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: CONCLUSIONS : QT interval prolongation in methadone maintenance patients hospitalized in a tertiary care center is a frequent finding .
### Output Text:
CONCLUSIONS : QT interval prolongation in <span class="chemical">methadone</span> maintenance patients hospitalized in a tertiary care center is a frequent finding . 