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 : With ethical approval , we studied 74 American Society of Anesthesiologists ( ASA ) , physical status class 1 and 2 patients scheduled for elective unilateral lower limb surgery .
### Output Text:
METHODS : With ethical approval , we studied 74 American Society of Anesthesiologists ( ASA ) , physical status class 1 and 2 patients scheduled for elective unilateral lower limb surgery . 