Apparatus for generating ions

A method of generating reference ions in an ion source for an analytical instrument, the ion source comprising:

• an ionisation region;
• an ionisation device configured to ionise analyte in the ionisation region so as to generate analyte ions;
• a volume housing a reference compound;
• a heater configured to heat the reference compound;
• a transfer line that is independent of the ionisation device and that connects the volume to the ionisation region;
• and
• a carrier gas source configured to supply a carrier gas;
• the method comprising:
• the heater heating the reference compound such that vaporised molecules and/or molecule clusters of the reference compound are produced in the volume;
• the carrier gas source supplying a carrier gas such that at least some of the vaporised reference compound molecules and/or molecule clusters are transported from the volume to the ionisation region via the transfer line;
• and
• at least some of the vaporised reference compound molecules and/or molecule clusters being ionised in the ionisation region so as to generate reference ions.