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: Nitro - L - arginine methyl ester : a potential protector against gentamicin ototoxicity .
### Output Text:
<span class="chemical">Nitro - L - arginine methyl ester</span> : a potential protector against <span class="chemical">gentamicin</span> ototoxicity . 