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: After the clipping of the aneurysm the isoflurane concentration was reduced to 0 . 75 % .
### Output Text:
After the clipping of the aneurysm the <span class="chemical">isoflurane</span> concentration was reduced to 0 . 75 % . 