Mass Spectrometry is introduced in this unit. Instrumentation,
fragmentation patterns, isotopic influence, and identification techniques
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.