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: Early postoperative delirium incidence risk factors were then assessed through three different multiple regression models .
### Output Text:
Early postoperative delirium incidence risk factors were then assessed through three different multiple regression models . 