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: BACKGROUND : There recently has been a resurgence in the utilization of ketamine , a unique anesthetic , for emergency - department procedures requiring sedation .
### Output Text:
BACKGROUND : There recently has been a resurgence in the utilization of <span class="chemical">ketamine</span> , a unique anesthetic , for emergency - department procedures requiring sedation . 