Question

Two ionization methods – electron ionization and chemical ionization. Briefly discuss the process involved in ion formation for each method, and discuss the advantages/disadvantages of each method in identification of an unknown compound.

Answer 1

Electron ionization (EI, formerly known as electron impact ionization^[1] and electron bombardment ionization^[2]) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions.^[3] EI was one of the first ionization techniques developed for mass spectrometry.^[4] However, this method is still a popular ionization technique. This technique is considered a hard (high fragmentation) ionization method, since it uses high energetic electrons to produce ions. This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds.

Advantages	Disadvantages
Simple	Molecule must be volatile
Sensitive	molecule must be thermally stable
Fragmentation aids Identification of molecules	Extensive fragmentation- can't interpret data
Library-searchable fingerprint spectracal ionization (CI) is an ionization technique used in mass spectrometry. Chemical ionization (CI) is an ionization technique used in mass spectrometry. Chemical ionization is a lower energy process than electron ionization (EI). The lower energy yields less or sometimes no fragmentation, and usually a simpler spectrum. Chemical Ionization Advantages: Milder ionization than EI ; reduces fragmentation. Requires lower energy than EI. Often gives molecular weight information through molecular-like ions such as [M+H]+, even when EI would not produce a molecular ion. Disadvantage :- no fragment library Need volatile samples,need thermal stability,picimolar samples, low mass compound(>1000DA)	Useful mass range is low (<1000 Da)