We need to mix isoprene into room temperature air so that isoprene would have 40 ppb in volume. This would mean that we would have isoprene in the order of few micrograms per cubic meter. The mixed air is blown into a tube where a separate sensor is housed. Is there any easy way to achieve such a small concentration or are mass flow controllers the only option?
This will depend on the smallest volume of isoprene you can accurately measure and the volume of air you are willing to mix it in to achieve your desired concentration.
I would suggest doing a serial dilution since it will be hard to measure the isoprene at the nanoliter scale and a large volume of air to the same precision, e.g. 40nL pure isoprene vapor into .99999996L of air. You could measure a reasonable volume of liquid (maybe 100uL) and then evaporate that liquid into some volume of air. If you put 1L in a vessel, seal it and remove 24.15cm3 of air and then inject 100uL of liquid isoprene you'll end up with 247.5ppt (I think I did that calculation right... you should check). Then pull some fraction of that air into a syringe and inject that into another volume of air. Mix and repeat until you reach your desired concentration. You could then compress that volume into a small cylinder and use it as a calibration source.
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