Data Analysis: What does a mass spectrum tell me?
 How
do we interpret the data from the GCMS? Below is an image of a mass
spectrum (Plural: mass spectra). The x-axis is mass and
the y-axis is abundance or quantity. Each chemical creates a unique
pattern, or fingerprint, depending on if and how it breaks into
ions.
The detector counts the ions of different masses after they pass
through the filter. Recall from the animation that the original
molecule had a mass of five. On the mass spectrum this largest ion
is the molecular ion. It has not lost any pieces.
The smaller ions, masses 1, 2, 3 and 4, are called the fragment
ions. This mass spectrum shows that a fraction of the chemical
passed through the MS without breaking apart. It also shows that
the chemical is most likely to break into ions in a combination
of 1 and 4 rather than a combination of 2 and 3.
Scientists can compare the mass spectrum of an unknown compound
to a library of mass spectra of known compounds. This is the same
way police compare the fingerprint of an unknown suspect to a library
of known fingerprints. If the mass spectra is not in the library,
scientists can also use the fingerprint to work backwards to determine
the original structure of the chemical.
Data from Mary’s Peak
This is the mass spectrum of the pesticide DDT. Air containing
DDT is most likely to have come from the west where DDT is still
used in Asia.
DDT mass spectrum
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This is the mass spectrum of retene, a combustion product of burning
vegetation. Air containing retene is most likely to have come from
the east from early season forest fires in the Pacific coast range
and the Cascade mountains.
Retene mass spectrum
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Let’s take a look at the chemical in the air sample collected
at Mary’s Peak.
Mass spectrum from Mary's Peak air sample
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