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: Patients were eligible for DSE if they had used cocaine within 24 hours preceding the onset of chest pain and had a normal ECG and tropinin I level .
### Output Text:
Patients were eligible for DSE if they had used <span class="chemical">cocaine</span> within 24 hours preceding the onset of chest pain and had a normal ECG and tropinin I level . 