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: Although the exact mechanism of hepatotoxicity of these agents is not known , the results suggest that trifluoroacetyl - altered liver proteins are involved .
### Output Text:
Although the exact mechanism of hepatotoxicity of these agents is not known , the results suggest that <span class="chemical">trifluoroacetyl</span> - altered liver proteins are involved . 