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: METHODS : An observational , prospective study was conducted on a consecutive cohort of patients admitted to our ICU within and for at least 24 h after major surgical procedures .
### Output Text:
METHODS : An observational , prospective study was conducted on a consecutive cohort of patients admitted to our ICU within and for at least 24 h after major surgical procedures . 