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: Complementary techniques such as western immunoblotting and immunohistochemistry were performed to investigate the validity of the data obtained using the proteomic approach .
### Output Text:
Complementary techniques such as western immunoblotting and immunohistochemistry were performed to investigate the validity of the data obtained using the proteomic approach . 