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 area under the plasma concentration time curve at 90 min was 4 - 12 times greater than for oral drug , suggesting the existence of an absorption - limiting process in the intestine , and providing an alternate form of administration for quaternary drugs .
### Output Text:
The area under the plasma concentration time curve at 90 min was 4 - 12 times greater than for oral drug , suggesting the existence of an absorption - limiting process in the intestine , and providing an alternate form of administration for quaternary drugs . 