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: Group OD ( n = 22 ) received a once - daily dose of tobramycin and group TD ( n = 21 ) received the same dose divided into two doses daily .
### Output Text:
Group OD ( n = 22 ) received a once - daily dose of <span class="chemical">tobramycin</span> and group TD ( n = 21 ) received the same dose divided into two doses daily . 