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: The extent of neuronal injury was determined by 2 , 3 , 5 - triphenyltetrazolium staining .
### Output Text:
The extent of neuronal injury was determined by <span class="chemical">2 , 3 , 5 - triphenyltetrazolium</span> staining . 