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: STZ also significantly increased the level of ROS , nitrite , Ca ( 2 + ) and reduced the mitochondrial activity in synaptosomal preparation illustrating free radical generation and excitotoxicity .
### Output Text:
<span class="chemical">STZ</span> also significantly increased the level of ROS , <span class="chemical">nitrite</span> , <span class="chemical">Ca</span> ( 2 + ) and reduced the mitochondrial activity in synaptosomal preparation illustrating free radical generation and excitotoxicity . 