In chemical analysis total amount of matter is not analysed, but only a smaller part of it. This small part is called analysis sample and it must represent the whole amount of the matter that is being analysed.
Citing this page:
Generalic, Eni. "Sample." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.