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: METHODS : This double - blind study examined the incidence and severity of postoperative nausea and vomiting and pain in the first 24 h after sevoflurane anaesthesia in 216 adult day surgery patients .
### Output Text:
METHODS : This double - blind study examined the incidence and severity of postoperative nausea and vomiting and pain in the first 24 h after <span class="chemical">sevoflurane</span> anaesthesia in 216 adult day surgery patients . 