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: Laboratory measurement of pre - procedure serum calcium levels in selected donors may identify cases requiring heightened vigilance .
### Output Text:
Laboratory measurement of pre - procedure serum <span class="chemical">calcium</span> levels in selected donors may identify cases requiring heightened vigilance . 