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: Comparison of the subjective effects and plasma concentrations following oral and i . m . administration of flunitrazepam in volunteers .
### Output Text:
Comparison of the subjective effects and plasma concentrations following oral and i . m . administration of <span class="chemical">flunitrazepam</span> in volunteers . 