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: We suggest that investigators involved in similar trials pay close attention to pulmonary pressure if hypoxaemia occurs after prophylactic administration of ibuprofen .
### Output Text:
We suggest that investigators involved in similar trials pay close attention to pulmonary pressure if hypoxaemia occurs after prophylactic administration of <span class="chemical">ibuprofen</span> . 