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: BACKGROUND : patients undergoing electroconvulsive therapy ( ECT ) often receive succinylcholine as part of the anesthetic procedure .
### Output Text:
BACKGROUND : patients undergoing electroconvulsive therapy ( ECT ) often receive <span class="chemical">succinylcholine</span> as part of the anesthetic procedure . 