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: There was pain on i . m . injection of flunitrazepam significantly more often than with isotonic saline .
### Output Text:
There was pain on i . m . injection of <span class="chemical">flunitrazepam</span> significantly more often than with isotonic saline . 