Mass spectrometry has grown into one of the most powerful tools a
chemist has for the identification of unknown compounds. Though not
practical for many substances, the technique is useful for a large
majority of organic compounds. Coupled with a gas chromatograph, it
is possible in many cases to collect mass spectra of pure compounds
from submitted samples (of unknown purity) with very little manual
sample preparation.
Mass spectrometry is a very, very broad field; in this chapter, we
will only investigate that portion of the field that is commonly applied
in an analytical forensic laboratory. However, it should be born in
mind that many other mass spectrometry techniques exist, and they
are useful, too.
For analytical purposes, the mass spectrum generally contains data
corresponding to the masses of decomposition products of a molecular
ion. The nature of the decomposition reactions relates quite specifically
to the structure of the molecular ion. In a sense, by measuring the
mass spectrum, a set of reactions of the molecular ion is inferred.
We then use this chemistry to deduce the structure of the unreacted
molecular ion.
In brief summation, measurement of the mass spectrum requires ionization of a sample with sufficient energy to cause decomposition reactions, filtering the reactant and product ions based on mass, detection of the ions after filtering and storing the data. Very few mass spectrometry systems are operated without computer data systems, and the data system used in this laboratory will be briefly covered in a subsequent chapter.