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: Melatonin pre - treatment affected in a dual manner barbiturate narcosis , however , no dose - effect correlation was found .
### Output Text:
<span class="chemical">Melatonin</span> pre - treatment affected in a dual manner <span class="chemical">barbiturate</span> narcosis , however , no dose - effect correlation was found . 