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: He received a 10 - day course of bupropion as an aid for smoking cessation 10 weeks prior to presentation .
### Output Text:
He received a 10 - day course of <span class="chemical">bupropion</span> as an aid for smoking cessation 10 weeks prior to presentation . 