Mass spectrometry explorer
Read the mass spectra of real compounds: find the molecular ion, count carbon atoms from the M+1 peak, recognise chlorine and bromine from M+2 and M+4 peaks and see which bond breaks to give each fragment.
Learning goals
- Explain mass-spectrometric ionisation and fragmentation
- Interpret mass-to-charge ratio
- Identify a molecular-ion peak
- Interpret M+1, M+2 and M+4 isotope patterns
- Suggest major fragment ions without rearrangement
Mass spectrum of butan-2-one, CH₃COCH₂CH₃, built from reference intensities: m/z 29 (17%), m/z 43 (100%), m/z 57 (8.0%), m/z 72 (25%). The cursor is on m/z 72: the molecular ion, M⁺.
- Cursor, m/z
- 72
- Relative intensity
- 25.0 %
- Ion
- M
Try this
0 of 4 donePut the cursor on the M+1 peak and use it to count the carbon atoms. (not done yet)
About 1.1% of carbon atoms are ¹³C, so for a molecule with n carbon atoms the M+1 peak is about 1.1n% of the M peak.
Compare the M and M+2 peaks of chloroethane with those of bromoethane. (not done yet)
³⁵Cl : ³⁷Cl is about 3 : 1 and ⁷⁹Br : ⁸¹Br about 1 : 1, so one Cl atom gives M : M+2 ≈ 3 : 1 and one Br atom gives M : M+2 ≈ 1 : 1.
Find the M+4 peak of dichloromethane. (not done yet)
With two Cl atoms the molecule can hold ³⁵Cl³⁵Cl, ³⁵Cl³⁷Cl or ³⁷Cl³⁷Cl, so M : M+2 : M+4 ≈ 9 : 6 : 1.
Find the base peaks of pentan-2-one and pentan-3-one. (not done yet)
Both break next to C=O to form a stable acylium ion: CH₃CO⁺ (m/z 43) from pentan-2-one, C₂H₅CO⁺ (m/z 57) from pentan-3-one. Fragments tell isomers apart.