CI-MS is a mass spectroscopic technique in which the analyte reacts with ions of a reagent gas.

In ordinary electron ionization mass spectrometry, EI-MS, the analyte is bombarded with energetic electrons to produce ions. The energetic ions break down into many fragmentation products.

In chemical ionization mass spectrometry, the analyte is in a large excess of a reagent gas, such as methane, ammonia, or isobutane.

The electron beam preferentially ionizes the reagent gas. Collisions with other reagent gas molecules create an ionization plasma.

The plasma then reacts with the analyte to form positive and negative ions.

CI is a lower-energy process than EI. It often yields less fragmentation and a simpler spectrum. A CI spectrum usually has an easily identifiable molecular ion.

When you analyze mass spectra, you first identify the molecular ions. Then it is fairly easy to identify the fragments.

With EI, there may be no molecular ions or they may be too weak to confirm.

In contrast, CI is a softer ionization method than EI. Fragmentation is less likely to occur, so the molecular ions are more prominent.

For example, sugars can be made more volatile by converting their OH groups to trimethylsilyl (TMS) ethers.

ROH + (CH₃)₃SiCl → RO-Si(CH₃)₃

Sucrose has 8 OH groups.

It forms an octa-TMS ether with molecular formula C₃₆H₈₆O₁₁Si₈.

The EI spectrum shows no detectable molecular ion.

In contrast, the CI spectrum with ammonia as the reagent gas shows a prominent molecular ion (M+NH₄)⁺.

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