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 major determinant of the symptoms may have been dopamine / serotonin imbalance in the central nervous system .
### Output Text:
The major determinant of the symptoms may have been <span class="chemical">dopamine</span> / <span class="chemical">serotonin</span> imbalance in the central nervous system . 