The phrase "to contaminate the sample" means that something unwanted or impure has been introduced into the sample, which can affect its accuracy or reliability for testing or analysis. Full definition
KEEPING CLEAN When studying ancient specimens, researchers work in ultraclean rooms to avoid contaminating samples with DNA from modern people or the environment. (sciencenews.org)
Care should be taken to avoid contaminating the sample with artifact. (riverbarkvet.com)
But the number of contaminated samples and level of contamination peaked by July 2011 and then decreased. (sciencemag.org)