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: Its incidence was determined in a prospective study involving a total of 100 Nigerian patients ( 50 out - patients and 50 in - patients ) .
### Output Text:
Its incidence was determined in a prospective study involving a total of 100 Nigerian patients ( 50 out - patients and 50 in - patients ) . 