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: An investigation was undertaken to determine the dosage of etomidate required to maintain sleep in adults undergoing surgery under regional local anesthesia .
### Output Text:
An investigation was undertaken to determine the dosage of <span class="chemical">etomidate</span> required to maintain sleep in adults undergoing surgery under regional local anesthesia . 