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: The presence of REM sleep was shown not to be a determining factor either for secretory pulse amplitude and frequency , as , for both , mean nocturnal values were similar with and without prior clomipramine ingestion .
### Output Text:
The presence of REM sleep was shown not to be a determining factor either for secretory pulse amplitude and frequency , as , for both , mean nocturnal values were similar with and without prior <span class="chemical">clomipramine</span> ingestion . 