spike 

https://doi.org/10.1351/goldbook.08047
Material with known quantity values added to an analytical sample.
Notes:
  1. The material can be a reference material or a certified reference material.
  2. The known quantity value is often a fraction or concentration.
  3. A spike may be used to estimate the recovered quantity value ratio or compensate for systematic measurement error.
  4. "Spike" used as a verb is the addition of a spike to a sample.
Source:
PAC, 2021, 93, 997. 'Metrological and quality concepts in analytical chemistry (IUPAC Recommendations 2021)' on page 1017 (https://doi.org/10.1515/pac-2019-0819)