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: Flow and metabolism were measured 5 - 13 days after the subarachnoid haemorrhage by a modification of the classical Kety - Schmidt technique using xenon - 133 i . v . Anaesthesia was maintained with an inspired isoflurane concentration of 0 . 75 % ( plus 67 % nitrous oxide in oxygen ) , during which CBF and CMRO2 were 34 . 3 + / - 2 . 1 ml / 100 g min - 1 and 2 . 32 + / - 0 . 16 ml / 100 g min - 1 at PaCO2 4 . 1 + / - 0 . 1 kPa ( mean + / - SEM ) .
### Output Text:
Flow and metabolism were measured 5 - 13 days after the subarachnoid haemorrhage by a modification of the classical Kety - Schmidt technique using <span class="chemical">xenon</span> - 133 i . v . Anaesthesia was maintained with an inspired <span class="chemical">isoflurane</span> concentration of 0 . 75 % ( plus 67 % <span class="chemical">nitrous oxide</span> in <span class="chemical">oxygen</span> ) , during which CBF and CMRO2 were 34 . 3 + / - 2 . 1 ml / 100 g min - 1 and 2 . 32 + / - 0 . 16 ml / 100 g min - 1 at PaCO2 4 . 1 + / - 0 . 1 kPa ( mean + / - SEM ) . 