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: Gamma Knife pallidotomy is as effective as radiofrequency pallidotomy in controlling certain of the symptoms of Parkinson ' s disease .
### Output Text:
Gamma Knife pallidotomy is as effective as radiofrequency pallidotomy in controlling certain of the symptoms of Parkinson ' s disease . 