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: Various doses of yohimbine were used to determine the ideal dose for each patient .
### Output Text:
Various doses of <span class="chemical">yohimbine</span> were used to determine the ideal dose for each patient . 