WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.3 Chemical calculations

Interpretation of mass spectra When a vaporised compound passes through a mass spectrometer, an electron is knocked off a molecule to form a positive ion. This ion is called the molecular ion and its mass gives the relative formula mass of the compound. The molecular ions are energetically unstable, and some of them will break up into smaller pieces or fragments. All sorts of fragmentations of the original molecular ion are possible – which means that you will get a wide range of lines in the mass spectrum. The mass spectrum of chlorine Chlorine is made up of two isotopes 35 Cl and 37 Cl. Chlorine gas consists of molecules, not individual atoms, but the mass spectrum of chlorine is:

Link

Mass spectrometry page 173

Study point

For the chlorine spectrum you can’t make any predictions about the relative heights of the lines at m / z 35/37 compared with those at 70/72/74. That depends on what proportion of the molecular ions break up into fragments.

C

Peaks in the ratio 3:1

Peaks in the ratio 9:6:1

A

D

Knowledge check In the mass spectrum of hydrogen, explain why peaks due to hydrogen atoms are present although hydrogen gas contains only H 2 molecules. 4

B

Relative

percentage

E

abundance

Mass/charge ratio

35 36 37

70 71 72 73 74

Molecular ion region, Cl 2 +

Atomic ion region, Cl +

When chlorine is passed into the ionisation chamber, an electron is knocked off the molecule to give a molecular ion, Cl 2 + . These ions won’t be particularly stable, and some will fall apart to give a chlorine atom and a Cl + ion. (This is known as fragmentation .) So peak A is caused by 35 Cl + and peak B by 37 Cl + . As the 35 Cl isotope is three times more common than the 37 Cl isotope, the heights of the peaks are in the ratio of 3:1. In the molecular ion region think about the possible combinations of 35 Cl and 37 Cl atoms in a Cl 2 + ion. Both atoms could be 35 Cl, both atoms could be 37 Cl, or you could have one of each sort. So peak C ( m / z 70) is due to ( 35 Cl − 35 Cl) + Peak D ( m / z 72) is due to ( 35 Cl − 37 Cl) + or ( 37 Cl − 35 Cl) + Peak E ( m / z 74) is due to ( 37 Cl − 37 Cl) + Since the probability of an atom being 35 Cl is 3 4 and that of being 37 Cl is 1 4 , then molecule 35 Cl − 35 Cl 35 Cl − 37 Cl or 37 Cl − 35 Cl 37 Cl − 37 Cl probability 3 4 × 3 4 3 4 × 1 4 or 1 4 × 3 4 1 4 × 1 4

Key terms

Molecular ion The positive ion formed in a mass spectrometer from the whole molecule. Fragmentation “Splitting of molecules, in a mass spectrometer, into smaller parts.”

⎧ 35 DRAFT ⎨ ⎩ 9 16 6 16 1 16 and ratio of peaks C:D: E is 9 : 6 : 1 The molecular ion region of mass spectra can give information about the isotopes in the molecule. Bromine has two isotopes. The molecular ion region of its mass spectrum is shown on the right. A B C 158 160 162 Relative percentage abundance Mass/charge ratio Peaks in the ratio 1:2:1