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: Among a total of 60 patients included in the study , 16 patients developed acute renal failure ( ARF ) on the second day after contrast material was injected ( 26 . 6 % ) .
### Output Text:
Among a total of 60 patients included in the study , 16 patients developed acute renal failure ( ARF ) on the second day after <span class="chemical">contrast</span> material was injected ( 26 . 6 % ) . 