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: DSE was performed with intravenous dobutamine infusion with an incremental doses of 5 , 10 , 20 , 30 , and 40 microg / kg / min every 5 minutes .
### Output Text:
DSE was performed with intravenous <span class="chemical">dobutamine</span> infusion with an incremental doses of 5 , 10 , 20 , 30 , and 40 microg / kg / min every 5 minutes . 