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: Chlordiazepoxide at doses up to 10 mg / kg failed to significantly influence the rigidity produced by alfentanil .
### Output Text:
<span class="chemical">Chlordiazepoxide</span> at doses up to 10 mg / kg failed to significantly influence the rigidity produced by <span class="chemical">alfentanil</span> . 