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: The aim of this study was to determine if the prophylactic value of misoprostol was dose - dependent .
### Output Text:
The aim of this study was to determine if the prophylactic value of <span class="chemical">misoprostol</span> was dose - dependent . 