Test of a miniature double-focusing mass spectrometer for real-time plasma monitoring

Transcription

1 trends in analytical chemistry, vol. 21, no. 8, Test of a miniature double-focusing mass spectrometer for real-time plasma monitoring Jorge Andres Diaz*, Andres E. Mora Vargas Escuela de Física, Universidad de Costa Rica and Centro Nacional de Alta Tecnología (CENAT), San José, Costa Rica Franklin Chang Diaz, Jared P. Squire, Verlin Jacobson, Greg McCaskill Advanced Space Propulsion Laboratory (ASPL), JSC-NASA, Houston, TX, USA Henry Rohrs, Rajiv Chhatwal Mass Sensors Inc., St. Louis, MO, USA A miniature double-focusing mass spectrometer with an 8 mm radius was tested at the Advanced Space Propulsion Laboratory (ASPL) at NASA Johnson Space Center (JSC). The purpose of these experiments was to provide residual gas-analysis and single-ion-monitoring capabilities for the vacuum chamber where NASA s new plasma propulsion system, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), is being developed. The instrument used here was an alpha (a) version of the Integrated Leak Detector System (ILD 50) and represents a smaller version of the original 20 mmradius Compact Double Focusing Mass Spectrometer prototype (CDFMS) developed at the University of Minnesota. The instrument successfully provided gas-composition information on the vacuum test chamber and several different plasma formulations, provided resolving powers close to the theoretical value of 40 at full width half maximum (FWHM) and demonstrated an operational mass range of 1 50 Da. # 2002 Published by Elsevier Science B.V. All rights reserved. 1. Introduction *Corresponding author. Tel.: ; Fax: jdiaz@conare.ac.cr; diazcharverri@racsa.co.cr Over the last few years, miniature mass spectrometers have been extending the utility of mass spectrometry (MS) as an analytical technique to applications beyond the normal operating conditions of standard laboratory MS instruments [1,2]. This development has primarily focused on providing portability, distributed sensing, real-time continuous monitoring, and in-situ field analysis. Lower cost is also a key driver [3]. Recent symposia [4,5], workshops [6], special feature journal editions [7] and review articles [8] have focused on miniature mass spectrometers. The goal of this research is to develop smaller mass spectrometers but with mass ranges and sensitivities comparable to larger laboratory instruments. Most mass analyzers have been the target of miniaturization. Many groups are working on small ion traps and a commercial instrument based on a small cylindrical trap has been released (MKS Instruments, MA, USA). A miniature quadrupole on a silicon chip is being developed in England [4]. A small quadrupolar array has been commercialized (Ferran Scientific Inc., CA, USA). Time-of-flight instruments are in development for, among other things, sample analysis on Mars and the head of a comet, and detection of biological weapons. Miniature sector instruments have been developed and commercialized as well [9]. A good overview is provided in [9] and in the abstracts from the Harsh-Environment Mass Spectrometry Conference [1,2]. Typically, small mass spectrometers are looking for volatile compounds and operate in the /02/$ - see front matter # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S (02)

2 516 trends in analytical chemistry, vol. 21, no. 8, 2002 range Da. For example, most laboratory instruments are too expensive and too heavy to be used for remote monitoring or setting up distributed arrays of spectrometers. Examples of environmental areas where distributed arrays of mass spectrometers would be useful are airports, subways, pipelines and volcanoes. They could be used to monitor the air for low concentrations of chemical markers that would provide early warning in the case of chemical attacks, unsafe levels of pollutants, chemical leaks and eruptions. Complete mass spectrometers (vacuum system, sensor, and electronics) that weigh less than 20 lbs (9 kg), occupy less than 1200 cubic inches (0.02 m 3 ), and cost less than $10,000 could begin to address these monitoring problems. The big advantages of mass spectrometry over other techniques are its sensitivity and its ability to identify any compound. A goal of 1 10 ppm sensitivity across a mass range of 200 Da covers most gas-monitoring problems in industrial and environmental monitoring. Other techniques, such as those involving light spectroscopy or electrochemistry, tend to be very good at measuring single compounds or classes of compounds, but they are not as universally applicable as MS. The large size, large power requirements, and high cost of mass spectrometers have prevented their widespread use in chemical-sensing applications. Miniaturization addresses these three issues. Smaller mass spectrometers are now light enough to be carried by a single person a commercial example is the HapSite (Inficon, NY, USA). Innovation in electronics has led to the reduction of the size and the cost of the driving circuitry for all types of mass spectrometers. For example, small sector instruments, which rely on fixed magnetic fields and scanning ion energies, use as little as 25 Watts of power. Finally, by taking advantage of advances in manufacturing and the reduction of cost of raw materials, miniaturization can dramatically reduce costs. A special feature of small mass analyzers is their tolerance to higher operating pressures as a result of the reduction in the length of ion path. Mass spectrometers are designed to operate in the molecular flow regime. This is when the mean free path of the ions or molecules is greater than the characteristic dimension of the analyzer. For example, for air at room temperature and a mass analyzer with a critical dimension of 5 mm, the molecular flow regime is achieved at just 10 mtorr. Thus, pumping requirements are reduced for small mass spectrometers. It is this characteristic that allows the employment of a special mass analyzer with very short ion path to be used in plasma monitoring where the normal operating pressure is in mtorr range. The Advanced Space Propulsion Laboratory (ASPL) at Johnson Space Center (JSC) is actively developing a new type of rocket technology, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) [10]. This engine is powered not by conventional chemical reactions but by hot plasma at temperatures reaching 50,000 C or more. The plasma is confined using magnetic fields and is expelled through a magnetic nozzle creating a force that pushes the spacecraft forward [11]. The technology can achieve higher speeds and propellant efficiency and is targeted to enable human exploration of Mars and beyond. The plasma rocket requires vacuum to operate. Therefore, for ground-based testing on Earth, the large VX-10 test chamber has been built at the ASPL (Fig. 1). The VX-10 set-up includes components to generate, confine and accelerate the plasma and a very large vacuum chamber to try different configurations. To produce vacuum, a combination of turbomolecular, cryogenic and diffusion pumps backed with conventional mechanical rough pumps are used. Nominal pressure is in the range Torr before plasma operations and up to Torr during plasma operation. The propellants are gas mixtures of hydrogen, deuterium, helium and argon. There is also interest in testing and characterizing other gases, such as ammonia or xenon, as propellants. The VX-10 chamber required residual gasanalysis capabilities and single-ion monitoring of the plasma-propellant species to determine the

3 trends in analytical chemistry, vol. 21, no. 8, vacuum conditions before, during, and after firing the rocket and to evaluate the efficiency of each gas mixture as a propellant. The purpose of incorporating a miniature mass spectrometer is precisely to provide these two capabilities to the test set-up working under harsh conditions and relatively high vacuum pressures. In the future, the goal is to integrate the selected miniature mass spectrometer to the VASIMR main control panel to provide feedback information on the rocket in order to manage the amount of mixture that is being injected to the plasma. Now available are several commercial miniature quadrupole mass spectrometers, such as the Micropole (Ferran Scientific Inc., CA, USA) and the Transpector XPR2 (Inficon, CA, USA), which could be useful for target plasma monitoring in the semiconductor-manufacturing industry, but they are not a good option for the VASIMR test, since their performance is sensitive to the very high radio-frequency (RF) fields generated to produce the plasma in the VX-10 chamber. Therefore, a new miniature magnetic sector mass spectrometer was selected to perform these tasks for the VASIMR test. 2. Instrumentation and experiments The miniature mass spectrometer used at ASPL is an a version of what is called the Integrated Leak Detector (ILD) 50 (Mass Sensors Inc., MO, USA). It was developed to detect helium and other low-mass, permanent-gas signatures (Fig. 2) with a mass range 2 50 Da and Fig. 2. Miniature Double-Focusing Mass Spectrometer: Integrated Leak Detector (ILD 50) developed by Mass Sensors Inc. Fig. 1. VASIMR main components. Plasma Rocket being fired at VX-10 chamber. ASPL- JSC/NASA 8/30/01.

4 518 trends in analytical chemistry, vol. 21, no. 8, 2002 was targeted to be used in residual gas-analysis applications and ion monitoring for industrial process control. The physics behind the ILD 50 is the same as for the Compact Double-Focusing Mass Spectrometer (CDFMS) developed at the University of Minnesota [12], and field tested at Costa Rican volcanoes, Sandia National Laboratory [13] and NASA Kennedy Space Center [14]. The remarkable characteristic of the ILD 50 is that it has been reduced to 40% of the original size of the CDFMS with a radius of 8 mm for the mass analyzer Miniature double-focusing mass spectrometer design The design of the miniature mass analyzer is based on the mass separation and focusing capabilities of a 90 cylindrical double-focusing mass analyzer using crossed electric and magnetic fields [15]. Double focusing in general is achieved when ions emerging from an ion source with spreads in both energy and direction are efficiently focused to points in space which depend only on the ion mass-to-charge ratio (m/z). Traditionally, electric and magnetic sectors are placed in tandem to create a doublefocusing mass spectrometer [16 18]. Another way to obtain double focusing is by superimposing electric and magnetic fields with vector directions perpendicular to each other. This kind of geometrical arrangement is called Crossed Electric and Magnetic Fields or EB Superimposed Fields and has been used in the design and construction of Wien filters, cycloidal mass spectrometers [19] and ioncyclotron-resonance mass spectrometers [20]. These three types of crossed field analyzers have been employed in various small mass spectrometers, for example: a 2-cm 180 magnetic analyzer with Wien filter [21], a miniature (2.7 cm from slit to detector) cycloidal focusing mass spectrometer [22], a compact (1-cm radius) helium detector [23], and a back-pack portable ion-cyclotron-resonance instrument [24], but very few mass spectrometers have been designed with cylindrical EB superimposed fields having double-focusing capabilities. In the CDFMS design, the radial electric field of cylindrical symmetry is superimposed at right angles to the homogeneous magnetic field, and double focusing is achieved only with a special ratio of E/B. Fig. 3 describes the 90 cylindrical EB mass analyzer. The cylindrical electric sector is oriented parallel to the radial vector, rˆ. The magnetic field, B, is placed perpendicular to the electric field, E. Ifxˆ and ŷ defines the plane of the paper, B is oriented in the ẑ direction. The signs of the electric and magnetic fields are chosen such that the forces from the electric and magnetic fields are anti-parallel. In addition, if the radial positive force is defined as inward and the negative force is outward along the radius of curvature, then the equation of motion of an ion of mass, m, charge, q, and velocity, v, is given by: F ¼ qbv qe ¼ mv2 r ð1þ The radius of curvature, r, of an ion traveling through the superimposed EB field can be expressed by: r ¼ mv2 qbv qe ð2þ Fig. 3. Miniature Double-Focusing Mass-Analyzer design: Cylindrical 90 superimposed EB geometry.

5 trends in analytical chemistry, vol. 21, no. 8, From this expression, it can be seen that the crossed cylindrical sector field is able to provide the mass dispersion necessary to separate the different ions. Fig. 4 shows an ion simulation for ions of 27, 28 and 29 Da produced by an electron-impact-ionization ion source. The electron beam is generated by a V-shaped filament. The crossed field analyzer is able to separate effectively the ions by selecting only one at the detector. If v and B are kept constant for a particular mass, deriving the radius r with respect to v will give: dr dv ¼ Bv 2E r Bv E v ð3þ In order to correct for the energy spread, the dispersion in the analyzer of r with respect to v needs to be zero [25], that is dr/dv = 0, which is satisfied when Bv = 2E. Thus, energy focusing using the EB geometry is possible when the magnitude of magnetic force is twice the magnitude of the electric force or: F m F e ¼ 2 Energy Focusing ð4þ Angular spread focusing is calculated resolving the Mattauch and Herzog [26,27] equations that describe the trajectories for ions traveling through sector fields. In the case of the EB analyzer, directional focusing will occur at: l o ¼ l i 0:35 r o Direction Focusing ð5þ Where: l o =object distance (source slit to field boundary); l i =image distance (detector slit to field boundary); r o =central radius of cylindrical analyzer Therefore, double-focusing of the ion beam with the crossed field analyzer will occur when the magnitude of magnetic force is exactly twice the magnitude of the electric force, and the focal points of the ion source and the detector slits are placed symmetrically 0.35 times the design radius away from the sector-field boundaries. If r o is the fixed radius at the center of the cylindrical EB analyzer and B is kept constant, then the selected mass is given by: m z ¼ c o V with c o ¼ e B2 ro 2 8 : constant ð6þ The mass spectrum is then produced by sweeping the voltage across the range of the selected masses. To avoid the cumbersome use of electromagnets, as in the case of the earlier cylindrical crossed field spectrometers, the magnetic field can be achieved using a small permanent magnet. The theoretical resolving power (RP Theory )at full-width of half-maximum (FWHM) for the cylindrical EB analyzer is: RP Theory ¼ m Dm ¼ 2r o ðs o þ S i Þ ð7þ Fig. 4. Ion simulation for three ions, m/z=27, 28, 29 Da, using the 20 mm-radius CDFMS mass analyzer and an electron-impact-ionization ion source. (SIMION 6.0). where S 0 and S i are the widths of the entrance (object) and exit (image) slits of the analyzer.

6 520 trends in analytical chemistry, vol. 21, no. 8, Experimental set-up As mentioned before, the advantages of the ILD 50 are its small size and very short ion path, which makes it possible to operate at relatively high vacuum pressures. Fig. 5 shows the a version of the ILD 50 (a-ild 50) used for the VASIMR test. The radius r o is 8 mm and the two slits are 0.1 mm wide, giving a theoretical resolving power of 40 according to equation (7). The magnetic field was measured to be B 0.9 tesla. If the acceleration voltage, V, is given in volts, then, using equation (6), the selected mass will be given by: m=zðdaþ ¼ 625 V ð8þ If the smallest molecule to be monitored is hydrogen (2 Da), then the maximum acceleration voltage required is only V. The expected mass range for the unit was 50 Da, but these specifications needed to be confirmed during the experiments. The a-ild 50 is equipped with its own electronic package and Labview (National Instruments Inc., TX, USA) control software. In addition to mass scanning and data acquisition, the software allows tuning of the electrode voltages for the ion source, electron beam, mass analyzer and electron multiplier gain. The original voltage settings provided by the manufacturer were loaded to the Labview driver to start the test. Pressure was monitored using a Micro-Ion Gauge (Helix Technologies Inc., CO) close to the mass spectrometer. To prevent high-energy ions from reaching the mass spectrometer, a 90 T shape configuration was employed as shown in Fig. 6. The electronics package was interfaced to the PC via two National Instruments data-acquisition cards. All the components, including the computer, were Fig. 5. a-ild 50 unit used in ASPL test.

7 trends in analytical chemistry, vol. 21, no. 8, assembled inside a portable rack. This set-up constituted the RGA /single-ion monitoring station for the VX-10 chamber. 3. Results The unit was first tested using known gases. Air, helium, deuterium and argon were introduced to the system using a small needle-valve inlet and several mass spectra, similar to the one shown in Fig. 7, were taken of the residual gases present in the vacuum system. Then the original settings for the slope, the sector asymmetry and the sector ratio were tuned and the number of data points per mass was increased to 20 to improve the resolution of the device. Fig. 7 shows the mass spectrum collected of a sample containing small quantities of deuterium, hydrogen, air and argon injected through the needle valve. The unit was able to resolve all gases present up to argon (40 Da) with acceptable resolution. The resolving power at FWHM measured at the Ar + peak was 37, which is pretty close to the theoretical value of 40. Fig. 8 shows the spectrum of a gas sample from a bottle of 90% D 2 and 10% H 2 used as propellant for the VASIMR. The mass spectrum shows a large amount of ions at m/z=3 Da, most likely HD + ions. These ions were formed on the mass spectrometer s filament or the ion gauge, or in the plasma itself. The exchange of H and D atoms in the bottle would be extremely slow. However, both D 2 and H 2 dissociate into atoms when adsorbed on a hot filament, and, when the atoms recombine, they do so statistically. Also the m/z=3 signal could also come from H 3 +, which can be produced by the following ion-molecule reaction: H þ 2 þ H þ 2 ¼ H þ 3 þ H þ ð9þ In some respects, both H + + and H 3 + molecules could have an effect on the performance of the plasma rocket. From Fig. 8 as well, the resolving power measured at the deuterium peak was 38.1, giving a very constant resolving power (RP) over the mass range of the a-ild 50. Once the unit was characterized, it was then used to perform residual gas analysis in VASIMR s VX-10 chamber. The chamber was pumped down and a mass scan was taken when the pressure reached Torr. Fig. 9 shows the residual gases present inside the main Fig. 6. Diagram for the a-ild 50 test set-up.

8 522 trends in analytical chemistry, vol. 21, no. 8, 2002 Fig. 7. a-ild 50 mass spectrum. Air + H 2 /D 2 + Ar sample. Fig. 8. Propellant gas spectrum (H 2 /D 2 ) using the a-ild 50 unit.

9 trends in analytical chemistry, vol. 21, no. 8, chamber, indicating the presence of water, air and hydrocarbons. The a-ild 50 was able to detect ions up to 100 Da, but with a deteriorated resolving power at high masses. After several hours of pumping, these molecules were effectively removed from the system. The next experiment was single-ion monitoring of the gas used as the propellant for the plasma. Helium was first selected for this task. A mass scan was performed and the electrode voltages were fixed for the 4 Da peak. Then a DI-190 data-logger (DATAQ Instruments, OH, USA) was used to monitor the ion signal over time. Helium was introduced through the VX- 10 gas-inlet system and immediately the VASIMR was fired up, producing the plasma (Fig. 1). Total pressure close to the MS changed from 10 6 to 10 4 Torr. Fig. 10 shows the He + ion signal monitored before, during and after plasma generation. The a-ild 50 was operational even when the short pulses of hot plasma were being generated, while another commercial quadrupole mass spectrometer, an RGA-100 (Stanford Research Systems Inc., CA, USA) was damaged by the same test conditions. Problems with noise in the ion signal were observed. The signal-to-noise level limited the sensitivity of the a-ild 50 to a detection limit of only 5000 ppm. Since the level of the ion signal monitored was above 1% (10,000 ppm and above), this was not a limiting factor for this particular application, but is certainly an area of improvement for future ILD 50 instruments. The noise could have been produced by fluctuation on the small power supply boards that set each electrode voltage and the main scanning voltage or else by noise associated with the VASIMR electronics itself. At this point all the objectives for the a-ild 50 initial test at ASPL were accomplished. Although other gas propellants were to be tested, problems with a water leak on one of the RF antennas from VX-10 system prevented more tests. Such tests will be the focus of future experiments with a new b-ild 50 unit. The goal is to have a final version of the miniature Fig. 9. Residual Gas Analysis at VX-10 chamber after calibration.

10 524 trends in analytical chemistry, vol. 21, no. 8, 2002 Fig. 10. Real-time single-ion monitoring for VASIMR. Helium gas as plasma propellant. mass-spectrometer system permanently attached to the VX-10 chamber and linked to the main VASIMR control and data acquisition panel for future testing of the plasma engine. 4. Conclusion and future plans The a-ild 50 was able to provide the required information for residual gas analysis and single ion monitoring for the VX-10 test chamber at ASPL. The miniature mass spectrometer was almost able to attain its theoretical resolving power of 40 and exceeded the specified mass range of 50 Da. The unit provided important information, even under the extremely harsh conditions when the hot plasma was generated. Future plans are to continue with the testing of different gas propellants using a new b-ild 50 unit with improved electronics and data acquisition software. It is expected that this next generation of ILD50 units will correct the problems with the power supplies, hardware and electronics experienced during the initial experiments described in this article. A user-friendly software is also expected to enable the integration of the mass-spectrometer sensor as part of the VASIMR main control panel in order to monitor the performance of the different plasma propellants. Acknowledgements The authors would like to express their gratitude to all the people who made this collaboration possible both in Costa Rica and USA. Special thanks to Mass Sensors Inc. for its direct support on the project, providing the a-ild 50

11 trends in analytical chemistry, vol. 21, no. 8, unit. Jorge Andres Diaz (JAD) thanks Michael Grace and Igor Alexeff for their financial support on the two-week trip to ASPL at JSC- NASA in Houston and to Guy de Teramond for providing the contacts and necessary logistics at the beginning of the project. JAD and Andres E. Mora Vargas (AMV) acknowledge Ronald Chang-Diaz for his help and the preparation of AMV s internship at ASPL and to the people involved with this collaboration both at CENAT and ASPL. References [1] 2nd Harsh-Environment Mass Spectrometry Workshop, Tampa, FL, USA, March [2] 3rd Harsh Environment Mass Spectrometry Workshop. Pasadena, CA, March [3] H.W. Rohrs, W.R. Gentry, R.S. Chhatwal, Pittcon 2002 Conference, New Orleans, LA, USA,17 22 March [4] Miniature Mass Spectrometers Symposium, Pittcon 2002 Conference, New Orleans, LA, USA, March Also Chem. Eng. News 8 April (2002) 34 (report of symposium). [5] Recent Developments in Field Analysis Symposium, Pacifichem 2000, ACS, Honolulu, HI, USA, December [6] Field-Portable and Miniature Mass Spectrometry Workshop, Sanibel Conference, ASMS, Sanibel Island, FL, USA, January [7] Special Focus on Field Portable and Miniature Mass Spectrometers, J. Am. Soc. Mass Spectrom.12 (2001). [8] E.R. Badman, R.G. Cooks, J. Mass Spectrom. 35 (2000) 659. [9] Harsh-Environment Mass Spectrometry webpage (2002) [10] F.R. Chang-Diaz et al., Joint Propulsion Conference, Huntsville, AL, USA, July 2000, AIAA [11] A. Petro, Electric Plasma Propulsion could get us to Mars (2002) petro/ html. [12] J.A. Diaz. Ph.D. Thesis, University of Minnesota, July [13] J.A. Diaz, C.F. Giese, W.R. Gentry, Field Anal. Chem. Technol. 5 (2001) 157. [14] C.R. Arkin, T.P. Griffin, A.K. Ottens, J.A. Diaz, D.W. Follistein, F.W. Adam, W.R. Helms, J. Am. Soc. Mass Spectrom. (In press). [15] J.A. Diaz, C.F. Giese, W.R. Gentry, J. Am. Soc. Mass Spectrom. 12 (2001) 619. [16] A.O. Nier, J.L. Hayden, Int. J. Mass Spectrom. 6 (1971) 339. [17] J.P. Carrico, et al., J. Phys. E: Sci. Instrum. 7 (1974) 469. [18] M.P. Sinha, A.D. Tomassian, Rev. Sci. Instrum. 62 (1991) [19] W. Bleakney, J.A. Hipple, Phys. Review 53 (1938) 521. [20] L.G. Smith, Phys. Rev. 81 (1951) 295. [21] D.L. Swinger, Vacuum 21 (1971) 121. [22] C.F. Robinson, L.G. Hall, Rev. Sci. Instrum. 27 (1956) 504. [23] D.L. Swinger, Int. J. Mass Spectrom. Ion Phys. 17 (1975) 321. [24] H.F. Hemond, Rev. Sci. Instrum. 62 (1991) [25] R.W. Gentry, Personal notes (unpublished), 25 October [26] J. Mattauch, R. Herzog, Annalen der Physik 19 (1934) 345. [27] J. Mattauch, R. Herzog, Zeitschrift für Physik 89 (1934) 789.