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 : A study cohort of 8 , 770 patients age > or = 65 years with a new diagnosis of AF was identified from a provincewide database of Quebec residents with a myocardial infarction ( MI ) between 1991 and 1999 .
### Output Text:
METHODS : A study cohort of 8 , 770 patients age > or = 65 years with a new diagnosis of AF was identified from a provincewide database of Quebec residents with a myocardial infarction ( MI ) between 1991 and 1999 . 