One of the most important pieces of information a chemist seeks for identification of an unknown is the molar mass of the compound. The is even true of purely chemical methods of identification, since stoichiometry calculations depend on molar mass. If the mass spectrometer is being used to confirm an identity, the molecular mass should be computed for the proposed structure. On the other hand, if one is attempting to identify a total unknown, the molecular mass should be determined experimentally. Measurement of the molecular mass via EI mass spectrometry can get tricky; chemical ionization is a technique that offers less ambiguity in molecular mass assignment.