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: A mean overall dose of etomidate 17 . 4 microgram / kg / min . was required to maintain sleep , but great individual variation occurred , with older patients requiring less drug .
### Output Text:
A mean overall dose of <span class="chemical">etomidate</span> 17 . 4 microgram / kg / min . was required to maintain sleep , but great individual variation occurred , with older patients requiring less drug . 