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: BACKGROUND : Spontaneous reporting systems for adverse drug reactions ( ADRs ) are handicapped by under - reporting and limited detail on individual cases .
### Output Text:
BACKGROUND : Spontaneous reporting systems for adverse drug reactions ( ADRs ) are handicapped by under - reporting and limited detail on individual cases . 