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: Although such an association is of practical and theoretical importance , only one prospective study has tried to confirm it .
### Output Text:
Although such an association is of practical and theoretical importance , only one prospective study has tried to confirm it . 