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: There was no significant difference in the convulsive threshold between sevoflurane and enflurane .
### Output Text:
There was no significant difference in the convulsive threshold between <span class="chemical">sevoflurane</span> and <span class="chemical">enflurane</span> . 