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 level of GFAP , CD11b , TNF - a , ROS and nitrite levels were increased .
### Output Text:
The level of GFAP , CD11b , TNF - a , ROS and <span class="chemical">nitrite</span> levels were increased . 