Correction: At 4:17 - 5:16 the structure for Butanoic acid is given instead of Propanoic acid. The fragmentation at 57 also comes from the formation of the C3H5O fragment as a result of cleavage of the OH bond. Check out the Interpreting Mass Spectrum Fragmentation Patterns page on our website to see the analysis scienceready.com.au/pages/interpreting-mass-spectrum-fragmentation-patterns