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: Anaesthesia was maintained with intermittent etomidate in 2 - 4 mg doses .
### Output Text:
Anaesthesia was maintained with intermittent <span class="chemical">etomidate</span> in 2 - 4 mg doses . 