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: In a counterbalanced , placebo - controlled , double - blind study design , healthy subjects were administered a continuous subanesthetic S - ketamine infusion while differences in BOLD responses measured with fMRI were detected .
### Output Text:
In a counterbalanced , placebo - controlled , double - blind study design , healthy subjects were administered a continuous subanesthetic <span class="chemical">S - ketamine</span> infusion while differences in BOLD responses measured with fMRI were detected . 