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: Venous discomfort was assessed with a visual analogue scale ( VAS ) 5 - 15 sec after commencing propofol administration using an infusion pump ( rate 1000 micrograms . kg - 1 . min - 1 ) .
### Output Text:
Venous discomfort was assessed with a visual analogue scale ( VAS ) 5 - 15 sec after commencing <span class="chemical">propofol</span> administration using an infusion pump ( rate 1000 micrograms . kg - 1 . min - 1 ) . 