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: QTc length was weakly but significantly associated with methadone daily dose ( Spearman rank correlation coefficient , 0 . 20 ; P < . 01 ) .
### Output Text:
QTc length was weakly but significantly associated with <span class="chemical">methadone</span> daily dose ( Spearman rank correlation coefficient , 0 . 20 ; P < . 01 ) . 