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: Pretreatment of rats with thiosemicarbazide for 30 minutes abolished the anesthetic action as well as the respiratory depressant action of thiopental .
### Output Text:
Pretreatment of rats with <span class="chemical">thiosemicarbazide</span> for 30 minutes abolished the anesthetic action as well as the respiratory depressant action of <span class="chemical">thiopental</span> . 