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: Two hours after the sixth dose of TNA , she had an episode of generalized tonic clonic convulsions .
### Output Text:
Two hours after the sixth dose of <span class="chemical">TNA</span> , she had an episode of generalized tonic clonic convulsions . 