An Analogy for Teaching Interpretation of Mass Spectra Gary A. Mabbott Colby College, Waterville, ME 04901 Introductory chemistry courses and chemistry classes for nonscientists often include some mention of modern instrumental methods of analysis. Usually interpretation of spectra is beyond the scope of the course and the preparation of the students. However, for some subjects, such as introductory organic chemistry or forensic chemistry, a discussion of spectral and structural relationships seems to be appropriate. Students find mass spectra t o be conceptually easier to understand than forms of optical spectroscopy. Of course, there is always an initial credibility problem that we have as teachers whenever we claim that one good way of identifying a substance starts by smashing it into pieces. A little word eamecan be an effective and entertaining-wavof . introducingthe subject. I tell mv classes that in some ways atoms are to molecules as letters are to words. Imagine trying to reconstruct a word from various fragments. On note cards I have written all of the possible one-, two-, and three-letter fragments (one per card) for the word COMPOUND. Individuals in the class are asked to select and read a card as I write the list of fragments on the board. Out of five classes I have never had to record more than four fragments before someone messed the word. Of course, that L too simple. I point ourthat the game is easier than working with atoms and molecules since each letter reveals its identity when we look a t a fragment. Since with atoms we often work with their masses, we play the game again, but this time I assign eachletter a "mass". I give them a table (see figure) in which each letter of the alphabet corresponds to one of the first 26 prime numbers (see below). This time I show them all possible fragments for a word, but they get only the masses of each fragment. An example puzzle using the word FUN is shown below. Students catch on auicklv: F is recomized rieht awav. Thev have a tendencv to work fpdm the loamass e& ~ h a t " a ~ p r o &ishall right bui is not reallv what we do with mass s ~ e c t r a .Occasionallv, without prompting, someone will poini out that the F must be on the end, since the difference in mass between the largest rnms rwhirh represents the whole word) and the one next to it must represent the loss of a letter from the end. Of course, that kindbf reasoning is exactly what we want them to do. Applying that logic they are able to sequence the letters. We work through asecond puzzle in class, and a third is given as homework. There are other principles that can be demonstrated by analogy here. For example, since B is the only letter with an even mass. an odd mass fragment must have an odd number of non-B litters. This idea similar to the nitrogen rule that says that an odd molecular weight indicates a molecule with adodd number of nitrogens.' Another extension of the game can he used to introduce the determination of elemental composition from isotopic
abundances. If we allow capital letters to appear randomly anywhere in the word but a t only 1%of the frequency that lower case letters appear, we have an analogy to heavy isotopes. The additional information that the students are given is the percentage of whole words that are found with one capital letter. I t is easy to demonstrate to the class that out of a population of copies for a five-letter word, for example, 5% of the whole words will contain one capital letter. The students then see that they can work in the other direction to obtain the number of letters in the word. Later, when the class is shown real mass spectra, the calculation of carbon number from the ratio of abundances for the M 1peak to the molecular ion peak is easier to follow. A table of isotopic abundances for other common elements is distributed and the students are shown how to deduce the remaining elements (as in McLafferty's text).'
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Students like the game and the challenge of solving a real spectral puzzle. In the forensic chemistry class they have interpreted mass spectra for simple molecules such as hydrogen cyanide and chloral hydrate on their own.
L E n E R 'MASSES" A
B
1 2
C
D
F G
E
3 5
7
N O P 41 43 47 53
11
R
S
59 61
J 23
H I 17 19
13
T
U
V
W
67 71 73 79
K
L
M
29 31 37
X
Y
Puzzle t i . (FUN) Fragment masses: 11.41.71,82.112.123 Puzzle #2. (EASY) Fragment masses: 1,7,8,61.62,69. 89.150,151.158 Puzzle W . (MOLECULE) Fragment masses: 3.7. 10,31.37,38.41.43,71.74.80, 81, 84,105, 109,111,112,118,119,121, 143,150,155,186, 192,193,223,230
Presented in part at the 194th ACS meeting in New Orleans, NSFCSlP Svmoosium. Auausl31. 1987. ~ciaiferty,F: w.-interpietation of Mass Spectra; University Science: Mill Valley, CA, 1980. 1052
Journal of Chemical Education
Table of letter "masses"
Z
83 89 97
and three examDles of meir use to decode words.
