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 study also revealed significant inhibition of NOS enzymatic activity by lead in cell - free preparations .
### Output Text:
The study also revealed significant inhibition of NOS enzymatic activity by <span class="chemical">lead</span> in cell - free preparations . 