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 performed by injecting 20 to 40 mL of a mixture of long - acting ( ropivacaine ) and short - acting ( prilocaine ) anesthetic .
### Output Text:
Anesthesia was performed by injecting 20 to 40 mL of a mixture of long - acting ( <span class="chemical">ropivacaine</span> ) and short - acting ( <span class="chemical">prilocaine</span> ) anesthetic . 