overhead projector demonstrations
edited by
DORISKOLB Bradley Univenily Peoria, IL61625
Mass Spectrometry Analogy on the Overhead Projector Nancy C. Grim and Jerry L. ~ a r ~ u l s ' Miami University Oxford. OH 45056
The overhead projector is a useful tool to introduce analogies into the large classroom. An analogy for the principle of separation by mass spectrometry has been developed. Equipment Needed The mass s ectrometry analogy needs a supply of nickel shot spheres,5' BB shot, a marble, a very strong small, rectangular magnet,3 two glass microscope slides, clear tape, a n overhead projector marker, a 25-cm x 20-cm x 0.5-cm Plexiglas sheet, and three 96-well clear plastic well plates to catch the rolling spheres. Figure 1indicates how to cut the Plexiglas into pieces: one 18-cm x 23-cm (El, two l-cm x 21-cm (C), one l-cm x 18-cm (Dl, and two l-cm x 6-cm pieces (A, B). The two 6-cm long pieces should have a Vshaped notch cut out that is centered at 3 cm and is aDprokmately 0.5 cm (A) or 0.8 cm (B) deep. Firmre 2 shows how the com~onentsare arraneed to Derfo&the analogy. The three l-cm wide pieces (c,>) arenot glued in place because flexibilitv is needed to adiust the angle of the Plexiglas plate and the placement of h e magnet to achieve optimum separation of the spheres. Notched piece A should be centered near the top of the plate. Notched piece B should be approximately 2.5 cm below piece A. The two glass microscope slides (GI should be taped on both sides to form a V-shaped trough and then set into the grooved pieces to make a ramp. The tip of the microscope slide V should rest approximately 2.5 cm below B. The edge of the magnet should be lined up with the center line of the ramp. The magnet (MI should be about 4 cm below the glass slide tip. These measurements are approximate. Determine the best position by trial and error with your set-up. The 96-well plates (F) are placed at the bottom and side of the Plexiglas plate (E)to catch the spheres.
c6 1
a
6
c
m
S
20 cm
c
m --c1+14
mm
Figure 1. Template for Plexiglas sheet.
Performing the Mass Spectrometry Analogy Practice Session
Select four nickel spheres that are different in size and are smaller than the marble. If the nickel sphere is too large, it will move the magnet as it rolls past; if it is too small, it may stop just above the magnet. Adjusting the speed of the sphere by adjusting the height of the plate or 'Author to whom correspondence should be addressed. 'Nickel shot comes in a variety of sizes. We used 13-mm,7-mm, 6-mm,and 5-mm diameter nickel shot spheres. 3Neodymium magnets can be obtained from The Magnet Source, Master Magnetics, Inc., 607 S. Gilbert, Castle Rock, CO 80104. These magnets are rejected from use in computer disk drives and vary in size. The magnet that we used is approximately0.25-in.thick, 0.5-in. wide, and 0.75-in. long. 930
Journal of Chemical Education
Figure 2. Expected results for mass spectrometry analogy.
the microscope ramp will help to prevent these types of problems. Spheres are placed a t the top of the ramp and then allowed to roll down the ramp, down the plate past the magnet, and then into the well plate. Watching where the spheres leave the plate a t the bottom, mark the locations with a n overhead pen. The marble is not attracted to the magnet and will follow a straight path down the plate. Larger spheres have more mass and, therefore, are deflected less than smaller spheres. Figure 2 illustrates a typical set of results. Classroom Presentation
You begin by asking the students to watch the following demonstration and hypothesize as to what phenomenon is occurring. You point out that you have five spheres of different sizes. Students can see these spheres in a Petri dish on the edge of the projector. Do not mention t h a t the 41rwin Talesnick can be contacted at the Department of Science
Education-Chemistry, Queen's University, Kingston, Ontario. Canada K7L 3N6.
square they see is a magnet. Roll the marble down the plate and mark on the plate where it crossed just before falling into the well plate. Next, roll one of the nickel spheres down the plate, again marking the location it crossed just before falling into the well plate. Ask the class why the behavior was different. Depending upon the nature of the responses, you may wish to reveal that a magnet is involved or you may wish to repeat the ex~ e r i m e nwith t the other snheres. Before rolline each s ~ h e r e down the incline, ask the &ass to predict its hegavior. ' When all of the spheres have been rolled down the plate, ask students if a predictable pattern has been observed. If vou have not alreadv indicated a m a m e t is involved. ask leading questions until the presence of a magnet is suggested. Ask if they can think of any use for such a technique. At this point, you can discuss mass spectrometry and its applications such a s determining average atomic masses or its use as a n analytical tool. Acknowledgment We would like to acknowledge Irwin ~ a l e s n i c kwho ~ developed the macroscale version of the mass spectrometry analogy.
Volume 72 Number 10 October 1995
931
