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: Syncope and QT prolongation among patients treated with methadone for heroin dependence in the city of Copenhagen .
### Output Text:
Syncope and QT prolongation among patients treated with <span class="chemical">methadone</span> for <span class="chemical">heroin</span> dependence in the city of Copenhagen . 