Optical spectrometry:
In optical spectrometry, the source of the spectrometer is the device from which the radiation is emitted. This may be a flame or plasma source.

Mass spectrometry:
The source is the device within the mass spectrometer in which ionization of sample molecules occurs. The source may be under vacuum, or it can operate at atmospheric pressure. A chromatographic method may interface with the source, or samples may be introduced via a probe or an automated sample introduction system. Ions are accelerated out of the source into the mass analyzer of the instrument.