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:

High levels of mercury found in mountain lions linked to coastal fog | EVISA's News
Instrument database: Thermo Scientific - MALDI LTQ Orbitrap Spectrometer
FDA/EPA recommends pregnant women to restrict their fish consumption because of methylmercury content | EVISA's News
Material database: NIES - National Institute for Environmental Studies - CRM 10 b: Rice Flour-Unpolished, Medium Level Cadmium
Instrument database: Agilent Technologies Inc. - 6475 Triple Quadrupole LC/MS
Instrument database: Varian Inc. (Part A) - 1200 and 1200L Triple Quadrupole GC/MS
Link database: Chemical Fact Sheet: Manganese
Instrument database: Finnigan MAT - MAT261 Thermal ionization mass spectrometer
EU bans certain organotin compounds | EVISA's News
Link database: Chemical Fact Sheet: Polonium
Speciation analysis of rhenium by capillary electrophoresis-inductively coupled plasma mass spectrometry | EVISA's News
ICP-MS: A versatile detection system for trace element and speciation analysis | EVISA's News
Instrument database: - SentroProc NIR QC
Material database: Canadian Certified Reference Materials Project (CCRMP) - GTS-2a Gold Ore Mill Tailing
Instrument database: Isoprime Ltd. - 100 IRMS
Link database: US FDA: Selenium Regulations
Instrument database: Bruker Daltonics - maXis II™ / maXis II™ ETD - high-resolution LC-QTOF
Instrument database: Beckman Coulter Inc. - System Gold 125 166 HPLC
EVISA's activities during the year 2010 | EVISA's News
Instrument database: Agilent Technologies Inc. - 5975 Inert MSD