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: A case of postoperative anxiety due to low dose droperidol used with patient - controlled analgesia .
### Output Text:
A case of postoperative anxiety due to low dose <span class="chemical">droperidol</span> used with patient - controlled analgesia . 