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: In 29 patients , the pallidotomies were performed with the Leksell Gamma Knife and in 22 they were performed with the standard radiofrequency ( RF ) method .
### Output Text:
In 29 patients , the pallidotomies were performed with the Leksell Gamma Knife and in 22 they were performed with the standard radiofrequency ( RF ) method . 