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: Quinine - associated hemolytic - uremic syndrome probably occurs more often than is recognized .
### Output Text:
<span class="chemical">Quinine</span> - associated hemolytic - uremic syndrome probably occurs more often than is recognized . 