Organic Chemistry Lesson 18 Mass spectrometry

Mass Spectrometry Determine the molecular mass of a compound from the molecular ion peak. Analyse fragmentation patterns in a mass spectrum to find the structure of a compound

Mass spec in organic chemistry Way back in topic 2 we looked at elemental analysis using mass spectrometry. What did each peak on the mass spectrum represent in that case? Now we are going to look at using mass spec to ‘see’ organic molecules. When organic molecules go through the mass spectrometer, they fragment. The masses of the fragments are then found and the structure can be deduced. The heaviest ‘fragment’ is called the molecular ion peak. This is not actually a fragment but a cation of the whole molecule.

Try drawing these fragments.

Fragmentation

Fragmentation Note the simplification of this graph

Note the more realistic graph than the previous slide Note the more realistic graph than the previous slide. Note fragmentation and discuss M+1. Also note points of low or negligible significance i.e M+2 (from C-14 or two C-13) and m/z where z=2.

Plenary Try the following question

Key points Organic molecules fragment in the mass spectroscopy. The mass of each fragment is detected. This information can be used to help deduce structure. Remember, each spectroscopic technique has pros and cons. Usually at 2 or more techniques are used together to deduce full structure.