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: Epileptic foci delineated and activated by enflurane were surgically ablated and the patients are now seizure - free .
### Output Text:
Epileptic foci delineated and activated by <span class="chemical">enflurane</span> were surgically ablated and the patients are now seizure - free . 