Laboratory Experiment pubs.acs.org/jchemeduc
Assembly of a Vacuum Chamber: A Hands-On Approach To Introduce Mass Spectrometry Guillaume Bussière,* Robin Stoodley, Kano Yajima, Abhimanyu Bagai, Aleksandra K. Popowich, and Nicholas E. Matthews Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada S Supporting Information *
ABSTRACT: Although vacuum technology is essential to many aspects of modern physical and analytical chemistry, vacuum experiments are rarely the focus of undergraduate laboratories. We describe an experiment that introduces students to vacuum science and mass spectrometry. The students first assemble a vacuum system, including a mass spectrometer. While assembling the vacuum system students learn about the components and the conditions necessary to operate a mass spectrometer. Then students use their assembled system to explore mass spectrometry; they use the mass spectrometer to diagnose vacuum conditions, then collect the mass spectrum of an unknown organic molecule, and propose the identity of the molecule.
KEYWORDS: Upper-Division Undergraduate, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Hands-On Learning/Manipulatives, Mass Spectrometry, Qualitative Analysis
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laboratories are separated from lectures, and it is therefore possible to offer experiments that are “outside the box”: not necessarily tied to the contents of a lecture course. Our experiment is interdisciplinary in nature, covering aspects of instrumental analysis, organic chemistry, and surface science. We think this experiment benefits all of our undergraduate students. Mass spectrometry knowledge is an asset in practically all fields of chemistry. Students continuing to graduate study in analytical or physical chemistry are frequently required to assemble vacuum systems or related hardware. For students who opt for a career in industry, Fahey and Tyson9 have found that practical experience with mass spectrometry is valued by employers.
acuum technology is commonly used in science; routine chemistry uses include mass spectrometry, surface chemistry, nanotechnology, and spectroscopy. Recognizing this importance, ACS guidelines1 suggest undergraduate instruction include student use of mass spectrometry and vacuum systems. Typically, this is achieved by offering undergraduate experiment(s) in mass spectrometry in tandem with other techniques such as gas chromatography (GC)2,3 and liquid chromatography (LC),4 or together with infrared spectroscopy (IR) and/or nuclear magnetic resonance (NMR) for structure determination.5 It is rare that vacuum science is specifically taught.6 To address this we have developed an experiment which aims to introduce vacuum science in combination with mass spectrometry in our third year undergraduate chemistry laboratory. The goal is to assemble a vacuum system and then use it to explore aspects of mass spectrometry. Our mass spectrometer design is similar to that of Henchman and Steel7 with the exception that our experiment provides students with a hands-on experience. The students assemble the system themselves. This is likely the best way to introduce vacuum science as it can be a very technical topic. The benefits of a hands-on approach to teach science are well-known, well documented, and time-tested. Studies have shown expected and unexpected learning benefits (ref 8 and references therein). This experiment was offered for the first time in the fall term of 2012 as a part of our third year integrated laboratories, which span the traditional disciplines of analytical, inorganic, organic, and physical chemistry. In our department the third year © XXXX American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL OVERVIEW Students assemble a vacuum system from parts provided, including vacuum pumps, pressure sensors, sample introduction hardware, and a basic mass spectrometer (a residual gas analyzer, RGA). Once assembled and pumped down to a low pressure, students record the mass spectrum (a) of the chamber contents at steady-state pressure, (b) while introducing an air leak, (c) while argon or helium is leaked into the chamber, and (d) of an unknown halogenated organic compound. Equipment Required
The major components required and their approximate cost are outlined in Table 1. Information about the specific system components we used and possible substitutions is given in the
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Table 1. Major Vacuum System Components and Their Cost Component Pumping hardware
Pressure measurement hardware Mass spectrometry hardware and software Sample introduction hardware Vacuum chamber Total a
Details 2 rotary vane pumps 1 turbo pump and controller Miscellaneous accessoriesa Ionization gauge and accessoriesa Pirani gauge and accessoriesa 1−300 amu Extorr XT 300 residual gas analyzer (RGA) with software and computer Manual stainless steel leak valve and accessoriesa Miniature sample cylinder Four way cross 2.75 in. CF
Price, $USD 14885
2183 6618 1811
188 25685
Detailed breakdown can be found in the Supporting Information.
Supporting Information. Henchman and Steel have addressed in detail the design of a mass spectrometer that is suitable to be operated by undergraduate students.7 The system described in this paper differs in a few important ways: our system is not mobile, it is assembled and dismantled by students for each use, and we use liquid nitrogen and a removable miniature sample cylinder for sample introduction rather than a septum and a syringe. Vacuum System Assembly
An individual or a team of two students are provided with a four way cross-shaped vacuum chamber, a turbomolecular pump and controller, an ion gauge, a leak valve, a quadrupole mass spectrometer (residual gas analyzer), and appropriate gaskets and bolts. Wrenches are the only tools required. The laboratory manual contains photos and a schematic illustrating the orientation of all the necessary components. Most students complete assembling the vacuum chamber within 1 h. Once assembled, the first rotary vane pump is started, reducing the pressure in the chamber. When the pressure falls below 9.9 × 10−2 Torr, the turbomolecular pump is started. In the absence of major leaks, a pressure of 2 × 10−6 Torr is reached within approximately 10 min. At this point the vacuum system is ready to be used to collect various mass spectra as described in the following section. At the end of the laboratory period, students stop the vacuum system and disassemble the components. The miniature sample cylinder is rinsed with acetone, and the washings are disposed to the halogenated solvent waste container. The cylinder is then stored in an oven at 100 °C to be clean and ready for reuse.
Figure 1. (A) Mass spectrum of the residual gas in the vacuum chamber with minimal leakage. Under these conditions, desorption is the main source of molecules. *The mass to charge ratio of 28 is not assigned, but it is most likely due to N2 at relatively high pressure and CO at lower pressure. (B) Mass spectrum with an air leak. Under these conditions, molecules are coming from both the leak and desorption from the inside surface of the chamber. (C) Mass spectrum of argon with an air leak. (D) Mass spectrum of dichloromethane with minimal air leak (note X-axis scale expanded).
tempting to assign the peak at m/z = 28 to N2+, but with UHV equipment a contribution from CO+ cannot be ignored. In Figure 1A the m/z = 28:m/z = 32 ratio is exceeding 13:1, which is far from the 4:1 value expected for an air leak. With the low resolution quadrupole mass spectrometer used for this experiment it is impossible to directly assign the peak at m/z = 28 to N2+, CO+, or even C2H4+ because they all have a mass of 28 amu. Differentiating between N2+ and CO+ with a quadrupole mass spectrometer is a challenging task,11,12 but it is well-known that at very low pressure CO desorption from stainless steel chamber walls is observed.10,13 In the absence of a major leak the main source of all molecules is desorption. Two different surface chemistry phenomena can occur: electron-stimulated desorption (ESD)14 and thermal desorption.15 The presence of CO, CO2, H2O, and H2 is due to both of these two processes. Air is introduced into the vacuum chamber by slowly opening the leak valve until the pressure increases to 1.0 × 10−5 Torr. Figure 1B shows this mass spectrum, and the N2:O2 ratio (m/z 28:32) is 5.6:1. A small argon peak (m/z = 40) is also notable. The N2:Ar ratio (m/z 28:40) in Figure 1B is 64:1. The intensity ratios for N2+, O2+, and Ar+ peaks are comparable to
Mass Spectral Studies
For the rest of the laboratory period, students collect mass spectra under different experimental conditions to explore practical aspects of mass spectrometry. First the students record a mass spectrum of the residual gases within the vacuum chamber with no sample introduced and in the absence of major leaks. If nitrogen (m/z = 28) and oxygen (m/z = 32) are detected in a ratio of approximately 4:1, this indicates a leak and students must tighten the chamber bolts. In the absence of leaks the base pressure of the system rapidly reaches 2 × 10−6 Torr. As shown in Figure 1A the base peak in these conditions is water (m/z = 18). Other peaks are H+ (m/z = 1), H2+ (m/z = 2), water fragments O+ and HO+ (m/z = 16 and 17), and CO2+ (m/z = 44). This mass spectrum is consistent with that of a typical stainless steel chamber under vacuum.10 It might be B
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the ratios of the abundance of these gases in air. This is not the case for H2, H2O, and CO2, which are coming mostly from desorption rather than the air leak. While the leak valve is kept open and the pressure is kept at 1 × 10−5 Torr, gases such as argon or helium stored in nearby cylinders can be sprayed into the sample inlet. The recorded mass spectrum then changes to represent a mixture of air with the gas being used, as shown in Figure 1C. After 1 or 2 min the signal of the gas that is sprayed becomes the base peak on the mass spectrum. Argon is a particularly interesting sample for this exercise due to its atomic nature and the presence of a relatively strong signal at m/z = 20. The only possible rationale for this peak is an argon atom of charge z = +2. Student assignment of this peak emphasizes to them the fact that the property measured by the mass spectrometer is the mass-tocharge ratio and not just the mass. A miniature sample cylinder is prepared in a fume hood to contain approximately 100 μL of an organic compound whose identity is unknown to the student(s). The rationale for teaching mass spectrometry using halogenated compounds has been thoroughly explained in papers published in this Journal.3,16,17 After attaching the cylinder to the sample inlet, air in the sample cylinder is removed by using a second rotary pump while the unknown is frozen by immersing the bottom of the cylinder in liquid nitrogen. After the air is removed, the valve to the pump is closed and the liquid nitrogen Dewar is removed, allowing the unknown to melt. The introduction of the unknown into the vacuum chamber is achieved by opening the leak valve. To identify the unknown molecule, students record a mass spectrum under the default operating conditions of the electron ionization source; Figure 1D shows a mass spectrum of dichloromethane as an example. In addition, students investigate the changes in the intensity distribution of the mass spectra caused by changing the ionization parameters. They complete this task by reducing the electron energy, which reduces kinetic energy of the ionizing electrons. Reducing the electron energy has two main effects: it reduces the number of ions detected, and it changes the fragmentation pattern. Students are expected to comment on these two observations in their laboratory report. The software that accompanies our Extorr mass spectrometer (Vacuum Plus) allows for a quick adjustment of the operating conditions and data acquisition, making the completion of all assigned tasks by students possible within the 4 h lab period.
ASSESSMENT In their laboratory report, students should identify all major peaks in the mass spectrum of normal operating condition (Figure 1A). They are expected to explain why these molecules are present in the vacuum system even if no sample is being introduced and why the water peak is the base peak under these conditions. They also identify species that are being detected when air is leaked in (Figure 1B). They discuss relative intensities and why the N2:O2 intensity ratio is approximately 4:1. They answer why the H2, CO2, and water to nitrogen peak intensity ratios differ from the ratios of their concentrations in air. Finally students propose the identity of their unknown. They support their conclusions with the fragmentation pattern observed and use the isotope abundance ratio to explain spectral intensities.
HAZARDS All unknown chemicals used by students in this experiment are halogenated and should be considered toxic. All are possible carcinogens. Preparation of an unknown must be done in a fume hood, and a lab coat, gloves, and protective eyewear must be worn. The amount of unknown used in this experiment is only 100 μL and should not pose unreasonable risk to the students or educators. High voltage is also a safety concern, and it is crucial to build the apparatus on an electrically grounded metal table or bench. Catastrophic turbomolecular pump failure is a low probability event, but nonetheless the pump must be securely anchored to the bench or table to prevent the possibility of it breaking loose upon sudden seizure. Liquid nitrogen can cause frostbite and must be handled with appropriate cryogen-proof gloves. Laboratory must be well ventilated for work with liquid nitrogen and pressurized gases. Gas cylinders must be well anchored.
Notes
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CONCLUSION In 2012−2013 when the experiment was introduced, 65 thirdyear chemistry students completed the experiment. This was achieved by offering the experiment three times a week for 18 weeks using a single setup. In this way the maximum capacity for the experiment is 108 students/year. Thirty-five students answered an end of term survey. When asked to describe what they liked about the experiment, about two-thirds mentioned that they liked assembling the vacuum system, and 14 students specifically used the term “hands-on”. Survey data suggest that, by building the apparatus, students better appreciate the significance and role of each component in a mass spectrometer. The popularity of the experiment was evident; many students selected this as their favorite experiment of the year. This introductory experiment can be adapted by other educators and can be a useful complement to already existing advanced experiments involving GC− or LC−MS.
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ASSOCIATED CONTENT
S Supporting Information *
Notes for instructors; lab manual for students; prelab assignment; mass spectra. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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*E-mail:
[email protected]. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the University of British Columbia Teaching and Learning Enhancement Fund for financial support of the development of new experiments for third-year chemistry laboratories. We thank Sailesh Daswani for help with figures and graphics. We also thank the Chemistry Department Science Teaching and Learning Fellow, Kerry Knox, for organizing the laboratory surveys.
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REFERENCES
(1) American Chemical Society, Committee on Professional Training. Undergraduate Professional Education in Chemistry: ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs; American Chemical Society: Washington, DC, 2008. C
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(2) Giarikos, D. G.; Patel, S.; Lister, A.; Razeghifard, R. Incorporation of Gas Chromatography-Mass Spectrometry into the Undergraduate Organic Chemistry Laboratory Curriculum. J. Chem. Educ. 2013, 90, 106−109. (3) O’Malley, R. M.; Lin, H. C. The Separation and Identification of Some Brominated and Chlorinated Compounds by GC/MS. J. Chem. Educ. 1999, 76, 1547−1551. (4) Xu, X.; Veenstra, T. D. Factors That Contribute to Assay Variation in Quantitative Analysis of Sex Steroid Hormones Using Liquid and Gas Chromatography-Mass Spectrometry. J. Chem. Educ. 2012, 89, 230−233. (5) Liota, L. J.; James-Pederson, M. Identification of an Unknown Compound by Combined Use of IR, 1H NMR, 13C NMR, and Mass Spectrometry: A Real-Life Experience in Structure Determination. J. Chem. Educ. 2008, 85, 832−833. (6) Girard, J. E.; Diamant, C. T. Instrumental Analysis Lecture and Laboratory: A Survey. J. Chem. Educ. 2000, 77, 646−648. (7) Henchman, M.; Steel, C. Design and Operation of a Portable Quadrupole Mass Spectrometer for the Undergraduate Curriculum. J. Chem. Educ. 1998, 75, 1042−1049. (8) Haury, D. L.; Rillero, P. Perspectives of Hands-On Science Teaching; Eric Clearinghouse for Science, Mathematics, and Environmental Education: Colombus, OH, 1994. (9) Fahey, A.; Tyson, J. Instrumental Analysis in the Undergraduate Curriculum. J. Anal. Chem. 2006, 78, 4249−4254. (10) Adams, R. O. A review of the stainless steel surface. J. Vac. Sci. Technol., A 1983, 1, 12−18. (11) Hein, S. J.; Ramus, T. L.; Wang, Y.; Xu, L.; Gu, M. Differentiating and Quantitative Analysis of N2 and CO in the Presence of Each Other With a Quadrupole Mass Spectrometer; 60th ASMS Conference, Vancouver, 2012. (12) Wang, Y.; Gu, M. The Concept of Spectral Accuracy for MS. Anal. Chem. 2010, 82, 7055−7062. (13) Rezaie-Serej, s.; Outlaw, R. A. Thermal desorption of CO and H2 from degassed 304 and 347 stainless steel. J. Vac. Sci. Technol., A 1994, 12, 2814−2819. (14) Ramsier, R. D.; Yates, J. T., Jr. Electron-stimulated desorption: Principles and applications. Surf. Sci. Rep. 1991, 12 (6−8), 246−378. (15) King, D. A. Thermal desorption from metal surfaces: A review. Surf. Sci. 1975, 47 (1), 384−402. (16) Schildcrout, S. M. No, the Molecular Mass of Bromobenzene Is Not 157 amu. J. Chem. Educ. 2000, 77, 1433−1434. (17) O’Malley, R. M. The Determination of the Natural Abundance of the Isotopes of Chlorine. J. Chem. Educ. 1982, 59, 1073−1076.
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