1. C2.8 Instrumental Analysis
Mass spectrometry
C2.8 Instrumental Analysis

2. What is it?
Mass spectrometry is the main method used for finding the mass of organic compounds

3. Background information
Task: Cut out the statements and link them to the correct areas of the mass spectrometer.

4. Worked example
We will draw the complete and labelled spectrum for pentane.

5. The molecular ion
When an organic compound is placed in the mass spectrometer it loses an electron and forms a positive ion, the molecular ion (M+)
This molecular ion is the peak with the largest m/z value on the spectrum and gives the molecular mass of the compound.
The spectrometer detects the mass/charge (m/z) ratio and since the charge is almost always 1+, it is equal to the mass.
Molecular ion

6. Knowledge Check
Give the structural formula of the molecular ion for each of the following molecules:
Ethane
Ethanol
Methylpropane
Compound A has the empirical formula CH3. Mass spectra analysis shows compound A to have a molecular ion peak at 30. Using this information draw the structure of compound A.
(Don’t forget the molecular ion always has a positive charge. It has lost an electron!)

7. FRagmentation
In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation.
The other peaks in the mass spectrum are caused by fragment ions.
Fragmentation peaks can be used to identify the structure of an organic molecule by considering which combination of atoms could be contributing to the mass of the fragment.

8. Knowledge check
Complete the following: A mass spectrometer can be used to ______________. The largest peak in the spectrum is known as the ___________. This is the ion with the greatest ________________ ratio. The other peaks in the spectrum are known as ____________. These are happen when ___________________.

9. Knowledge check
Complete the following: A mass spectrometer can be used to determine the structure of a molecule. The largest peak in the spectrum is known as the molecular ion. This is the ion with the greatest mass to charge ratio. The other peaks in the spectrum are known as fragments. These are happen when parts of the molecule break apart.

10. Worked Example
Predict a possible mass spectrum for butan-2-ol including any possible fragments.
Predict the mass spectrum for propanal including any possible fragments.

11. Knowledge check
At what m/z would you expect to see the largest value for propan-1-ol?
What group has been lost from propan-1-ol to give a peak with m/z 43?
A compound was found to have a molecular ion with a mass to charge ratio of Which of the following molecular formulae could it have? C9H12O or C10H16? You will need to work out the accurate Mr of each of these molecules.
A primary alcohol containing five carbons produces the following m/z peaks: 87, 70, 56, 29. Identify this alcohol
Use the spectra below to identify the molecular ion peak and hence the molecular mass of compound A and compound B.

12. Knowledge check 60.08 OH C9H12O m/z 87.11 - CH3CH2C(CH3)CH2OH+
At what m/z would you expect to see the largest value for propan-1-ol?
What group has been lost from propan-1-ol to give a peak with m/z 43?
A compound was found to have a molecular ion with a mass to charge ratio of Which of the following molecular formulae could it have? C9H12O or C10H16? You will need to work out the accurate Mr of each of these molecules.
A primary alcohol containing five carbons produces the following m/z peaks: 87.11, 70.10, 56.08, Identify this alcohol.
Use the spectra below to identify the molecular ion peak and hence the molecular mass of compound A and compound B.
60.08 OH
C9H12O
m/z CH3CH2C(CH3)CH2OH+
m/z CH3CH2C(CH3)CH2+
m/z CH3CH2C(CH3) +
m/z CH3CH2+
A – 73
B - 100