This is the 15659th most frequent Russian word.
примесью
"Примесью" means "impurity" or "admixture."
Here, 'примесью' indicates a minor component that influences the overall result or mixture.
Краска была странного оттенка, с примесью зелёного цвета.
The paint had an odd hue, with a tint of green.
Within this sentence, 'примесью' describes the component detected in the mix, again in instrumental case.
В воздухе ощущался запах с примесью газа.
There was a smell in the air with a trace of gas.
The word 'примесью' is used in instrumental case here to denote the substance mixed with the main component.