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: The mean initial ( preprocedure ) BIS score was 97 . 1 ( + / - 2 . 3 ; range 92 to 99 ) .
### Output Text:
The mean initial ( preprocedure ) BIS score was 97 . 1 ( + / - 2 . 3 ; range 92 to 99 ) . 