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: Two groups of 22 similar patients were studied : one group received 6 mL prilocaine 2 % ; and the other received 3 mL bupivacaine 0 . 5 % .
### Output Text:
Two groups of 22 similar patients were studied : one group received 6 mL <span class="chemical">prilocaine</span> 2 % ; and the other received 3 mL <span class="chemical">bupivacaine</span> 0 . 5 % . 