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: Unpremedicated patients were given fentanyl 1 microgram / kg followed by etomidate 0 . 3 mg / kg .
### Output Text:
Unpremedicated patients were given <span class="chemical">fentanyl</span> 1 microgram / kg followed by <span class="chemical">etomidate</span> 0 . 3 mg / kg . 