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: In some cases , a high serum concentration of citalopram ( > 600 nmol / L ) in elderly patients has been associated with increased somnolence and movement difficulties .
### Output Text:
In some cases , a high serum concentration of <span class="chemical">citalopram</span> ( > 600 nmol / L ) in elderly patients has been associated with increased somnolence and movement difficulties . 