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Why has not the pressure been mentioned in most articles?
Posted: Thu Oct 27, 2016 1:28 pm
by Masih
The volume of a gas is strongly dependent on its temperature and pressure. When the GC is used to analyze a gas, e.g. CH4 or H2, is it important to report the pressure of the bioreactor where the sample is taken and then injected to the GC?
For instance, suppose that the calibration curve is plotted as the CH4 volume (micro liter) versus peak area. A sample of 50 micro liters is taken from the bottle or bioreactor and then injected to the GC. Based on the calibration curve the volume of CH4 is measured to be Vs. Thus, the total volume of CH4 in the bottle is:
Vt=Vs/50*Vgas
where Vgas is the total gas volume in the bottle which is dependent on the bottle (bioreactor) pressure, while in many articles it has not been reported.
Re: Why has not the pressure been mentioned in most of the arti
Posted: Thu Oct 27, 2016 1:33 pm
by Peter Apps
Welcome to the forum.
Your are right about gas quantity depending on pressure and temperature. The reason that the pressure in the vessel is not mentioned is that taking a sample with a syringe (without a closure on the needle) or by bleeding though a 6- port valve reduces the pressure of the sample to local atmospheric pressure as excess pressure in the vessel bleeds off through the needle or the valve vent.
Peter
Re: Why has not the pressure been mentioned in most of the arti
Posted: Thu Oct 27, 2016 1:39 pm
by Masih
Welcome to the forum.
Your are right about gas quantity depending on pressure and temperature. The reason that the pressure in the vessel is not mentioned is that taking a sample with a syringe (without a closure on the needle) or by bleeding though a 6- port valve reduces the pressure of the sample to local atmospheric pressure as excess pressure in the vessel bleeds off through the needle or the valve vent.
Peter
Thank you Peter,
is the equation Vt=Vs/50*Vgas correct? if so, Vgas is the volume of the vessel contains gas, for example, if the total vessel volume is 120 ml that contains 20 ml liquid, Vgas is equal to 100 ml. Now, suppose that the total vessel volume is 130 ml with the same liquid volume and gas moles, so Vgas=110 ml. For the same sample (50 micro liters) injected to the GC, Vs is constant, but based on the equation Vt=Vs/50*Vgas, the volume of CH4 is calculated to be different results from the pressure difference between two states.
Re: Why has not the pressure been mentioned in most articles?
Posted: Fri Oct 28, 2016 9:34 am
by nick576
From what I understand about your question. Yes knowing the pressure inside the reactor is important for quantifying the amount of CH4 in moles inside the reactor. As Peter said for analysis (injecting with syringe or through a 6 port valve) the sample is equilibrated to atmospheric but post analysis this would need back correcting to the pressure inside the reactor for working out exactly how much CH4 (or any other gas) there really is inside your reactor headspace of 100ml.