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: In contrast , doses of 1 mg / kg BN 52021 given 30 min before or 10 mg / kg administered 5 min before i . v . injection of bupivacaine were ineffective .
### Output Text:
In contrast , doses of 1 mg / kg <span class="chemical">BN 52021</span> given 30 min before or 10 mg / kg administered 5 min before i . v . injection of <span class="chemical">bupivacaine</span> were ineffective . 