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: Changes in intragastric pressure after the administration of suxamethonium 1 . 5 mg kg - 1 i . v . were studied in 32 children ( mean age 6 . 9 yr ) pretreated with either physiological saline or alfentanil 50 micrograms kg - 1 .
### Output Text:
Changes in intragastric pressure after the administration of <span class="chemical">suxamethonium</span> 1 . 5 mg kg - 1 i . v . were studied in 32 children ( mean age 6 . 9 yr ) pretreated with either physiological saline or <span class="chemical">alfentanil</span> 50 micrograms kg - 1 . 