Certain chemicals refers to specific substances or compounds that have been identified or known to exist. It indicates that there are particular substances being referred to, rather than any random or unspecified chemicals. Full definition
To do that, this drug is taken daily and, over time, can affect the levels of certain chemicals in the brain. (businessinsider.com)
When dogs (and humans) exercise there's a spike in certain chemicals in the brain. (malenademartini.com)
For example, although rare, there is a chemical reaction rule, which defines a product's country of origin based on where certain chemical reactions have occurred in the production process. (edc.trade)