A model of mass spectrometry

Q1.How easily do your students understand mass spectrometry?

Q2.Do you use any modelling activities to help explain mass spectrometry?

Q3.Do you see a need for mass spectrometry models?

In this modelling activity, the first 26 prime numbers are assigned to the letters of the alphabet. Each letter of the alphabet now has a ‘relative mass’ number. The activity is provided, with gratitude, courtesy of Tracy Suggs, the National Science Foundation and Cornell Center for Materials Research.

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z
1	2	3	5	7	11	13	17	19	23	29	31	37	41	43	47	53	59	61	67	71	73	79	83	89	97

A word represents a molecule, ie letters ‘bonded’ together.

Another ‘word’ is passed through the word spectrometer. All possible 1-, 2-, 3- and 4-letter fragments were detected.

The following peaks were in the ‘mass’ spectrum.

1

7

8

37

38

45

67

74

75

112

Using the code below, see if you can work out the identity of the word. 

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z
1	2	3	5	7	11	13	17	19	23	29	31	37	41	43	47	53	59	61	67	71	73	79	83	89	97

Check your answer.

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Why is this a useful precursor to teaching mass spectrometry?

• Students are familiar with words. The exercise helps them make the connection between atomic mass and the fragment masses. The exercise encourages students to consider what letters are adjacent to each other.

How does the order of letters in a word relate to bonding in a molecule?

• Students’ knowledge of words means they can often work out a word from simply knowing the individual letters.In a similar manner, a knowledge of bonding and how many bonds each atom can make can enable spectrometrists to more easily piece together molecular fragments. 

What could be a precursor to this activity?

• An easier activity begins with a stack of cards containing 1-, 2- and 3-letter fragments of a word, eg the word TEAM can be broken down into T, TE, TEA, E, EA, EAM, A, AM, M. These word fragments are written on cards, shuffled and placed face down. As students turn over the cards one by one they must work out the original word. You can try this activity with different elements of the periodic table. 

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Where are students most likely to go wrong?

• As mentioned in the animation, once students have worked out the individual letters that make up the word, they are likely to simply have a guess and end up with MATE or MEAT instead of TEAM. The ordering of the letters is important and can be worked out by looking at the other letter ‘peaks’.

I understand the activity. But why would I use it and not simply use proper mass spectra?

• The fact that TEAM, MATE and MEAT give rise to such similar peaks enables you to take the discussion into lots of different directions such as looking at isomers, connectivity in organic molecules etc. The activity is also fun and, for some students who may find chemistry challenging or be inclined to disengage, this can be a great way to spark interest.