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 : Perioperative data from consecutive cardiac surgery patients were prospectively collected between September 2005 and June 2006 in a university - affiliated clinic ( n = 1188 ) .
### Output Text:
METHODS : Perioperative data from consecutive cardiac surgery patients were prospectively collected between September 2005 and June 2006 in a university - affiliated clinic ( n = 1188 ) . 