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: Twenty - three h postoperatively he developed a brief self - limited seizure .
### Output Text:
Twenty - three h postoperatively he developed a brief self - limited seizure . 