Demonstration pubs.acs.org/jchemeduc
Illustrating the Basic Functioning of Mass Analyzers in Mass Spectrometers with Ball-Rolling Mechanisms Ryo Horikoshi,*,† Fumitaka Takeiri,‡ Riho Mikita,‡ Yoji Kobayashi,‡ and Hiroshi Kageyama‡ †
Department of Environmental Science and Technology, Faculty of Design Technology, Osaka Sangyo University, Nakagaito, Daito, Osaka 574-8530, Japan ‡ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *
ABSTRACT: A unique demonstration with ball-rolling mechanisms has been developed to illustrate the basic principles of mass analyzers as components of mass spectrometers. Three ball-rolling mechanisms mimicking the currently used mass analyzers (i.e., a quadrupole mass filter, a magnetic sector, and a time-offlight) have been constructed. Each mechanism was designed to separate balls with three different weights (representing three ionized analytes with different weights) in an attempt to imitate the separation process employed by real mass analyzers. This demonstration helped students understand the basic principles of mass analyzers. KEYWORDS: High School/Introductory Chemistry, Demonstration, Humor/Puzzles/Games, Mass Spectrometry
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INTRODUCTION Mass spectrometry, one of the most widely used analytical techniques,1 is briefly covered in most of the general chemistry textbooks.2−4 Mass spectrometry is used to identify and quantify compounds in a sample while also allowing the estimation of molecular chemical structures.1−4 Mass spectrometry is carried out in a mass spectrometer consisting of an ion source, a mass analyzer, and an ion detector (Figure 1).1 Among the components, the mass analyzer is used to separate ionized analytes according to their mass-to-charge (m/z) ratios. Three types of mass analyzers are commonly used, namely, quadrupole mass filter, magnetic sector, and time-of-flight.5−9 Mass analyzers allow the separation of ionized analytes by
taking advantage of their characteristic behavior under electric and/or magnetic fields.1 Ball-rolling mechanisms have been built in the classroom, not only for the enjoyment of constructing and watching them in action but also for learning physical principles, such as uniform acceleration and potential−kinetic energy.10,11 We conceived the idea of using ball-rolling mechanisms for teaching the basic functioning of mass analyzers. Herein, we describe a demonstration with three ball-rolling mechanisms for teaching the function of mass analyzers. The mechanisms were designed by mimicking real mass analyzers such as quadrupole (1), magnetic sector (2), and time-of-flight (3) mass analyzers, respectively. Each mechanism allowed the separation of balls with three different weight (representing three different ions with varying weights) in an attempt to imitate the separation processes used by real mass analyzers.
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PREPARATION The photographs and corresponding schematics of ball-rolling mechanisms 1−3 are shown in Figures 2−7. Detailed constructions of the mechanisms 1−3 are provided in the Supporting Information (Videos S1−S3). The instructor requires approximately 3 h or less to construct mechanisms 1−3. Ball-rolling mechanisms 1−3 were constructed from readily available materials purchased from do-it-yourself or dollar stores. Total cost is approximately $100, which appears Figure 1. (a) Components of a mass spectrometer and (b) types of components. Arrows indicate the route followed by the ionized analytes. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: May 2, 2017 Revised: August 9, 2017
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DOI: 10.1021/acs.jchemed.7b00297 J. Chem. Educ. XXXX, XXX, XXX−XXX
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parallel and do not physically move, thereby creating a cylindrical pathway between them. In the pathway of the electric fields, ionized analytes travel with an oscillating behavior. By controlling the RF and dc voltages, a quadrupole mass filter allows the ionized analytes with a specific m/z value to avoid collision with the four electrodes and reach the detector or suggest considering a stable trajectory. Meanwhile, ions with different m/z collide with the electrodes or consider an unstable trajectory. Magnetic Sector
Magnetic sector analyzers comprise two fan-shaped magnets. According to Fleming’s left-hand rule, the accelerated ionized analytes are subjected to a centripetal force and are deflected while passing through a magnetic field between the fan-shaped magnets. Consequently, the ionized analytes follow an arc within a plane perpendicular to the magnetic field. The degree of deflection is proportional to the m/z value of the ionized analytes, with lighter ions being deflected more than heavier ones. Time-of-Flight
A time-of-flight is based on the fact that different weighted objects require a different amount of time to move in a vacuum. Time-of-fight is the time required for an ionized analyte to fly from the ion source to the detector. In the case of accelerated ionized analytes, these times are proportional to their (m/z)1/2 values. In this manner, the m/z values of ionized analytes can be calculated. Ion Detectors
The most common detector used in mass spectrometers is an electron multiplier that employs the secondary-electron emission principle for amplification. When an ionized analyte selected by the mass analyzer collides with the first dynode in an ion detector, the dynode emits a few electrons. These electrons emitted from the first dynode strike a second dynode from which more secondary electrons are released. The number of released electrons is multiplied upon successive dynode strikes to form a cascade of electrons (i.e., an electric current). This electric current is finally detected by an ammeter in the ion detector.
Figure 2. Assembled ball-rolling mechanism representing a quadrupole mass filter (1) mass analyzer with a distance between the two sponge rubbers of ca. 1.5 cm and styrene ball sizes of (a) 1.0, (b) 2.0, and (c) 4.0 cm.
to be expensive. However, it should be noted that the mechanisms can be repeatedly used and that most of the constituent materials are reusable in other situations.
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DEMONSTRATION Videos S4−S6 (Supporting Information) show the ball separations performed by the ball-rolling mechanisms 1−3. These performances were used by the instructors to demonstrate how ionized analytes are separated by the mass analyzers.
BACKGROUND
Ion Sources
The analytes must be first transformed into gas phase ions by appropriate techniques before being separated and detected. Several techniques such as electron ionization, chemical ionization, fast atom bombardment, electrospray ionization, and matrix-assisted laser desorption/ionization are employed for generating gas phase ions (Figure 1). These techniques are briefly explained in the Supporting Information. The choice of the ionization technique depends on the properties of the analytes in a sample.
Quadrupole Mass Filter
Mechanism 1 contained two inclined corrugated sponge rubbers arranged parallel to each another. The corrugated sponge rubbers represented the trajectory of an ion allowing separation of styrene foam balls with different sizes (weights) that represented the ionized analytes (Figures 2 and 3). By changing the distance between the two corrugated sponge rubbers, styrene foam balls with different sizes were separated. When the two sponge rubbers were located at a distance of ca. 0.5 cm, the styrene foam balls 1.0 cm in diameter rolled along with the corrugated sponge rubber and subsequently reached the end of the sponge (Figure 3a). In contrast, the foam balls 2.0 and 4.0 cm in diameter do not reach the end of the sponge but rather dropped off the path delimited by the corrugated sponge. When the distance between the two sponge rubbers
Mass Analyzers
Mass analyzers separate ionized analytes by taking advantage of their differing behavior under electric and/or magnetic fields. Quadrupole Mass Filter
A quadrupole mass filter is based on the trajectories of ions in a combination of radio frequency (RF) and direct current (dc) electric fields. The electric fields are applied between two pairs of rod-shaped electrodes.7 The four electrodes are arranged in B
DOI: 10.1021/acs.jchemed.7b00297 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 5. Schematic of the assembled ball-rolling mechanism representing a magnetic sector (2) mass analyzer. Figure 3. Schematic of the assembled ball-rolling mechanisms representing a quadrupole mass filter (1) mass analyzer with varying distances between the two sponge rubbers: (a) ca. 0.5, (b) ca. 1.5, and (c) ca. 3.5 cm.
iron balls rolling down from the wooden slope, thus changing their course. The extent of course change was roughly proportional to the sizes of the iron balls. Thus, iron balls 1.5 cm in diameter largely changed their course, rolling into a metal can placed far from the slope (Figure 4a). In contrast, iron balls 2.0 cm in diameter slightly changed their course and rolled into a metal can located next along the trajectory (Figure 4b). The iron balls 2.5 cm in diameter mostly followed a straight course and rolled into the nearest metal can from the original trajectory (Figure 4c). The main differences between mechanism 2 and the corresponding mass analyzer are associated with the shape and size of the magnet used. The actual magnet used is a fanshaped magnet, which is usually large. Similar to mechanism 1, the iron balls roll on the upside-down wooden box in the mechanism; however, the actual ionized analytes in a magnetic sector mass analyzer fly between the two fan-shaped magnets.
was fixed to ca. 1.5 cm, only the styrene foam balls 2.0 cm in diameter were able to roll along the corrugated sponge rubber (Figures 2 and 3b). A distance of ca. 3.5 cm was suitable for isolating 4.0 cm diameter balls (Figure 3c). The actual trajectories of the ions in the mass analyzer are not as simple as those illustrated through the use of the corrugated sponge rubbers in mechanism 1. Instead, it is difficult to calculate them analytically and trace them experimentally.7 The foam balls roll along with the sponge in the mechanism; however, the actual ionized analytes in a quadrupole mass filter mass analyzer fly through a cylindrical pathway. The foam balls roll along with the sponge in the mechanism; however, the actual ionized analytes in a quadrupole mass filter mass analyzer fly through the cylindrical pathway. First, we attempted to construct a model of the four poles; however, a four-pole model was not constructed in order to make the participants observe the motion of balls better.
Time-of-Flight
Mechanism 3 consisted of a measuring tape, a suspended bell, an aluminum long rail, a simple sheet metal bookend equipped with another bell, and a wooden ring linked to a small container with a string (Figures 6 and 7). This mechanism allows the separation of three different wooden balls based on their weight. The measuring tape acted as a starting gate maintaining the ring stopping at the starting position. When the measuring tape was rolled up, it released the ring and subsequently knocked the bell, and the instructor pushed the start button of a stopwatch at this moment. The ring slid on the rail by pulling with the fully falling container, thereby subsequently knocking the bell suspended under the bookend, and the instructor pushed the stop button of the stopwatch at this moment. The weight of wooden balls can be roughly calculated from the times recorded on the stopwatch (Supporting Information). Lighter wooden balls reached the bell faster. The container served as a weight, allowing the loading of different weights to regulate the speed of the ring and the tension of the string. The main difference between mechanism 3 and the corresponding mass analyzer is the number of particles flying simultaneously. In the actual time-of-flight mass analyzer, many molecular ions (of various sizes) travel simultaneously. The wooden ball travels by the wooden sled in the mechanism; however, the ionized analytes travel through a vacuum in the mass analyzer.
Magnetic Sector
Mechanism 2 consisted of a neodymium magnet, an upside down wooden box, a wooden down slope, and three metal cans (Figures 4 and 5) able to separate iron balls with different sizes. When located inside the wooden box, the magnet attracted the
Figure 4. Assembled ball-rolling mechanism for representing a magnetic sector (2) mass analyzer for iron balls varying sizes: (a) 1.5, (b) 2.0, and (c) 2.5 cm in diameter. C
DOI: 10.1021/acs.jchemed.7b00297 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 7. Schematic of the assembled ball-rolling mechanism representing a of time-of-flight (3) mass analyzer. The letters and numbers in parentheses correspond to those used in Figure 6.
laboratory, instead of those described in this article. In case the mechanisms do not work properly, instructors need to adjust and improve them. Owing to their simple structures, the mechanisms can be easily assembled and adjusted. During the demonstration of mechanism 1, the styrene foam balls being dropped off rarely returned to the gap between two corrugated sponge rubber parts, and rolled along it. To avoid this return, several cork disks were put on the sponge rubbers to eject the dropped off styrene foam balls. The iron balls used in mechanism 2 sometime rolled into different metal cans, and this can be overcome by a fine adjustment of the ball starting point, the angle of the slope, or the positions of the metal cans. Regarding mechanism 3, instructors must increase the slope of the aluminum rail in case the wooden ring moves too fast. In contrast, if the wooden ring moves too slowly, instructors must load some weights on the container. The students’ comprehension is summarized in the Supporting Information. The participants showed enhanced understanding on the functioning of mass analyzers and were able to describe the separation process of ionized analytes in these devices. The mechanisms were teaching aids for lectures and did not illustrate the m/z values of ionized analytes. Several demonstrations and activities introducing the principles of analytical instruments have been previously reported although they were limited to atomic force microscopy and colorimetry techniques.12−14 Illustrating the basic principles of analytical instruments with unique mechanisms made of readily available components will be the subject of additional studies in the future.
Figure 6. Assembled ball-rolling mechanism representing time-offlight (3) mass analyzer: (a) start point, (b) immediately after the start, (c) after the start, and (d) goal point. (i) Wooden ring, (ii) measuring tape, (iii) suspended bell, (iv) aluminum long rail, (v) simple sheet metal bookend, (vi) suspended bell, and (vii) small container. Red and black triangles show the positions of the wooden ring and the container, respectively. The former goes up the rail while the latter goes down.
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HAZARDS The neodymium magnet may damage watches. When attracted to the neodymium magnet, the iron balls can pinch the hands or fingers. The sharp edges of the aluminum angle and the aluminum long rail can cause serious injury. Thus, the author recommends covering these edges with rubber protectors.
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DISCUSSION To assemble the mechanisms 1−3, instructors can use alternative materials found in their home, classroom, and D
DOI: 10.1021/acs.jchemed.7b00297 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(5) Miller, P. E.; Denton, M. B. The Quadrupole Mass Filter: Basic Operating Concepts. J. Chem. Educ. 1986, 63 (7), 617−622. (6) Leary, J. J.; Schmidt, R. L. Quadrupole Mass Spectrometers: An Intuitive Look at the Math. J. Chem. Educ. 1996, 73 (12), 1142−1144. (7) Steel, C.; Henchman, M. Understanding the Quadrupole Mass Filter through Computer Simulation. J. Chem. Educ. 1998, 75 (8), 1049−1054. (8) Vergne, M. J.; Hercules, D. M.; Lattimer, R. P. A Development of Polymer Mass Spectrometry. J. Chem. Educ. 2007, 84 (1), 81−90. (9) Dopke, N. C.; Lovett, T. N. Illustrating the Concepts of Isotopes and Mass Spectrometry in Introductory Courses: A MALDI-TOF Mass Spectrometry Laboratory Experiment. J. Chem. Educ. 2007, 84 (12), 1968−170. (10) Uniform Acceleration: Ball rolling down an incline-xmdemo 111. https://www.youtube.com/watch?v=LQMQLcV0Mwg (accessed Jul 2017). (11) Yeany, B. Homemade Science: High road low road track race, potential-kinetic energy tracks. https://www.youtube.com/watch?v=_ GJujClGYJQ&list= PLrwBjIP07Qj1O882WT9iUwv1v5iKDXNeD&index=1 (accessed Jul 2017). (12) Goss, V.; Brandt, S.; Lieberman, M. The Analog Atomic Force Microscope: Measuring, Modeling, and Graphing for Middle School. J. Chem. Educ. 2013, 90 (3), 358−360. (13) Hajkova, Z.; Fejfar, A.; SmejKal, P. Two Simple Classroom Demonstrations for Scanning Probe Microscopy Based on a Macroscopic Analogy. J. Chem. Educ. 2013, 90 (3), 361−363. (14) Albert, D. R.; Todt, M. A.; Davis, H. F. A Low-Cost Quantitative Absorption Spectrophotometer. J. Chem. Educ. 2012, 89 (11), 1432−1435.
SUMMARY The demonstration described above was designed to introduce the basic principles of mass analyzers to high school students. The students were required to continuously check the mechanisms and the schematics of the corresponding mass analyzers. They enjoyed the demonstration and exhibited an improvement in the understanding of the overall principles. The mechanisms involved simple, readily available components, such as stationery, commodities, or toys. In addition, these models can be easily disassembled and reassembled so that they are suitable for use in classroom demonstrations. The drawback of this system is that the m/z values of the ionized analytes cannot be illustrated by mechanisms 1−3. Illustrating the basic principles of analytical instruments with similar mechanisms is the subject of future research.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00297. Instructions for construction of the ball-rolling mechanisms, images from the demonstrations, note for instructors, a student handout, students’ comprehension, and reference books (PDF, DOCX) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI) Video S4 (AVI) Video S5 (AVI) Video S6 (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ryo Horikoshi: 0000-0002-8609-9173 Hiroshi Kageyama: 0000-0002-3911-9864 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant JP16H06438. R.H. and F.T. would like to acknowledge contributions from T. Okumoto and M. Okumoto and their students at Tezukayama Senior High School. R.H. would like to thank J. Seta (Bruker Daltonics K.K.) and H. Chihara (Osaka Sangyo University) for their helpful discussions. R.H. thanks Enago (www.enago.jp) for the English language review.
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REFERENCES
(1) Watson, J. T.; Sparkman, O. D. Introduction to Mass Spectrometry: Instrumentation, Applications, and Strategies for Data Interpretation, 4th ed.: John Wiley & Sons. Ltd.: Chichester, UK, 2011. (2) Ebbing, D. D.; Gammon, S. D. General Chemistry, 11th ed.; Cengage Learning: Boston, 2015. (3) Petrucci, R. H.; Herring, F. G.; Madura, J. D.; Bissonnette, C. General Chemistry: Principles and Modern Applications, 10th ed.; Pearson Canada Inc.: Toronto, 2011. (4) Davis, E. R.; Frey, R.; Sarquis, M.; Sarquis, J. L. Modern Chemistry; Holt, Rinehart and Winston: Austin, TX, 2009. E
DOI: 10.1021/acs.jchemed.7b00297 J. Chem. Educ. XXXX, XXX, XXX−XXX
