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 : This study comprised 24 cognitively intact , health elderly adults ( 12 APOE epsilon4 carriers ) at an outpatient geriatric psychiatry research clinic .
### Output Text:
METHODS : This study comprised 24 cognitively intact , health elderly adults ( 12 APOE epsilon4 carriers ) at an outpatient geriatric psychiatry research clinic . 