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: Correlation with rating recall after one week was best when first - time ratings were requested as late as one day after injection ( R ( 2 ) = 0 . 79 ) indicating that both rating retrievals utilized similar memory traces .
### Output Text:
Correlation with rating recall after one week was best when first - time ratings were requested as late as one day after injection ( R ( 2 ) = 0 . 79 ) indicating that both rating retrievals utilized similar memory traces . 