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: To demonstrate the relationship between peripheral iron overload and dopaminergic neuron loss in rat substantia nigra ( SN ) , in the present study we used fast cyclic voltammetry , tyrosine hydroxylase ( TH ) immunohistochemistry , Perls ' iron staining , and high performance liquid chromatography - electrochemical detection to study the degeneration of dopaminergic neurons and increased iron content in the SN of iron dextran overloaded animals .
### Output Text:
To demonstrate the relationship between peripheral <span class="chemical">iron</span> overload and dopaminergic neuron loss in rat substantia nigra ( SN ) , in the present study we used fast cyclic voltammetry , <span class="chemical">tyrosine</span> hydroxylase ( TH ) immunohistochemistry , Perls ' <span class="chemical">iron</span> staining , and high performance liquid chromatography - electrochemical detection to study the degeneration of dopaminergic neurons and increased <span class="chemical">iron</span> content in the SN of <span class="chemical">iron dextran</span> overloaded animals . 