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: Myocardial concentrations of calcium also increased significantly ( 12 . 0 vs . 5 . 0 mg . per 100 Gm . of fat - free dry weight ) .
### Output Text:
Myocardial concentrations of <span class="chemical">calcium</span> also increased significantly ( 12 . 0 vs . 5 . 0 mg . per 100 Gm . of fat - free dry weight ) . 