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: At the end of each pretreatment period , animals were used for ( i ) determining their locomotor response to acutely injected nicotine ( 0 . 2 mg / kg , s . c . ) and ( ii ) measuring the density of L - [ 3H ] nicotine and [ 3H ] spiperone binding sites in the striatum .
### Output Text:
At the end of each pretreatment period , animals were used for ( i ) determining their locomotor response to acutely injected <span class="chemical">nicotine</span> ( 0 . 2 mg / kg , s . c . ) and ( ii ) measuring the density of L - [ 3H ] <span class="chemical">nicotine</span> and [ 3H ] <span class="chemical">spiperone</span> binding sites in the striatum . 