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:

USGS Study Reveals Mercury Contamination in Fish Nationwide | EVISA's News
Instrument database: Shimadzu Europe - GCMS-QP2010 Ultra
Report Finds Mercury Contamination Permeates Wildlife Systems | EVISA's News
Instrument database: Analytik Jena GmbH - contrAA 800 F - HRCS- Atomic absorption spectrometer for flame and hydride techniques
Iron-reducing bacteria could detoxify hexavalent chromium | EVISA's News
Component database: CSols Ltd - CSols LabCache™ – Laboratory Database
Instrument database: Organomation Assosciates Inc. - ROT-X-TRACT-LC Extractor for Sample Preparation
Instrument database: Shimadzu Europe - Nexera Mikros - Micro-Flow LC-MS solution
Half of Piedmont drinking wells may exceed NC's hexavalent chromium standards | EVISA's News
Seabass populations can be differentiated by their Mercury Isotope Distribution | EVISA's News
Arsenic-Containing Phosphatidylcholines Discovered in Herring Caviar | EVISA's News
Instrument database: Knauer GmbH - WellChrom Diode Array Detector K-2800
2014 Winter Conference on Plasma Spectrochemistry | EVISA's Agenda of Events
Iron levels linked to Alzheimer's progression | EVISA's News
Journals database: Science
Mercury 'speciation analysis' by using thermal desorption analysis | EVISA's News
Instrument database: Toptica Photonics AG - TA-SHG 110
Journals database: Nutrition and Food Science
High levels of mercury in newborns likely from mothers eating contaminated fish | EVISA's News
Half of the rice sold in the UK breaches limits on arsenic for children | EVISA's News