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: Troponin appears to have an equivalent diagnostic accuracy compared with CK - MB for diagnosing necrosis in patients with cocaine - associated chest pain and suspected MI .
### Output Text:
Troponin appears to have an equivalent diagnostic accuracy compared with CK - MB for diagnosing necrosis in patients with <span class="chemical">cocaine</span> - associated chest pain and suspected MI . 