WO2005031287A2

WO2005031287A2 - Method and apparatus employing turbo pump-foreline pump configuration, for example, in mass spectrometer

Info

Publication number: WO2005031287A2
Application number: PCT/US2004/031450
Authority: WO
Inventors: Adi A. Scheidemann
Original assignee: Oi Corporation
Priority date: 2003-09-25
Filing date: 2004-09-24
Publication date: 2005-04-07
Also published as: WO2005031287A3

Abstract

A vacuum system for an analytical instrument such as a mass spectrometer employs a turbo pump-foreline pump configurations, where a volume between the turbo-drag pump and the foreline pump is sufficiently large to maintain the operation of the turbo-drag pump during periods when the foreline pump in an OFF state or in a low speed operation, thereby conserving power.

Description

METHOD AND APPARATUS EMPLOYING TURBO PUMP-FORELINE PUMP CONFIGURATION, FOR EXAMPLE, IN MASS SPECTROMETER

BACKGROUND OF THE INVENTION

Field of the Invention This disclosure is generally related to analytical instruments employing a vacuum, and more particularly to pumps to create vacuum (i.e., near vacuum) conditions in an enclosure, such as a vacuum chamber of a mass spectrometer.

Description of the Related Art Portable mass spectrometry is a very powerful analytical tool that has not seen widespread application, at least in part due to the high power consumption of the vacuum system needed to run the mass spectrometer. Providing vacuum for a portable mass spectrometer, with low power consumption is a major technical challenge. Current instruments (such as Inficon's Hapsite) employ a vacuum system that includes: 1) a membrane inlet system to reduce the gas load by holding back the N2 fraction in air, and thus to reduce the gas load on the pump; 2) a vacuum container housing that houses the mass spectrometer (MS); and 3) high vacuum pumps such as a non-evaporative-getter pump (NEG). While such a system has the advantage of being robust and light weight, the NEG pump needs to be recycled frequently due to saturation of the NEG cartridge. For this recycle process, the vacuum container is connected to a docking station, vacuum is provided via a turbo pump, and the NEG pump/cartridge is heated to release the gases trapped in the NEG pump. This recycle process presents a significant drawback to successful commercial use of the MS devices employing such vacuum systems. Turbo pump-foreline pump configurations are known, both with and without a membrane inlet system (e.g., Tim Short and David Fries' work on underwater mass spectrometry with battery powered remote operated MS instruments are an example). While such configurations have been used in the past, traditional systems consume substantial amounts of energy, hindering commercial acceptance and portability. There is a need for a small, energy efficient, and reliable pump system, suitable for use in portable instruments.

SUMMARY OF THE INVENTION While turbo pump-foreline pump configurations have been used in the past, traditional systems keep both pumps (i.e., turbo-drag pump and foreline pump) running during system operation. As will become apparent from the below description, it is not necessary to continuously run the foreline pump. Conventional turbo pump-foreline pump configurations teach the designer of such systems to use short connections (i.e., "foreline gas container") between the turbo-drag pump and foreline pump to ensure a short pump down time during start-up of the system. Thus, conventional practice teaches thus use of a foreline gas container having a relatively small volume. In contrast, a vacuum system may employ a foreline gas container having a relatively large volume, in order to allow the foreline pump to be turned OFF or slowed down during normal system operation, thereby reducing power consumption while maintaining sufficient vacuum in a vacuum chamber via the trubo-drag pump and the "buffered" vacuum. In one aspect, a vacuum system comprises a turbo pump-foreline configuration having a large volume foreline gas container between the foreline pump and the turbo-drag pump. In another aspect, a vacuum system comprises a controller that turns OFF a foreline pump from time-to-time during operation of the vacuum system. In yet another aspect, a vacuum system comprises employs a turbo-drag pump that remains in an ON state during system operation and a foreline pump that is in an ON state during a first time and an OFF state during a second time, the first and the second time occurring during operation of the system. In a further aspect, a method of operating a vacuum system comprises buffering a vacuum in a foreline vacuum container, and selectively applying the buffered vacuum to a turbo-drag pump during system operation. In yet a further aspect, a method of operating a vacuum system comprises operating a turbo-drag pump during system operation, and selectively operating a foreline pump during system operation such that the foreline pump is in an OFF state for some period during system operation.

BRIEF DESCRIPTION OF THE DRAWINGS 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 vacuum system employing a turbo pump-foreline configuration, and incorporated into a mass spectrometer system.

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 systems, pump, valves, and pressure reducers 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 vacuum system 10 employing a turbo pump- foreline pump configuration. The vacuum system 10 system includes a sample inlet system 12, means to reduce gas flow 14, vacuum chamber 16, turbo-drag pump 18, foreline gas container 20, optional valve 22, mechanical foreline pump 24, high vacuum gauge 26, pressure gauge 28, exhaust 29, and controller 30. The vacuum system 10 may be part of a mass spectrometer 32, including an ion source 34 (e.g., electro-spray, atmospheric pressure ionization), with transfer optics, to provide charge particles in the vacuum chamber 16, a magnet (electromagnet, permanent magnet) 36 positioned to deflect the path 38 of the charged particles in the vacuum chamber 16, and an ion detector 40 (e.g., see US Patent No. 6,576,899) positioned to detect the position of the deflected particles in the vacuum chamber 16. In particular, the sample inlet system 12 may take the form of a basic sniffer tube to transport the sample gas to the gas flow reduction means

14, or may take the form of a fan blowing gas over the gas flow reduction means 14 to ensure that representative samples of the ambient environment are taken. The gas flow reduction means 14 may include a membrane to hold back N2 while allowing volatile organic vapors to pass into the mass spectrometer 32, and a gas flow reducing element. The gas flow reducing element may, for example, take the form of capillary tubing, flow restricting valve, and/or pressure reducer. The vacuum chamber 16 will typically contain the particles to be tested by the instrument. For example, in a mass spectrometer, the vacuum chamber 3 is where the charged particles (i.e., ions) pass the magnet 36 and impact the detector 40. The turbo-drag pump 18 maintains the vacuum condition in the vacuum chamber 16, for example, a vacuum condition sufficient to allow a free path flight of charged particles in the mass spectrometer 32. The foreline gas container 20 may take the form of a tube or container connecting the mechanical foreline pump 24 to the turbo-drag pump 18. As discussed in detail below, it is advantageous to employ a foreline gas container 20 capable of holding a relatively large volume of gas, for example greater than approximately 0.5 liter. If, the foreline vacuum container 20 is not otherwise capable of holding the vacuum when the foreline pump 24 is stopped, the system 10 may employ the valve 22, to open and close this line, accordingly. The mechanical foreline pump 24 maintains the vacuum condition in the foreline vacuum container 20 which is used to operate the turbo drag pump 18. The high vacuum gauge 26 detects the vacuum condition (e.g., level) in the vacuum chamber 16. The pressure gauge 28 detects the vacuum condition (e.g., level) in the foreline gas container 20. A suitable controller (not shown) such as a microprocessor may be coupled to the vacuum gauge 26 and/or pressure gauge 28 to monitor the vacuum condition in the vacuum chamber 16 and the foreline gas container 20, respectively, to control the foreline pump 24 accordingly. It is desirable to reduce the power used by an instrument, for example, a mass spectrometer, particularly where the instrument is intended to be portable. Modern turbo pumps have very high compression ratios. This may be taken advantage of by using large inner diameter tubing - or an intermediate vessel - to connect the foreline pump 24 to the turbo-drag pump 18, rather than a short and small volume tubing suggested by conventional designs. Thus, the volume of the foreline gas container 20 (i.e., connect the foreline pump 24 to the turbo-drag pump 18) is significantly increased over previous designs. For example, a volume of greater than approximately 0.5 liter may be suitable If both pumps are running, the pressure P in the foreline gas container 20 is determined by the gas flow rate F of the gas into the foreline gas container 20 (i.e., flow out of the turbo-drag pump 18) and the pumping speed S of the foreline pump 24, where:

P = F / S. If the foreline pump 24 is stopped, the pressure in the foreline gas container 20 rises (according to gas flow rate and volume V), once the maximum foreline pressure for the turbo-drag pump 18 is reached, the turbo- drag pump 18 stops working and gas flows back into the high vacuum recipient. (Such a "brake down" in the vacuum systems happens rather quickly in traditional instruments since the volume of the foreline gas container 20 is small and traditional turbo-drag pumps 18 require low backing pressure (e.g., < 1 Torr)). Operating the vacuum system 10 without continually running the foreline pump 24 may conserve substantial amounts of power. For example, the foreline pump 24 may be in the OFF state during approximately 50% or more of the operating time that the turbo-drag pump 18 is operating. Likewise, operating the vacuum system 10 while selectively adjusting the speed of the foreline pump 24 may also converse substantial amounts of power. In order to allow the foreline pump 24 to be turned OFF during operation of the vacuum system 10, the volume of the foreline gas container 20 is increased over existing designs. Additionally, a turbo-drag pump 18 which can operate with foreline pressures of 10Torr or even higher (e.g., Alcatel 30+), may be advantageously employed. By increasing the volume of the foreline gas container 20, we can turn the foreline pump 24 OFF during operation of the vacuum system 10, "buffering" the gas into the foreline gas container 20. Once the pressure in the foreline gas container 20 approaches the maximal operating foreline pressure (or a lower value set by the user) the foreline pump 24 will re-start and evacuate the container 20. The time constant T in this filling is given by gas flow rate F and the amount of gas AoG stored in the volume and the volume of the container. The amount of gas is given by:

AoG = P * V. Thus

T = AoG / F. As shown below, this time constant T can be long if the system 10 is designed accordingly. Operating the foreline pump 24 only when the vacuum system 10 requires the foreline pressure to be brought down, reduces the power consumption of the system 10, and any instrument in which the system is installed, for example, a mass spectrometer. The controller 30 may be a microprocessor executing a software program to turn the foreline pump 24 ON and OFF based on the vacuum condition in the foreline gas container 20 measured by the pressure gauge 28. Alternatively, the controller may be hardware based, for example, an Application Specific Integrated Circuit (ASIC). Thus the system 10 turns the foreline pump 24 ON and OFF during operation of the system, and employs an enlarged foreline gas container 20 to essentially "buffer" the vacuum. Power consumption may be reduced even further by varying the speed of the foreline pump 24, to optimized the power it takes to rough the "foreline vacuum container" back out. Some exemplary suitable values for variables associated with a portable (or transportable) instrument may be: Pressure in MS: 1 * 10E-6 Ton- Pumping speed: 20 liters / sec Gas Flow into MS: 2 * E-5 Torr * liters / sec Gas Flow out of Turbo pump = Gas Flow into MS Maximum foreline pressure desired during operation: 5 Torr Maximum foreline pressure according to manufacturer for turbo -drag pumps 10-30 Torr. Volume "Foreline gas container": 1 liter Base pressure achieved by foreline pump: 1 Torr Amount of gas to be stored in "Foreline gas container" at 5 Torr pressure: 4 Torr * liter Time to fill "foreline gas container" from base pressure to 5 Torr = Time = { Amount of gas to be stored in "Foreline gas container" } / { Gas Flow out of Turbo pump } Time = 4 Torr / 2 * E-5 Torr * liters / sec = 4 * E4 sec = 11 hours. Such a system 10 may be capable of operating for approximately one working day without turning the foreline pump 24 ON. Although specific embodiments of and examples for the reader and method of the invention 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 instruments, not necessarily the exemplary mass spectrometer 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, 20O3; 60/358,124, filed February 20, 2002; 60/116,710, filed January 22, 1999; 60/061 ,394, filed October 7, 1997; and 60/484,801 , filed July 3, 2003; 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 March 18, 2003, are incorporated herein 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 vacuum systems and methods of operating vacuum systems that operate 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

CLAIMS I/We claim:

1. A vacuum system, comprising: a foreline pump; a turbo-drag pump that is operated in part via a vacuum produced by the foreline pump; and means for reducing operational ON time of the foreline pump.

2. The vacuum system of claim 1 wherein the means for reducing operational ON time of the foreline pump, comprises: a large volume foreline gas container coupled to provide a fluid flow path between the foreline pump and the turbo-drag pump.

3. The vacuum system of claim 2 wherein the large volume foreline gas container has a volume of at least approximately 0.5 liter.

4. The vacuum system of claims 1 , 2 or 3 wherein the means for reducing operational ON time of the foreline pump, comprises: a controller that turns OFF the foreline pump from time-to-time during operation of the vacuum system.

5. The vacuum system of claims 1 , 2 or 3 wherein the means for reducing operational ON time of the foreline pump, comprises: a controller that places the foreline pump in an ON state during a first time and in an OFF state during a second time, the first and the second times occurring during operation of the turbo-drag pump.

6. The vacuum system of claim 5 wherein the controller places the foreline pump in the OFF state at least 50% of the time during which the turbo-drag pump is operating.

7. The vacuum system of claims 1 , 2 or 3 wherein the means for reducing operational ON time of the foreline pump, comprises: a controller that places the foreline pump in a high speed state during a first time and in a low speed state during a second time, the first and the second times occurring during operation of the turbo-drag pump.

8. The vacuum system of any of claims 1 , 2, 3 or 4 wherein the turbo-drag pump is coupled to provide a vacuum in an analytic device.

9. The vacuum system of any of claims 1 , 2, 3 or 4, further comprising: a vacuum chamber of a mass spectrometer, wherein the turbo- drag pump is coupled to provide a vacuum in the vacuum chamber.

10. The vacuum system of claim 9, further comprising: a first pressure sensor producing signals indicative of a pressure in the foreline gas container; and a second pressure sensor producing signals indicative of a pressure in the vacuum chamber.

1 1. The vacuum system of any of claims 2, 3 or 4, further comprising: a valve between the foreline gas container and the turbo-drag valve to selectively provide fluid communication therebetween.

12. . A method of operating a vacuum system, comprising: operating a turbo-drag pump in an ON state during a first period; during at least one portion of the first period, operating a foreline pump in a first state; and during at least another portion of the first period, operating the foreline pump in a second state, where the second state is different from the first state.

13. The method of claim 12, further comprising: buffering a vacuum produced by the foreline pump in a larve volume foreline vacuum container during the first period, and selectively applying the buffered vacuum to a turbo-drag pump during one of the portions of the first period.

14. The method of claims 12 or 13 wherein operating a foreline pump in a first state comprises placing the foreline pump in an ON state and wherein operating a foreline pump in a second state comprises placing the foreline pump in an OFF state.

15. The method of claim 14 wherein the at least one portion of period during which the foreline pump is in the ON state is less than approximately half of the first period.

16. The method of claims 12 or 13 wherein operating a foreline pump in a first state comprises placing the foreline pump in an high speed state and wherein operating a foreline pump in a second state comprises placing the foreline pump in a low speed state.

17. The method of claims 12 or 13, further comprising: determining a pressure in the foreline gas container; determining a pressure in a vacuum chamber in which the turbo- drag pump produces a vacuum condition; and determining whether to place the foreline pump in the first or the second states based at least in part on the determined pressures.

18. A processor-readable medium storing instructions for causing a processor to operate a vacuum system in accordance with any of claims 12 through 17.