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.

The term "source" was found in the following pages:

Material database: International Atomic Energy Agency (IAEA) - IAEA-313 226Ra, Th and U in Stream Sediment
Journals database: Environmental Science & Technology
Link database: Encyclopedia of Earth: Rice (Oryza sativa L.) as a source of microelements and toxic contaminants
Adisseo obtains FDA approval on pure organic selenium feed additive | EVISA's News
Man made mercury pollution is reaching even deep ocean organisms | EVISA's News
Instrument database: Bergmann Messgeräte Entwicklung KG (BME) - ALBATROS ESI-TOF
Instrument database: Thermo Scientific - Orbitrap Eclipse Tribrid Mass Spectrometer
Instrument database: Analytik Jena GmbH - multi N/C® UV HS
Instrument database: Nu Instruments Ltd. - Plasma 1700
USGS Study Reveals Mercury Contamination in Fish Nationwide | EVISA's News
Instrument database: Jenway - 6305 - UV/Visible Spectrophotometer
Instrument database: Shimadzu Europe - UV-2600 - Spectrophotometer
Instrument database: Thermo Scientific - Exactive™ Plus Orbitrap Mass Spectrometer
Material database: International Atomic Energy Agency (IAEA) - IAEA-155 Trace Elements in Whey Powder
9th Nordic Conference on Plasma Spectrochemistry | EVISA's Agenda of Events
Toxic Methylmercury-Producing Microbes More Widespread Than Realized | EVISA's News
Instrument database: Applied Biosystems - 4800 Plus MALDI TOF/TOF™ Analyzer
Instrument database: Aurora Biomed - AI 2200 AA/Plasma Emission Spectrometer
LC-ICP-MS: The most often used hyphenated system for speciation analysis | EVISA's News
Journals database: ACS Food Science & Technology