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: Because the protocol was long and involved interventions which might affect the determinations , we also studied in nine patients using an identical protocol except that ouabain administration was omitted .
### Output Text:
Because the protocol was long and involved interventions which might affect the determinations , we also studied in nine patients using an identical protocol except that <span class="chemical">ouabain</span> administration was omitted . 