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: After 30 hours of quinine infusion the patient felt palpitation and electrocardiography ( ECG ) recording showed premature ventricular contraction ( PVC ) > 5 x / minute , trigemini , constant type - - sinoatrial block , positive U wave .
### Output Text:
After 30 hours of <span class="chemical">quinine</span> infusion the patient felt palpitation and electrocardiography ( ECG ) recording showed premature ventricular contraction ( PVC ) > 5 x / minute , trigemini , constant type - - sinoatrial block , positive U wave . 