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: Recoveries of heart rate and left ventricular end - diastolic pressure were not shown within 60 min after SNP had been stopped .
### Output Text:
Recoveries of heart rate and left ventricular end - diastolic pressure were not shown within 60 min after <span class="chemical">SNP</span> had been stopped . 