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: Twenty patients were asked to quantify the severity of pain after receiving standard lidocaine in one femoral area and buffered lidocaine in the opposite femoral area .
### Output Text:
Twenty patients were asked to quantify the severity of pain after receiving standard <span class="chemical">lidocaine</span> in one femoral area and buffered <span class="chemical">lidocaine</span> in the opposite femoral area . 