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: A prospective evaluation of the efficacy and safety of vancomycin was conducted in 54 consecutive patients over a 16 - month period .
### Output Text:
A prospective evaluation of the efficacy and safety of <span class="chemical">vancomycin</span> was conducted in 54 consecutive patients over a 16 - month period . 