The M+1 & M+2 Peaks | CIE A Level Chemistry Revision Notes 2025

Determine Number of Carbon Atoms Using M+1 Peak

The [M+1] peak is caused by the presence of the carbon-13 (¹³C) isotope in the molecule
Carbon-13 makes up approximately 1.1% of all carbon atoms
Therefore, the [M+1] peak is much smaller than the M peak as the isotope is less common
- The ratio of ¹³C to ¹²C is approximately 1:99
Thus, the greater the number of carbon atoms present in a molecule the greater the height of the [M+1] peak
The number of carbon atoms, n, in a compound can be deduced using the [M+1] peak and the following formula:
- $n = \frac{100 \times abundance of [M + 1]}{1.1 \times abundace of M^{+} ion}$

Determine the number of carbon atoms of compound X with the following mass spectrum:

Analytical Techniques Spec 1_Determine Number of Carbon Atoms Using M+1 Peak, downloadable AS & A Level Chemistry revision notes

Answer:

The M⁺ ion peak is at m/e 58 with a relative abundance of around 85
The [M+1] peak is at m/e 59 with a relative abundance of 3
Therefore, the number of carbon atoms (n) is:
- $n = \frac{100 \times 3}{1.1 \times 85} = 3.21$
There are therefore 3 carbon atoms present in compound X

The presence of bromine or chlorine atoms in a compound gives rise to a [M+2] and possibly [M+4] peak

Chlorine exists as two isotopes, ³⁵Cl and ³⁷Cl
A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain
- ³⁵Cl = M⁺ peak
- ³⁷Cl = [M+2] peak
- The ratio of the peak heights is 3:1 (as the relative abundance of ³⁵Cl is 3x greater than that of ³⁷Cl)
A compound containing two chlorine atoms will have three molecular ion peaks due to the different combinations of chlorine isotopes they can contain
- ³⁵Cl + ³⁵Cl = M⁺ peak
- ³⁵Cl + ³⁷Cl = [M+2] peak
- ³⁷Cl + ³⁷Cl = [M+4] peak
- The ratio of the peak heights is 9:6:1

Analytical Techniques Mass Spectrum Chlorine, downloadable AS & A Level Chemistry revision notes

Mass spectrum of compounds containing one chlorine atom (1) and two chlorine atoms (2)

Bromine too exists as two isotopes, ⁷⁹Br and ⁸¹Br
A compound containing one bromine atom will have two molecular ion peaks
- ⁷⁹Br = M⁺ peak
- ⁸¹Br = [M+2] peak
- The ratio of the peak heights is 1:1 (they are of similar heights as their relative abundance is the same!)
A compound containing two bromine atoms will have three molecular ion peaks
- ⁷⁹Br + ⁷⁹Br= M⁺ peak
- ⁷⁹Br+ ⁸¹Br = [M+2] peak
- ⁸¹Br + ⁸¹Br= [M+4] peak
- The ratio of the peak heights is 1:2:1

Analytical Techniques Mass Spectrum Bromine, downloadable AS & A Level Chemistry revision notes

Mass spectrum of compounds containing one bromine atom (1) and two bromine atoms (2)

Two stable isotope of bromine have relative masses of 79 and 81

Which is the correct pattern of peaks in the mass spectrum of molecular bromine?

Analytical Techniques Question Worked example - Analysing bromine spectra, downloadable AS & A Level Chemistry revision notes

Answer:

The correct answer is D
Bromine is a diatomic molecule there will be 5 peaks on the mass spectrum of bromine
Bromine consists of molecules, not individual atoms
When bromine is passed through the mass spectrometer, an electron is given off to give the molecular ion, Br₂⁺
Some of these will fragment to make Br + Br⁺
- Br₂⁺→ Br + Br⁺
The Br atom passes through the machine, and the Br⁺ ions will give lines at 79 and 81
There will also be a line for the unfragmented Br₂⁺ion
This will give 3 molecular ion peaks
- Br₂⁺ ion containing the isotopes 79 + 79 = 158
- Br₂⁺ containing the isotopes 79 + 81 = 160
- Br₂⁺ containing the isotopes 81 + 81 = 162