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: Fifteen patients demonstrating unexpected prolonged apnoea lasting several hours after succinylcholine have been treated by a new preparation of human serum cholinesterase .
### Output Text:
Fifteen patients demonstrating unexpected prolonged apnoea lasting several hours after <span class="chemical">succinylcholine</span> have been treated by a new preparation of human serum cholinesterase . 