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: Three audiometric threshold measurements ( 0 . 25 - 18 kHz ) were made : the first before IBPB , the second 2 - 6 h after surgery and the third on the first day after operation .
### Output Text:
Three audiometric threshold measurements ( 0 . 25 - 18 kHz ) were made : the first before IBPB , the second 2 - 6 h after surgery and the third on the first day after operation . 