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 followed up at 6 - monthly intervals for not less than 18 months ( range 18 - 43 months ) .
### Output Text:
Patients were followed up at 6 - monthly intervals for not less than 18 months ( range 18 - 43 months ) . 