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 : We conducted a prospective , randomized , double - blind study in the emergency department of a central - city teaching hospital .
### Output Text:
METHODS : We conducted a prospective , randomized , double - blind study in the emergency department of a central - city teaching hospital . 