Mass Spectrometry

Overview:

Mass Spectrometry is introduced in this unit. Instrumentation, fragmentation patterns, isotopic influence, and identification techniques are addressed.
The student is introduced to the fundamental way in which the mass spectrometer works and the concept of fragmentation. The initial formation of the radical cation is introduced and the way in which this structure subsequently fragments to produce the free radicals and cations is discussed. Previously studied concepts of stable free radicals and cations are used to facilitate determination of the most likely fragments to be formed. The classic fragmentation patterns are predicted for select structures to give the student some experience in this area.

The base peak and parent ion, key terms expected in mass spectrometry, are presented and explained. The importance of the base peak as a reference is introduced. The information gained from the parent ion, if one happens to be present, is a highly beneficial item of data. However, absence of the parent ion is expected under certain circumstances, particularly when larger alcohols are present. The unit addresses identification of alcohols as a functional group by the mass differences indicated by signal combinations. In this manner a student quickly learns that it is not just the individual signals from which information is derived, but it is also the combination of signals that present important information.

A series of steps is outlined in order to help the student learn an approach pattern for solving these mass specs. The importance of the signal position representing the relative weight of the fragment and the height of the signal indicating the intensity of the signal or frequency of occurrence of that particular fragment are items of evidence for determining the identification of an unknown compound. One of the parts of the pattern or puzzle is found in isotopic patterns presented in characteristic mass specs. How to recognize and identify atoms such as chlorine, bromine, and iodine by their combination and/or location of signals is addressed. Students are taught to quickly scan the mass spec in the event such patterns are present. Isotopic patterns which result in the production of small signals are discussed so students can understand that those tiny signals they see actually mean something.

A series of simple spectra are examined to provide an initial experience in solving mass spec. Most students will expect to see these mass spec presented in combination with other spectral data such as infrared and ultraviolet spectroscopy.