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: Ketoprofen but not acetaminophen impaired platelet function in patients with SAH .
### Output Text:
<span class="chemical">Ketoprofen</span> but not <span class="chemical">acetaminophen</span> impaired platelet function in patients with SAH . 