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: There was a difference in metoprolol potency with higher racemic metoprolol IC50 values in PMs ( 72 + / - 7 ng . ml - 1 ) than EMs ( 42 + / - 8 ng . ml - 1 , P less than . 001 ) . ( ABSTRACT TRUNCATED AT 250 WORDS )
### Output Text:
There was a difference in <span class="chemical">metoprolol</span> potency with higher racemic <span class="chemical">metoprolol</span> IC50 values in PMs ( 72 + / - 7 ng . ml - 1 ) than EMs ( 42 + / - 8 ng . ml - 1 , P less than . 001 ) . ( ABSTRACT TRUNCATED AT 250 WORDS ) 