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: A postmarketing surveillance study was conducted to determine the safety and efficacy of a fixed - ratio combination containing 10 mg of timolol maleate and 25 mg of hydrochlorothiazide , administered twice daily for one month to hypertensive patients .
### Output Text:
A postmarketing surveillance study was conducted to determine the safety and efficacy of a fixed - ratio combination containing 10 mg of <span class="chemical">timolol maleate</span> and 25 mg of <span class="chemical">hydrochlorothiazide</span> , administered twice daily for one month to hypertensive patients . 