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: Electroencephalographic activity was recorded in 11 of these patients , and all patients had a formal neurological evaluation and brain imaging studies .
### Output Text:
Electroencephalographic activity was recorded in 11 of these patients , and all patients had a formal neurological evaluation and brain imaging studies . 