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: The authors report two observations on patients with delayed reactions in whom intradermoreactions ( IDR ) and patch tests to a series of ionic and non ionic PRC were studied .
### Output Text:
The authors report two observations on patients with delayed reactions in whom intradermoreactions ( IDR ) and patch tests to a series of ionic and non ionic <span class="chemical">PRC</span> were studied . 