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


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


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


What is the chemical and where is it coming from?