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: Each subject received a single 50 mg dose of clomipramine given orally 2 hours before blood collection .
### Output Text:
Each subject received a single 50 mg dose of <span class="chemical">clomipramine</span> given orally 2 hours before blood collection . 