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: Thirty - five patients with primary pulmonary hypertension and 85 matched controls were recruited over 32 months ( 1992 - 1994 ) in Belgium .
### Output Text:
Thirty - five patients with primary pulmonary hypertension and 85 matched controls were recruited over 32 months ( 1992 - 1994 ) in Belgium . 