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: Dobutamine infusion at 10 micrograms / kg per min was discontinued after six studies secondary to a 50 % incidence rate of adverse symptoms .
### Output Text:
<span class="chemical">Dobutamine</span> infusion at 10 micrograms / kg per min was discontinued after six studies secondary to a 50 % incidence rate of adverse symptoms . 