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: Oral administration of captopril at 100 mg / kg for 14 days failed to aggravate proteinuria pre - existing in SHR .
### Output Text:
Oral administration of <span class="chemical">captopril</span> at 100 mg / kg for 14 days failed to aggravate proteinuria pre - existing in SHR . 