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: OBJECTIVE : To assess outcomes based on troponin positivity in patients with cocaine chest pain admitted for exclusion of MI .
### Output Text:
OBJECTIVE : To assess outcomes based on troponin positivity in patients with <span class="chemical">cocaine</span> chest pain admitted for exclusion of MI . 