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: Toxicity of CAA was determined by protein content , cell number , LDH release , trypan blue exclusion assay and caspase - 3 activity .
### Output Text:
Toxicity of <span class="chemical">CAA</span> was determined by protein content , cell number , LDH release , <span class="chemical">trypan blue</span> exclusion assay and caspase - 3 activity . 