation
Priority date: 2003-08-26
Filing date: 2004-08-26
Publication date: 2005-03-10
Also published as: WO2005022096A3

Abstract

A mass spectrometer (10) replaces a filament with a light source (20), for example, a UV light source such as a UV-laser or UV-lamp to increase reliability and potentially reduce fragmentation, especially in the case of one-photon-ionization. The mass spectrometer may replace a micro-channel based position sensitive ion detection with a direct charge measuring device, such as a shift register type strip charge detector or a micro-machined faraday cup detector array.

Description

MASS SPECTROMETER WITH LIGHT SOURCE AND/OR DIRECT CHARGE MEASURING

BACKGROUND OF THE INVENTION

Field of the Invention This disclosure is generally related to analytical instruments, and particularly to mass spectrometers and mass spectrometer/gas chromatograph combinations.

Description of the Related Art Mass spectrometry (MS) is widely used in many applications ranging from process monitoring to life science applications. Over the course of the last 60 years a wide variety of instruments have been developed. Some aspects of MS are discussed in A.O. Nier, D. J. Schlutter, Rev. Sci. Instrum. 56(2):214-219, 1985; T.W. Burgoyne et. al., J. Am. Soc. Mass Spectrum 8:307- 318, 1997. Recently, the focus of new instrument development is two fold: on one hand a push for ever higher mass range with high mass resolution and MS/MS capability; and on the other hand small / desktop MS systems. MS instruments are often coupled with a gas chromatograph (GC) for analysis of complex mixtures. This is especially the case for analysis of volatile organic compounds (VOCs) and semi-VOCs compounds. These MS instruments have in common, a gas inlet systems (the GC would be part of this), an electron impact based ionizer (El) with ion extractor, some optic elements to focus the ion beam, ion separation, and ion detection, lonization can also be carried out via chemical ionization. Typical mass spectrometry applications, outside of laboratory settings, may include environmental monitoring of workplaces or other closed environments, or monitoring of process streams, such as discussed in Hans- Joachim Hϋbschmann, Handbook of GC/MS, Wiley-VCD, 2001. High reliability of the GC/MS or MS systems is paramount for such applications. Ion separation can be performed in the time or spatial domain. An example for mass separation in the time domain is a time of flight mass spectrometer. An example of spatial separation is seen in the commonly used quadrupole mass spectrometer. Here the "quadrupole filter" allows only one mass/charge ratio to be transmitted from the ionizer to the detector. A full mass spectrum is recorded by scanning the mass range through the "mass filter". Other spatial separation is based on magnetic fields were either the ion energy or the magnetic field strength is varied. Again the mass filter allows only one mass/charge ratio to be transmitted at a time and a spectrum can be recorded by scanning through the mass range. In current MS systems, the gas to be analyzed is injected into the ionizer, ionized through electron impact ionization, mass separated (for example in a quadrupole filter) and detected with a multiplier based detector. Such systems have reached a high level of sensitivity and mass resolution. Major drawbacks of current MS systems include the necessity of providing a (glowing) filament and the necessity of providing a high vacuum for the electron multiplier. Filaments tend to burn out -especially if vacuum condition change-, and further they degrade over a period of time and are cumbersome to replace. Ion detection for sensitive instruments is typically performed with an electron multiplier. These multipliers required high vacuum, have a limited life time and a loss of gain during their life time. Another component in the mass spectrometer that has traditionally been prone to failure is the high vacuum pump.

BRIEF SUMMARY OF THE INVENTION The present disclosure addresses these issues to provide a mass spectrometer with extremely high reliability, ideally suited for various monitoring applications. In one aspect, the filament may be replaced with a light source, for example, a UV light source such as a UV-laser or UV-lamp. The light source is much more reliable than a glowing filament and potentially includes the advantage of less fragmentation, especially in the case of one-photon- ionization. In another aspect, the micro-channel based position sensitive ion detection may be replaced with a direct charge measuring device. Such as device could be a shift register type strip charge detector or a micro-machined faraday cup detector array.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. Figure 1 is a schematic diagram of a mass spectrometer system according to one illustrated embodiment, the mass spectrometer system comprising an ionizer and extractor, optics, magnet, position sensitive ion detector, illumination source for providing light or photons to the ionizer, and further illustrating a data collection and processing system. Figure 2 is a schematic diagram of a mass spectrometer system in conjunction with a gas chromatograph and a vacuum system according to one illustrated embodiment. Figure 3 is a schematic diagram of a mass spectrometer system in conjunction with a gas inlet system and a vacuum system according to one illustrated embodiment. Figure 4 is a schematic diagram of a mass spectrometer system with gas inlet system and a reduced vacuum system according to one illustrated embodiment. DETAILED DESCRIPTION OF THE INVENTION In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with mass spectrometers, vacuum pumps, ion sources, silicon fabrication, transfer optics and electro static sector analyzers have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including, but not limited to." Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Further more, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention. Figure 1 shows a mass spectrometer system 10 according to one illustrated embodiment. The mass spectrometer system 10 comprises an ionizer and ion extractor 12, transfer optics 14, magnet 16, position sensitive ion detector 18 and illumination source 20. The illumination source 20 provides light or photons 22 to the ionizer and ion extractor 12 to ionize molecules and produce an ion beam. Suitable illumination sources 20 may include a lamp or LASER. The transfer optics 14, such as an electro-static sector, focuses the ion beam as is commonly understood in the art. The magnet 16 is positioned along the flight path of the ions, and causes deviation in the flight paths of the ions based on the charge of the particular ions. The position sensitive ion detector 18 detects ions and hence the deviation in the flight path of the ion. Suitable position sensitive ion detectors 18 may include, for example, a Faraday cup detector array or a strip charge detector with a Charge Coupled Device (CCD) type shift register. The mass spectrometer system 10 may include, or may be coupled to, a data collection interface 24 to receive data from the position sensitive ion detector 18, and to provide collected data to a data processor such as a programmed general purpose computing system 26 via a bus 28. The computing system 26 to control the instrument, to handle the data flow, and to analyze the results is not discussed in detail in this interest of brevity since such computing systems are an integral part of any mass spectrometer system 10. Figure 2 shows another illustrated embodiment of a mass spectrometer system 10. This alternative embodiment, and those alternative embodiments and other alternatives described herein, are substantially similar to previously described embodiments, and common acts and structures are identified by the same reference numbers. Only significant differences in operation and structure are described below. In particular, Figure 2 shows the mass spectrometer system 10 positioned in a vacuum chamber 30 that receives a gas flow 32 from a gas chromatograph 34 via a transfer system 36. The transfer system 36 may, for example, take the form of headed capillary tubing to connect the gas chromatograph 34 to the ionizer 12 of the mass spectrometer system 10. In some embodiments, the illumination source 20 is located inside the vacuum chamber 30, while in other embodiments the illumination source 20 may be located outside of the vacuum chamber 30. A high vacuum pump 38 and a foreline vacuum pump 40 produce the vacuum in the vacuum chamber 30. In some embodiments, the high and foreline vacuum pumps 38, 40 can be formed as an integrated unit. Alternatively, the high and foreline vacuum pumps 38, 40 can be formed as a single pump with sufficient pumping capacity and end pressure. Figure 3 shows the mass spectrometer system 10 positioned in a vacuum chamber 30 that receives a gas flow 32 via a gas flow pressure reduction device 42. the gas flow pressure reduction device 42 is particularly useful where the gas is sampled from a source having a higher pressure than the pressure in the vacuum chamber 30 of the mass spectrometer system 10. Thus, the gas flow needs to be adjusted to the meet the requirement of the mass spectrometer system 10. This gas flow reduction can be achieved through - capillary tubing, valves, pressure regulator, an intermediate chamber with additional pump (split-flow), or a combination of these elements. The gauges and/or transducers typically associated with measuring high vacuum and/or foreline vacuum levels are not illustrated in the interest of clarity since such elements will be apparent to those skilled in the art. Figure 4 shows the mass spectrometer 10 positioned in a vacuum chamber 30 that advantageously eliminates the high vacuum pump 38 from the embodiments of Figures 1-3. In operation, a gas is introduced into the vacuum chamber 30. The gas may or may not have undergone separation in a gas chromatograph34 (Figure 2). The ionizer and ion extractor 12 ionizes the gas employing light such as UV light from the illumination source 20, and extracts the resulting ions. Optional transfer optics 14 (e.g., electrostatic analyzer) may focus the resulting ion stream. The ion stream is injected into a magnet field produced by magnet 16 for mass separation, which deflects the flight paths of the ions. The position sensitive ion detector 18 (see Figure 1) detects the deflected ions. The concept of a double focusing mass spectrograph was first introduced by Mattauch and Herzog (MH) in 1940 and discussed more fully at J. Mattauch, Ergebnisse derexakten Naturwissenschaften, vol. 19, pages 170- 236, 1940. Double focusing refers to the instrument's ability to refocus both the energy spread as well as the spatial beam spread. Modern developments in magnet- and micro machining technologies allow dramatic reductions in the size of these instruments. The length of the focal plane ranges down to a few centimeters to build a mass spectrometer for VOC and semi-VOC. As illustrated in Figure 2, the mass spectrometer system 10 may be located inside a vacuum chamber 30. The gas to be analyzed can be separated by GC prior to the MS measurement (Figure 2) or, the gas may be directly injected into the MS (Figure 3) without separation. The ion detector 18 in a Mattauch- Herzog lay-out is a position sensitive detector. Numerous concepts have been developed over the last decades. Recent developments focus on solid state based direct ion detection as an alternative to previously used electro optical ion detection (EOID). In traditional instruments, an electro-optical ion detector converts «. the ions in a multi-channel-plate (MCP) into electrons, amplifies the electrons (in the same MCP), and illuminates with the electrons (emitted from the MCP) on a phosphorus film. The image formed on phosphorus film is recorded with a photo diode array via a fiber optic coupler (see US patent US5801380). Such a multiplier based detector has the above mentioned disadvantage of requiring a high vacuum (P<10E-5 Torr) for operation. A direct charge measurement can be based on micro-machined Faraday cup detector array. In particular, an array of individually addressable Faraday cups monitors the ion beam. The charge collected in individual elements of the array is provided to an amplifier via a multiplexer unit. This layout reduces the number of amplifiers and feedthroughs needed. This concept is described in further detail in A.A. Scheidemann, R.B. Darling, F.J. Schumacher, and A. Isakarov, Tech. Digest of the 14th Int. Forum on Process Analytical Chem. (IFPAC-2000), Lake Las Vegas, Nevada, Jan. 23-26, 2000, abstract I-067 and R.B. Darling, A.A. Scheidemann, K.N. Bhat, and T.C. Chen, Proc. of the 14th IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS- 2001), Interlaken, Switzerland, Jan. 21-25, 2001 , pp. 90-93 (see attachment). Alternatively - especially for low energy ions - a flat metallic strip (referred to as a strip charge detector (SCD)) on a grounded and insulated background can be used to monitor the ion beam. Again the charge is provided to the amplifier via a multiplexer. In a Mattauch-Herzog layout the detector array, composed of either Faraday cup detector array or strip charge detector, has to be placed at the exit of the magnet. This position is commonly referred to as the "focal plane". The Faraday cup detector array (FCDA) can be made in deep reactive ion etching DRIE or in case of the strip-charge detector (SCD) through vapor deposition. The dice with the active element (FCDA or SCD) is cut out of the wafer with conventional techniques such as laser cutting or sawing. While the concept of the Mattauch-Herzog configuration is well- known, the instrument proposed herein may have include one or both of two major changes not seen in the currently used instruments: 1. The proposed instrument may replace the filament with a light source. Light sources could be a UV-laser or UV-lamp. These sources can be inside or outside of the vacuum system. The light source is much more reliable than a glowing filament and potentially includes the advantage of less fragmentation, especially in the case of one- photon-ionization. 2. The proposed instrument may replace the micro-channel based position sensitive ion detection with direct charge measuring device. Such as device could be a shift register type strip charge detector or a micro-machined faraday cup detector array. Mass spectrometers operate under vacuum; the vacuum is maintained for a number of reasons: to ensure the filament does not burn out, to provide the required vacuum for the electron multiplier to avoid oxidation and discharges, and finally to provide a collision free flight path for the ions from the ionizer and ion extractor 12 to the ion detector 18. Of these three reasons only the last point (mean free path) applies to the mass spectrometer system 10 described herein since both ionizer and ion extractor 12 and ion detector 18 can be operated in air. Since a collision free flight path is desirable, reducing the flight path length would allow raising the gas pressure in the vacuum. Thus such a mass spectrometer system 10 could be operated under vacuum conditions maintained with a simple mechanical pump, such as the pumps routinely used in foreline applications (see Figure 4). The cost benefits are significant. In this respect it is noted that high vacuum pumps have traditionally been prone to failure, although relatively costly modern turbomolecular pumps have reached a high level of reliability if properly operated. Although specific embodiments of and examples for the mass spectrometer are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. The teachings provided herein of the invention can be applied to other analytical instruments, not necessarily the exemplary mass spectrometer, and the mass spectrometer/gas chromatograph combination generally described above. The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including but not limited to commonly assigned US provisional patent applications Serial Nos. 60/484,801 , filed July 3, 2003; 60/468,780, filed May 7, 2003; 60/358,124, filed February 20, 2002; 60/116,710, filed January 22, 1999; and 60/061 ,394, filed October 7, 1997, and U.S. nonprovisional patent application Serial Nos. PCT/US98/21000, filed October 6, 1998; PCT/US99/23307, filed October 6, 1999; 09/325,936, filed June 4, 1999; 09/744,360, filed January 22, 2001 ; and PCT/US03/05517, filed February 20, 2003, are incorporated by reference in their entirety. Aspects of the invention can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments of the invention. These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all analytical instruments that operated in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.

Claims

1. A mass spectrometer, comprising: a vacuum chamber; a non-glowing filament illumination source; an ionizer coupled to receive light from the illumination source to ionize molecules; a magnetic assembly positioned to affect a flight path of the ions from ionizer; and an ion detector positioned to detect ions having flight paths deflected by the magnetic assembly.

2. The mass spectrometer of claim 1 wherein the ion detector is a direct charge measuring device positioned to detect ions having flight paths deflected by the magnetic assembly.

3. The mass spectrometer of claim 1 wherein the magnetic assembly is positioned to deflect a flight path of the ions ejected into a vacuum of the vacuum chamber from the ionizer.

4. The mass spectrometer of claim 2 wherein non-glowing filament illumination source comprises an ultraviolet illumination source to produce light in an ultraviolet portion of the electromagnetic spectrum.

5. The mass spectrometer of claim 2 wherein non-glowing filament illumination source comprises an ultraviolet laser to produce coherent light in an ultraviolet portion of the electromagnetic spectrum.

6. A method of operating a mass spectrometer, comprising: providing at least a partial vacuum in a vacuum chamber; providing light from a non-glowing filament illumination source to an ionizer; ionizing molecules; ejecting the resulting ions into the vacuum of the vacuum chamber; deflecting a flight path of the ions ejected into the vacuum of the vacuum chamber from the ionizer; and detecting a position of the ions having the deflected flight paths.

7. The method of claim 6 wherein detecting a position of the ions having the deflected flight paths comprises detecting the position of the ions with a direct charge measuring device.

8. The method of claim 6 wherein providing light from a non- glowing filament illumination source to an ionizer comprises operating an ultraviolet illumination source to produce light in an ultraviolet portion of the electromagnetic spectrum.

9. The method of claim 6 wherein providing light from a non- glowing filament illumination source to an ionizer comprises operating an ultraviolet laser to produce coherent light in an ultraviolet portion of the electromagnetic spectrum.