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: METHOD : prospective study with 137 patients that underwent TEE with MZ associated to moderate sedation .
### Output Text:
METHOD : prospective study with 137 patients that underwent TEE with <span class="chemical">MZ</span> associated to moderate sedation . 