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: These changes were significantly attenuated by alpha - TC and DFO .
### Output Text:
These changes were significantly attenuated by <span class="chemical">alpha - TC</span> and <span class="chemical">DFO</span> . 