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: A standardized loading dose of VPA was administered , and venous blood was sampled at 0 , 1 , 2 , 3 , and 4 hours .
### Output Text:
A standardized loading dose of <span class="chemical">VPA</span> was administered , and venous blood was sampled at 0 , 1 , 2 , 3 , and 4 hours . 