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 AND OBJECTIVE : Despite advantages of induction and maintenance of anaesthesia with sevoflurane , postoperative nausea and vomiting occurs frequently .
### Output Text:
BACKGROUND AND OBJECTIVE : Despite advantages of induction and maintenance of anaesthesia with <span class="chemical">sevoflurane</span> , postoperative nausea and vomiting occurs frequently . 