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: Anesthesia was maintained with fentanyl , nitrous oxide , oxygen , and isoflurane .
### Output Text:
Anesthesia was maintained with <span class="chemical">fentanyl</span> , <span class="chemical">nitrous oxide</span> , <span class="chemical">oxygen</span> , and <span class="chemical">isoflurane</span> . 