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 long - lasting effect is consistent with that observed in preliminary reports of patients with chronic neuropathic pain and is not due to prolonged residence of adenosine in cerebrospinal fluid .
### Output Text:
The long - lasting effect is consistent with that observed in preliminary reports of patients with chronic neuropathic pain and is not due to prolonged residence of <span class="chemical">adenosine</span> in cerebrospinal fluid . 