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: The effect of azathioprine on both annexin V binding and forward scatter was significantly blunted in the nominal absence of extracellular Ca2 + .
### Output Text:
The effect of <span class="chemical">azathioprine</span> on both annexin V binding and forward scatter was significantly blunted in the nominal absence of extracellular <span class="chemical">Ca2</span> + . 