About mass spectrometry

1. AS 2.12 Mass spectra
a. interpret fragment ion peaks in the mass spectra of simple organic
compounds, eg the difference between propanal and propanone
Crowe2009
Connector:
1. Briefly describe the steps involved to produce a mass
spectrum. (A description of a spectrometer is not needed here.)
2. Look at the spectrum below
a) What element was analysed? b) Explain the spectrum

2. 1. Vaporisation of atoms or molecules;
2. Ionisation of atoms or molecules;
3. Acceleration of ions;
4. Deflection of ions;
5. Detection of ions.
The steps involved to produce a mass spectrum:

3. Mass spectrum of chlorine, Cl2
Isotopes Cl-35 & Cl-37 Molecules Cl2-70 Cl2-72 Cl2-74

4. Mass Spectra of Organic Compounds
1. The molecular ion
When an organic compound is bombarded by a stream
of high energy electrons in the spectrometer:
An electron is removed from the molecule, M, to form
the molecular ion, M+.
This ion will have the highest m/z value on the
spectrum, and the value will correspond to the
molecular weight of the compound.

5. Mass Spectra of Organic Compounds
2. Fragmentation patterns
The molecular ions are energetically unstable, and
some of them will break up into smaller pieces, e.g.
The uncharged free radical (Y ) won't produce a line
on the mass spectrum. Since only charged particles
are detected by the mass spectrometer.
All sorts of fragmentations of the original molecular
ion are possible, and these help with the identification
of the parent compound.

6. Mass Spectra of Organic Compounds
3. Fragmentation patterns
The base peak in a mass spectrum is the tallest
peak.
The most abundant ion is responsible for the base
peak.
The base peak is given a value of 100.
All other peaks are reported as percentages of the
abundance of the base peak

7. Methanol
The mass spectrum of methanol shows
peaks at 32,31, 29, 28 and 15.
Which line is the base peak?
32 represents the molecular ion, CH3OH+
Identify the other fragments.
CH3O+ CHO+ CO+ CH3
+

8. Ethanol and methoxymethane have the same molecular formula,
C2H6O, but their mass spectra are quite different: Identify them.

9. 45
29
15
17

10. 31
15
45

11. Work sheet – AS 2.12 mass spectra