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: Calcium concentrations 1 day postpartum were higher in cows treated with vitamin D3 about 32 days prepartum ( 8 . 8 mg / 100 ml ) than in control cows ( 5 . 5 mg / 100 ml ) .
### Output Text:
<span class="chemical">Calcium</span> concentrations 1 day postpartum were higher in cows treated with <span class="chemical">vitamin D3</span> about 32 days prepartum ( 8 . 8 mg / 100 ml ) than in control cows ( 5 . 5 mg / 100 ml ) . 