In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: METHODS : In this cross - sectional study interview , ECGs and blood samples were collected in a population of adult heroin addicts treated with methadone or buprenorphine on a daily basis .
### Output Text:
METHODS : In this cross - sectional study interview , ECGs and blood samples were collected in a population of adult <span class="chemical">heroin</span> addicts treated with <span class="chemical">methadone</span> or <span class="chemical">buprenorphine</span> on a daily basis . 