Chemistry Everyday for Everyone Tested Demonstrations
Electromagnetic Induction in Inductively Coupled Plasma submitted by:
Jimmy C. Yu Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong
checked by:
George F. Wollaston Department of Chemistry, Clarion University, Clarion, PA 16214-1232
Inductively coupled plasma (ICP) has become the dominant source for rapid spectroscopic multielement analysis (1). ICP-atomic emission spectrometry (AES) and ICP-mass spectrometry (MS) are two of the most popular techniques for the determination of trace metals in environmental samples (2). Our recently established multidisciplinary Environmental Science Programme requires the students to learn the principles of ICP-AES and to have hands-on experience with the instrument. Most of our environmental science students, however, do not have a strong background in physics. They are often puzzled by the term ICP, and they do not understand how the induction phenomenon supplies energy to sustain the high temperature. This is a simple demonstration to show what electromagnetic induction is. Experimental Procedure This demonstration requires a commonly available 5watt 27-MHz Citizens Band Radio1 (CB) as a radio-frequency (RF) generator, a 12-V dc 2-A power supply or a car battery, a magnetic mount CB antenna with a center or base loading coil (such as the Radio Shack’s 21-940), and a 20–40-W fluorescent tube. For easy access and good grounding, the antenna is best mounted horizontally on the side panel of a refrigerator in a laboratory. Turn the CB radio to a quiet nonemergency channel. While holding the fluorescent tube right next to the loading coil of the antenna, press the talk button on the CB’s microphone. The “Star Wars” glow will certainly draw the students’ attention, and they will beg you for an explanation. Discussion Induction may be defined as the production of an electromotive force by a change in the magnetic flux that threads a conductor (3). In this demonstration, the conductor is the mercury atoms inside the fluorescent tube and the fluctuating magnetic field is generated from the antenna emitting the 27-MHz electromagnetic radiation. This induction action produces an electromotive force, which causes the mercury atoms to collide with each other forming some excited atoms. When electrons in excited mercury atoms fall back to the ground state, energy in the form of ultraviolet (UV) radiation is released. The fluorescent coating in the tube absorbs
316
UV and emits the visible glow. This demonstration shows clearly how effective it is to transfer energy from RF to atomic species by induction. To relate the observations of this demonstration to the ICPAES instrumentation, you may ask the following questions: What if the power of the CB is increased to 2 kW? What if the antenna consisted only of an induction coil? What happens when slightly ionized argon gas is directed in the form of a jet inside the induction coil? What do we call an electrical conducting gas that contains a high concentration of cations and electrons?
The term ICP starts to make sense when the students think about these questions. To describe the ICP in more detail, a trip to see the spectrometer is helpful. The ICP torch is basically a quartz tube through which argon flows, with an external induction coil wrapped around the tip. The watercooled coil is powered by an RF generator typically running at 2 kW on 27 MHz (4). This coil induces an electromagnetic field within the torch. This field inductively heats the formed plasma to temperatures exceeding 5000 K. The argon gas that sustains the plasma is initially made electrically conductive by Tesla sparks before a self-sustained plasma results (5). Note 1. Citizens Band Radio is a license-free short-distance voice communications system used by private individuals. Inexpensive CBs are available at Radio Shack and many other department stores. A CB for mobile operation or an ac-powered base station works fine for this demonstration. Most hand-held walkie-talkies are not suitable because their power outputs are weaker than those of the mobile/base models. In the United States, 40 channels at frequencies from 26.965 to 27.225 MHz, have been allocated to CB. Channel 9 may be used only for emergency communications.
Literature Cited 1. Olesik, J. W. Anal. Chem. 1991, 63, 12A–21A. 2. Beauchemin, D.; Yves Le Blanc, J. C.; Peters, G. R.; Persaud, A. T. Anal. Chem. 1994, 66, 462R–499R. 3. McGraw-Hill Encyclopedia of Physics, 2nd ed.; Parker, S. P., Ed.; McGraw-Hill: New York, 1993; pp 338–339. 4. Day, R. A.; Underwood, A. L. Quantitative Analysis, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1991; p 458. 5. Meyer, G. A. Anal. Chem. 1987, 59, 1345A–1354A.
Journal of Chemical Education • Vol. 75 No. 3 March 1998 • JChemEd.chem.wisc.edu
