Interpreting Mass Spectrum Fragmentation Patterns
This is part of the HSC Chemistry course under Module 8 Section 2: Analysis of Organic Substances.
HSC Chemistry Syllabus
Investigate the processes used to analyse the structure of simple organic compounds addressed in the course, including but not limited to:
proton and carbon-13 NMR
infrared spectroscopy (ACSCH130)
Mass Spectrometry Fragmentation Patterns
This video explores how mass fragmentation patterns in the mass spectrum can be utilised to help us confirm the identity of a compound.
Understanding Mass Spectrometry
Mass spectrometry is a spectroscopic technique which measure the mass-to-charge ratio of molecules. The process involves a heater vaporising a sample molecule which is then ionised by an electron gun. This ionisation typically fragments the molecule.
The Role of Fragmentation
The fragments which are formed as a result of ionisation each produce unique signals in the mass spectrum. By analysing these fragmentation patterns, we can deduce and confirm the structure of a suspected compound. Analysis involves looking at the mass-to-charge (m/z) ratio, which directly indicates the molecular mass of the fragment.
Example Fragmentation Patterns (analysed in video)
- Pentane: this molecule's molecular ion peak at a mass-to-charge ratio of 72 matches its molar mass. The peak at 29 in the mass spectrum aligns with the `C_2H_5` fragment.
- Ethanol: Here, the base peak at 31 correlates with the `CH_2OH` fragment. Additionally, a peak at 45 signifies the loss of a hydrogen atom, likely from the alcohol group.
Propane: A peak at 15 in its spectrum often indicates a methyl group, resulting from cleavage between specific carbon atoms.
- Propanoic Acid: Signals at 45 and 57 in this acid's spectrum are indicative of the carboxylic acid group and a specific cleavage pattern.
Esters and Amines (analysed in video)
Esters: In molecules like methyl propionate, cleavages near oxygen atoms lead to distinct ion peaks.
- Ethanamine: Its molecular ion peak and base peak reveal specific fragments, aiding its identification.