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: Fifty - two patients suffering from intractable tinnitus entered this therapeutic trial , but only nine finished all five courses .
### Output Text:
Fifty - two patients suffering from intractable tinnitus entered this therapeutic trial , but only nine finished all five courses . 