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: Peak serum cocaethylene concentrations were associated with prolonged myocardial depression .
### Output Text:
Peak serum <span class="chemical">cocaethylene</span> concentrations were associated with prolonged myocardial depression . 