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: IKr and IKs blockers concentration - dependently prolonged corrected FPD ( FPDc ) , whereas Ca ( 2 + ) channel blockers concentration - dependently shortened FPDc .
### Output Text:
IKr and IKs blockers concentration - dependently prolonged corrected FPD ( FPDc ) , whereas <span class="chemical">Ca</span> ( 2 + ) channel blockers concentration - dependently shortened FPDc . 