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: CONCLUSION : As fentanyl exacerbated postoperative nausea and vomiting without an improvement in postoperative pain and also had adverse cardiorespiratory effects , it appears to be an unnecessary and possibly detrimental supplement to sevoflurane in day surgery .
### Output Text:
CONCLUSION : As <span class="chemical">fentanyl</span> exacerbated postoperative nausea and vomiting without an improvement in postoperative pain and also had adverse cardiorespiratory effects , it appears to be an unnecessary and possibly detrimental supplement to <span class="chemical">sevoflurane</span> in day surgery . 