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: In this experimental study we used 30 Sprague - Dawley rats , 27 of which had gentamicin instilled into the middle ear .
### Output Text:
In this experimental study we used 30 Sprague - Dawley rats , 27 of which had <span class="chemical">gentamicin</span> instilled into the middle ear . 