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: An antibody response to a protein - bound biotransformation product ( trifluoroacetyl adduct ) has been detected on halothane hepatitis patients .
### Output Text:
An antibody response to a protein - bound biotransformation product ( <span class="chemical">trifluoroacetyl</span> adduct ) has been detected on <span class="chemical">halothane</span> hepatitis patients . 