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 antinociceptive effect of 3 alpha - tropyl 2 - ( p - bromophenyl ) propionate [ ( + / - ) - PG - 9 ] ( 10 - 40 mg kg - 1 s . c . ; 30 - 60 mg kg - 1 p . o . ; 10 - 30 mg kg - 1 i . v . ; 10 - 30 micrograms / mouse i . c . v . ) was examined in mice , rats and guinea pigs by use of the hot - plate , abdominal - constriction , tail - flick and paw - pressure tests .
### Output Text:
The antinociceptive effect of <span class="chemical">3 alpha - tropyl 2 - ( p - bromophenyl ) propionate</span> [ ( + / - ) - <span class="chemical">PG - 9</span> ] ( 10 - 40 mg kg - 1 s . c . ; 30 - 60 mg kg - 1 p . o . ; 10 - 30 mg kg - 1 i . v . ; 10 - 30 micrograms / mouse i . c . v . ) was examined in mice , rats and guinea pigs by use of the hot - plate , abdominal - constriction , tail - flick and paw - pressure tests . 