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: DATA SOURCES : MEDLINE and Cochrane Library ( search dates , 1 January 2001 to 28 August 2008 ) , recent systematic reviews , reference lists of retrieved articles , and suggestions from experts .
### Output Text:
DATA SOURCES : MEDLINE and Cochrane Library ( search dates , 1 January 2001 to 28 August 2008 ) , recent systematic reviews , reference lists of retrieved articles , and suggestions from experts . 