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: Confocal microscopy was used to measure intracellular free - calcium concentrations ( [ Ca ( 2 + ) ] ( i ) ) in isolated myocytes .
### Output Text:
Confocal microscopy was used to measure intracellular free - <span class="chemical">calcium</span> concentrations ( [ <span class="chemical">Ca</span> ( 2 + ) ] ( i ) ) in isolated myocytes . 