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: BACKGROUND : In this retrospective comparative study , we aimed to compare the effectiveness of fentanyl , midazolam , and a combination of fentanyl and midazolam to prevent etomidate - induced myoclonus .
### Output Text:
BACKGROUND : In this retrospective comparative study , we aimed to compare the effectiveness of <span class="chemical">fentanyl</span> , <span class="chemical">midazolam</span> , and a combination of <span class="chemical">fentanyl</span> and <span class="chemical">midazolam</span> to prevent <span class="chemical">etomidate</span> - induced myoclonus . 