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: METHODS : A total of 128 patients undergoing general surgery were randomly allocated to receive normal saline ( control ) or remifentanil to a target Ce of 2 ng ml ( - 1 ) ( R2 ) , 4 ng ml ( - 1 ) ( R4 ) , or 6 ng ml ( - 1 ) ( R6 ) administered via TCI .
### Output Text:
METHODS : A total of 128 patients undergoing general surgery were randomly allocated to receive normal saline ( control ) or <span class="chemical">remifentanil</span> to a target Ce of 2 ng ml ( - 1 ) ( R2 ) , 4 ng ml ( - 1 ) ( R4 ) , or 6 ng ml ( - 1 ) ( R6 ) administered via TCI . 