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: Previous investigations have determined that , in response to clonidine , ultrasound production increases through the 2nd - week postpartum and decreases thereafter .
### Output Text:
Previous investigations have determined that , in response to <span class="chemical">clonidine</span> , ultrasound production increases through the 2nd - week postpartum and decreases thereafter . 