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: Therapeutic drug monitoring of tobramycin : once - daily versus twice - daily dosage schedules .
### Output Text:
Therapeutic drug monitoring of <span class="chemical">tobramycin</span> : once - daily versus twice - daily dosage schedules . 