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 present work has been planned in order to elucidate the effect of phenobarbital ( PB : 15 mg per rat of ingested dose ) on carcinogenesis when it is administered simultaneously with diethylnitrosamine ( DEN : 10 mg / kg / day ) .
### Output Text:
The present work has been planned in order to elucidate the effect of <span class="chemical">phenobarbital</span> ( <span class="chemical">PB</span> : 15 mg per rat of ingested dose ) on carcinogenesis when it is administered simultaneously with <span class="chemical">diethylnitrosamine</span> ( <span class="chemical">DEN</span> : 10 mg / kg / day ) . 