Features of NCI
Negative ions are mainly formed by capturing (or attaching) thermal electrons.
- NCI is a soft ionization method, like PCI, so a NCI spectrum is relatively simple.
- NCI is highly sensitive and highly selective for compounds with a positive electron affinity. All compounds cannot be negatively ionized. For example, hydrocarbons cannot be negatively ionized. Therefore, NCI allows us to perform highly selective and highly sensitive analysis, roughly equivalent to ECD, for these compounds.
- Reagent gas is used for this ionization: methane, iso-butane or ammonia. This gas makes it easy for sample molecules to capture electrons by slowing down the electron velocity.
- A semi-closed type of ion source is used. Pressure in the ion source is about 1 to 10 Pa.
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