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: This study was performed to determine cross - reactivity between enflurane and isoflurane with the hypersensitivity induced by halothane .
### Output Text:
This study was performed to determine cross - reactivity between <span class="chemical">enflurane</span> and <span class="chemical">isoflurane</span> with the hypersensitivity induced by <span class="chemical">halothane</span> . 