It should not matter if know the composition of your inlet. Flowing and chamber do not change the concentration of components in gas (once you have good mixing and in case your talking about ppm_mole).

Vi/Vt = niRTiPt/ntRTtPi .. You can see that there is pressure and temperature that cancel out. In other words, Vi/Vt = ni/nt which applies to your mixture at certain temperature and pressure assuming z is 1 i.e standard conditions or close

However, if you have elevated pressure and temperature in your chamber that really can alter the results, then you should include z:

Vi/Vt = zini/ztnt since your mole ratio is the same before and after (I mean before it enters chamber and in chamber and after it exists) then all you need is to know the ratio of the z for your certain component and for the mixture to convert to volume ration.

Once you find the volume ratio (multiply by 106) to get your ppm_vol