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 brain concentration of NA was not changed by morphine and fentanyl , but one of the doses of codeine ( 45 mg / kg ) slightly enhanced it .
### Output Text:
The brain concentration of <span class="chemical">NA</span> was not changed by <span class="chemical">morphine</span> and <span class="chemical">fentanyl</span> , but one of the doses of <span class="chemical">codeine</span> ( 45 mg / kg ) slightly enhanced it . 