The molecular peak is important for determining the molecular weight. However, an EI spectrum sometimes shows such a small molecular peak that it is hard to determine the molecular weight. Compared to an EI spectrum, a PCI spectrum shows an intense molecular-related peak with less fragmentation. Using methane or iso-butane usually produces a protoneted molecule, whose mass is M+1, where M is molecular mass. For example, the base peak of a methylstealate PCI spectrum is 239; molecular weight of methylstealate 238 plus mass of hydrogen ion 1.

Spectrum of PCI