Crayons, Boxes, and Books: A Model for Mass Spectrometry Thomas D. Crute and Stephanie A. Myers Augusta College, Augusta, GA30910 Mass Spectrometry (MS) has gained increased attention in recent vears e v e n the availabilitv of appropriate afford.. . able ins&ume&ation. The technique is discussed in a t least three different courses in our curriculum; general chemistry (for isotope analysis), organic chemistry (for structure determination), and instrumental analysis. Recent articles related to mass spectrometry have described research projects ( I ) , laboratory experiments (2,3, 4 ) , and comouterized aids (5. 6). An a m a r a t u s for illustratine the principles of mass s ~ k c t r o m e t ~ ~been h a s published (7). We have develo~edan analoev -" to illustrate some of the ~ r i n c i ples of MS using inexpensive, readily available materials. In its most basic form, mass spectrometry is easy: break a sample of molecules into pieces and weigh the pieces. lkaditional aooroaches to teachine mass s ~ e c t r o m e t r v .. can, in our experience, leave several misconceptions arising from students'inabilitv to visualize what is hameniug inside the instrument. OU; primary goal i n developing this analogy was to communicate that a large number of molecules are present in the sample, and while they may fragment i n many different ways, each actually fragments in one particular way. Therefore, we sought a simple model that would allow students to readily grasp the basic principles of mass spectrometry and found that a few crayons, a box, and a hook work well. Procedure Our apparatus consists of a sample for analysis (a number of crayons in which the paper has been peeled om, the instrument (a cardboard box), and a method of ionization (large and small hardback books). A typical sample consists of 10-20 cravons of the size that is obtained from a box of 64 crayons."~smallscore is etched into each crayon about one-fourth the distance from the end of the cravon. This score increases the likelihood of the crayon breaking at this point, and represents a n easily cleaved bond. The crayons are placed in a shallow cardboard box (we use a 30x 40-cm box which is 1 5 cm high). A CRC Handbook of Chemistry a n d Physics is dropped onto the sample of crayons five or six times from a height of 0.5-1 m. This is suffcient to give a variety of fragments for analysis. Analysis consists of sorting the fragments by size, corresponding to approximately similar masses. o n e or more crayons is likely to survive intact (molecular ion). Most of the crayons will break a t the place where scored (the weak bond). There will be several groups of fragments with similar masses and numerous very small crumbs. If desired, the procedure can be repeated using the small book representing a milder ionization method. Discussion Many parallels can be drawn between the above procedure and mass s~ectrometrv.Most obvious is that this procedure i s a des'tructive t&hnique. Students i n organic chemistrv are usuallv introduced to MS i n coniunction with infrared spectros~opy(IR) and nuclear magnetic resonance spectrometry (NMR), which are fundamentally different and nondestructive. While IR and NMR examine energy changes associated with electromagnetic radiation interacting with the sample, MS actually detects fragments of the sample. This demonstration addresses the
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Journal of Chemical Education
concerns from above by emphasizing that a large number of molecules (crayons) is present i n the sample and that each molecule (crayon) is capable of fragmenting i n many different ways. The fact that each molecule actually fragments in one particular way is pointed out when separating fragments. This concept may easily he lost when analyzing a single structure drawn on the chalkboard and predicting likely fragments. In a t , v ~ i c a mass l soectrum. certain sized fraements are more fawred then othrrs, and the rnolemlar Ion is usually in t h r :in;ilw~..\loit crnvons break ran:. This i s o;ir:iilt~lc~d a t the score (weak bondifavorable cagment) and can also break elsewhere. I t i s ~ o s s i b l eto have similarlv sized pieces by wav ol'dit~ereniSraemenring. which is thecosc in 115. It is :il.w e m a r m t th:lt no fra~mwnr-:Iaraer than the molecular ion aEe normally found, a i d then oniy if they are recombined fragments. This can actually be observed in the crayons as different colored crayon pieces may be fused together. If a smaller book is used to smash the crayons, the proportion of intact crayons increases. This demonstrates the case when soft ionization methods such as chemical ionization are used. The advantages to hard and soft ionization sources can be discussed. While this analogy is a fun and interesting way to demonstrate the fundamentals of mass spectrometry of organic compounds, there are several significant limitations. The first is a realization that a molecule is made up of discrete units (atoms), which fragment a t bonds to create only certain sized pieces that are multiples of the various constituent atoms. Acrayon, being one "continuous" unit for these purposes, is able to fragment essentially anywhere and produce an endless variety of fragment sizes. Thus in the fragment analysis one is only concerned with rough groupings of fragment sizes. A solution to this is to use a model that is constructed of discrete units such a s several SweetTart or Skittles candies glued together. While this improves the analogy, there is considerable effort involved in sample preparation. A second limitation is difficulty in showing isotopes. One remedy involves marking samples with white-out liquid paper or a small piece of tape to indicate the presence of a different isotope. Athird limitation is the method of detecting fragments. Only positively charged fragments are detected in the MS while we can examine all of the crayon fragments. The way that we detect them is also fundamentally different, but the fact t h a t we must examine the pieces somehow can lead into a discussion of how the MS accomplishes this task. We have found this analogy to supplement a more traditional discussion of mass spectrometry in a memorable way. If the limitations of the method are kept in mind, this activity can help students appreciate the basis of this powerful instrumental method.
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Literature Cited 1. Holdswolth, D.: Ching, G.S.: Hsmld. M. J. Chem. Educ 1992.69, 856. 2. Hamann.C.S.;Myen.D.P.:Rittle.K.J.:Wiri'~.E.F:Moc.O.A.J Cham.Ediie. 1991, 68, 438.
3. Hill. D. W.; McSharry.B. T.; 'hupek. L. S. J Chnn. Edur. 1988.6.5.907 4 . Mabbott.G.A. J. Cimm Educ 1990, 67, 441. 5. Arnold. C. J. J. C h l m Educ 1992.69. 811. 8. Lehman.T.:Veeenin.G. J. Chern. E d w 1993. 7OA816. 7 . Finet, D.J Chem. E d u i 1980.67, 232.
