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: Capsaicin ( 10 micro g ) was injected into the masseter muscle to induce pain in 11 healthy volunteers .
### Output Text:
<span class="chemical">Capsaicin</span> ( 10 micro g ) was injected into the masseter muscle to induce pain in 11 healthy volunteers . 