a) What sort of analytes are promising candidates for EI?
Analytes for electron ionization should be accessible to evaporation or sublimation without significant thermal decomposition.
These requirements are usually met by saturated and unsaturated aliphatic and aromatic hydrocarbons and their derivatives such as halides, ethers, acids, esters, amines, amides etc. Heterocycles generally yield useful EI spectra, and flavones, steroids, terpenes and comparable compounds can successfully be analyzed by EI, too.
Whereas low- and medium-polarity analytes are usually well suited for EI, highly polar or even ionic compounds should not be subjected to EI.
b) What is the upper limit for the analyte’s molecular weight to use EI?
There is – as with any other ionization method – no strict upper limit for molecular mass, nevertheless a range of up to 800-1000 u is a realistic estimate. Exceptions up to 1300 u are observed, if the analyte is perfectly nonpolar.