NCI: Why is the ion source changed between NCI and PCI?
With CI the ion source sensitivity is determined by the reagent gas pressure within the ionization chamber and the ability to extract ions from the chamber. When PCI is compared to NCI, higher reaction gas pressures produce better sensitivity with PCI while lower reaction gas pressure with higher ion extraction rates produce better sensitivity with NCI. The size of the hole for extracting ions from the ion source is therefore different between PCI and NCI. Replacement of the ion source box is therefore necessary when switching between the methods.