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: By routinely monitoring serum calcium levels , healthcare providers can improve the quality of life of this patient group .
### Output Text:
By routinely monitoring serum <span class="chemical">calcium</span> levels , healthcare providers can improve the quality of life of this patient group . 