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: Field potential duration ( FPD ) in human - induced pluripotent stem cell - derived cardiomyocytes ( hiPS - CMs ) , which can express QT interval in an electrocardiogram , is reported to be a useful tool to predict K ( + ) channel and Ca ( 2 + ) channel blocker effects on QT interval .
### Output Text:
Field potential duration ( FPD ) in human - induced pluripotent stem cell - derived cardiomyocytes ( hiPS - CMs ) , which can express QT interval in an electrocardiogram , is reported to be a useful tool to predict <span class="chemical">K</span> ( + ) channel and <span class="chemical">Ca</span> ( 2 + ) channel blocker effects on QT interval . 