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: In a randomised , double - blind , cross - over study , 30 healthy volunteers received a single intravenous bolus injection of 2 . 5 mg . kg - 1 propofol .
### Output Text:
In a randomised , double - blind , cross - over study , 30 healthy volunteers received a single intravenous bolus injection of 2 . 5 mg . kg - 1 <span class="chemical">propofol</span> . 