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:

Testing finds: Arsenic added to feedstuff finds its way into chicken meat | EVISA's News
Instrument database: ESA Inc. - Model 526 / 528 Variable Wavelength UV/VIS Detector
Instrument database: JEOL - JSX-3201M Element Analyzer
Instrument database: Shimadzu Europe - SPD-10Avp UV/VIS Detector
Mercury 2024: 16th International Conference on Mercury as a Global Pollutant | EVISA's Agenda of Events
Factors Affecting Methylmercury Accumulation in the Food Chain | EVISA's News
Journals database: Food Technology
Material database: National Institute for Environmental Studies (NIES) - NIES CRM 10 a: Rice Flour-Unpolished, Low Level Cadmium
User community on EVISA's Website doubling during the last 6 months | EVISA's News
Study links high levels of cadmium and lead in blood to pregnancy delay | EVISA's News
Instrument database: Thermo Scientific - TSQ Quantum Access MAX Triple Stage Quadrupole Mass Spectrometer
Instrument database: InProcess Instruments GmbH - GAM 500 UT
Instrument database: Shimadzu Europe - IRAffinity-1S - Fourier Transform Infrared Spectrophotometer
Instrument database: Australian Scientific Instruments Pty. Ltd. - SHRIMP II
Instrument database: Shimadzu Europe - ICPE-9000
Instrument database: Aurora Instruments Ltd. - AI 500 Atomic Absorption Spectrometer
Instrument database: Varian Inc. (Part A) - Scimitar 1000 FT-IR
Journals database: Journal of Nanoscience and Nanotechnology
Instrument database: Thermo Scientific - TSQ Endura™ Triple Quadrupole Mass Spectrometer
Instrument database: Hewlett-Packard - MSD 5970