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: Preoperative and postoperative assessments of these patients at 1 , 3 , 6 and 12 months follow - up , in " on " and " off " drug conditions , was carried out using Unified Parkinson ' s Disease Rating Scale , Hoehn and Yahr staging , England activities of daily living score and video recordings .
### Output Text:
Preoperative and postoperative assessments of these patients at 1 , 3 , 6 and 12 months follow - up , in " on " and " off " drug conditions , was carried out using Unified Parkinson ' s Disease Rating Scale , Hoehn and Yahr staging , England activities of daily living score and video recordings . 