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: Patients were observed on an inpatient basis for 18 to 24 hours , and vital signs , nausea , vomiting , retching , and appetite were assessed .
### Output Text:
Patients were observed on an inpatient basis for 18 to 24 hours , and vital signs , nausea , vomiting , retching , and appetite were assessed . 