Answer 1.2

a) How is a mass spectrum defined?

A mass spectrum is the two-dimensional representation of signal intensity (ordinate) versus m/z (abscissa). The intensity of a peak, as signals are usually called, directly reflects the abundance of ionic species of that respective m/z ratio which have been created from the analyte within the ion source.

b) Assign the following terms to the EI mass spectrum below: base peak, molecular ion peak, and fragment ion peak.

Mass spectrum of unknown small molecule. Spectrum by courtesy of NIST.

c) Can you identify the unknown compound from its spectrum?

Fragment ion peaks at m/z 12 and 16 point towards atomic ions of 12 and 16 u, i.e., 12C+. and 16O+. ions. The ion at m/z 28 corresponds to [M-16]+, and therefore indicates [M-O]+. Thus, the molecular ion seems to consist of C and O exclusively. The mass difference between M+. and m/z 16 is 28 u and should result from loss of a molecule of CO. Other neutral losses are not observed.
This spectrum belongs to carbon dioxide, CO2, 44 u.
The ion at m/z 22 is not a fragment ion, but is due to the doubly charged molecular ion, CO22+.