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: OBJECTIVE : To evaluate the effect of dosage regimen ( once - daily vs . twice - daily ) of tobramicyn on steady - state serum concentrations and toxicity .
### Output Text:
OBJECTIVE : To evaluate the effect of dosage regimen ( once - daily vs . twice - daily ) of <span class="chemical">tobramicyn</span> on steady - state serum concentrations and toxicity . 