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: Microinjection of fentanyl ( 2 . 5 micrograms / 50 nl ) directly into this pontine nucleus , on the other hand , elicited discernible electromyographic excitation .
### Output Text:
Microinjection of <span class="chemical">fentanyl</span> ( 2 . 5 micrograms / 50 nl ) directly into this pontine nucleus , on the other hand , elicited discernible electromyographic excitation . 