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: Experimental cranial pain elicited by capsaicin : a PET study .
### Output Text:
Experimental cranial pain elicited by <span class="chemical">capsaicin</span> : a PET study . 