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: In a second series , cytosolic Ca2 + activity ( Fluo3 fluorescence ) , cell volume ( forward scatter ) , and PS - exposure ( annexin V binding ) were determined by FACS analysis in erythrocytes from healthy volunteers .
### Output Text:
In a second series , cytosolic <span class="chemical">Ca2</span> + activity ( <span class="chemical">Fluo3</span> fluorescence ) , cell volume ( forward scatter ) , and <span class="chemical">PS</span> - exposure ( annexin V binding ) were determined by FACS analysis in erythrocytes from healthy volunteers . 