What does a mass spectrum tell me?


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.

How do we interpret the data from the GCMS? To the right is an image of a mass spectrum (Plural: mass spectra). Chemicals can either pass through the MS whole, or can break apart into smaller fragments. The x-axis represents the mass of what is being detected and the y-axis is abundance or how much is detected. Each chemical creates a unique pattern, or fingerprint, depending on if and how it breaks into 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.

Data from Mary’s Peak

Using the library, we know that this is the mass spectrum of the pesticide DDT and Retene. 

Here is the sample collected at Mary's Peak. 


Mass spectrum from Mary's Peak air sample
This is the mass spectrum for the pesticide DDT (on the left) and retene (on the right). Retene is 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. Air containing DDT is most likely to have come from the west where DDT is still used in Asia.

DDT mass spectrum

Retene mass spectrum


What is the chemical and where is it coming from?