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: Following instrumentation , halothane was discontinued and alfentanil ( 125 mu / kg ) administered iv during emergence from halothane anesthesia .
### Output Text:
Following instrumentation , <span class="chemical">halothane</span> was discontinued and <span class="chemical">alfentanil</span> ( 125 mu / kg ) administered iv during emergence from <span class="chemical">halothane</span> anesthesia . 