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: Thereafter , 2 mg / kg / min bupivacaine 0 . 5 % was infused .
### Output Text:
Thereafter , 2 mg / kg / min <span class="chemical">bupivacaine</span> 0 . 5 % was infused . 