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: Urine samples for Na and Cr also were checked at the same intervals .
### Output Text:
Urine samples for <span class="chemical">Na</span> and <span class="chemical">Cr</span> also were checked at the same intervals . 