PCI needs the following conditions, at the minimum:
- Reagent gas is supplied to ion source. As reagent gas, Methane, iso-butane or ammonia is commonly used.
- The pressure in the ion source is about 10 to 100Pa. To realize this pressure, the PCI ion source is almost closed up.
An EI spectrum of methane shows the base peak at m/z 16. This spectrum is very different from the spectrum under the appropriate PCI conditions, in which many complex ions are produced. If a methane reagent gas spectrum observed in PCI mode looks like an EI spectrum, you might check the pressure of the reagent gas or suspect incorrect mounting of the ion source.