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: Sublingual absorption of the quaternary ammonium antiarrhythmic agent , UM - 272 .
### Output Text:
Sublingual absorption of the <span class="chemical">quaternary ammonium</span> antiarrhythmic agent , <span class="chemical">UM - 272</span> . 