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: Intracranial pressure ( ICP ) was measured during alfentanil - induced rigidity in rats .
### Output Text:
Intracranial pressure ( ICP ) was measured during <span class="chemical">alfentanil</span> - induced rigidity in rats . 