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: None of the subjects treated with buprenorphine had QTc interval > 0 . 440 s ( ( 1 / 2 ) ) .
### Output Text:
None of the subjects treated with <span class="chemical">buprenorphine</span> had QTc interval > 0 . 440 s ( ( 1 / 2 ) ) . 