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: We investigated in this study the possible engagement of locus coeruleus of the pons in this phenomenon , using male Sprague - Dawley rats anesthetized with ketamine .
### Output Text:
We investigated in this study the possible engagement of locus coeruleus of the pons in this phenomenon , using male Sprague - Dawley rats anesthetized with <span class="chemical">ketamine</span> . 