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: The rate of NA turnover was not altered by analgesics except for the higher dose of fentanyl ( 0 . 2 mg / kg ) following which the disappearance of NA from the brain was diminished .
### Output Text:
The rate of <span class="chemical">NA</span> turnover was not altered by analgesics except for the higher dose of <span class="chemical">fentanyl</span> ( 0 . 2 mg / kg ) following which the disappearance of <span class="chemical">NA</span> from the brain was diminished . 