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: He was put on aspirin following surgery and took ibuprofen for fever for nearly a week prior to presentation .
### Output Text:
He was put on <span class="chemical">aspirin</span> following surgery and took <span class="chemical">ibuprofen</span> for fever for nearly a week prior to presentation . 