Chemical Ionisation (CI)

The Electronic Impact technique is a very aggressive (hard) technique, as it causes a lot of fragmentation of the molecular ion, giving a lot of structural information and little or no molecular information.

This makes it difficult to interpret the spectrum. In cases where it is of interest to have more information from the molecular ion and less from the fragmentation, it is better to resort to other less aggressive or soft ionisation techniques, as in the case of Chemical Ionisation, due to the little ‘excess’ energy that is deposited in the molecules when they are ionised. For this reason, the Chemical Ionisation technique is used as a complementary technique to the Electron Impact technique, since there are no spectra libraries due to their poor structural information (low reproducibility).

Chemical Ionisation is performed by the neutral molecule reaction of the analyte with ions from the previous ionisation of a reactive gas (usually methane) through Electronic Impact. The reactions that take place between positively charged reagent ions and sample molecules can be grouped mainly into four categories: Proton transfer, Charge exchange, Electrolyte addition and Anion abstraction.

It is often observed that the Chemical Methane Ionisation mass spectrum combines a MH+ ion signal of suitable intensity with appropriate fragmentation for structural elucidation.

As an alternative to the Electronic Impact technique, this is a technique for the analysis of relatively labile organic compounds (pharmaceuticals, metabolites, steroids, carbohydrates, lipids, pesticides...), i.e. compounds with low to medium polarity, high volatility, low molecular weight and high thermal stability.