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: Pulmonary hypertension after ibuprofen prophylaxis in very preterm infants .
### Output Text:
Pulmonary hypertension after <span class="chemical">ibuprofen</span> prophylaxis in very preterm infants . 