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

  • mass spectrometry

  • 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.