Headspace analysis propane, Turbomatrix-Clarus

[rb6banjo]

Got it fixed. It was an easy fix.

https://i.postimg.cc/FsCKy8c2/Methane-b ... alysis.jpg

Here's the 1-page summary. Some example chromatograms. Triplicate analysis of my 25:21 dilution (how I've described above). When I used an old calibration from May, 2024 (from 0 to 100% methane), I say that my samples are 70.1% (v/v) methane. My target was 69.6%. Pretty good agreement (101% recovery).

1.5% RSD on my triplicate measurements of the dilution of the same gas. Pretty good there too.

The water is a waste of water and time. A properly filled empty vial should work just fine.

[MichaelVW]

Thanks for everyone's suggestions. I finally found a leak (it had to be a leak!). The adapter where the column connects to the FID had been bent somehow. I think that I missed it because I had been looking for nitrogen leaks with the FID gasses off. When I finally checked again with the FID gasses on, I picked up hydrogen coming into the oven.

I have a good curve relating propane concentration to FID response.

The method's equation to convert measured concentration (as propane) to organic vapor pressure is:

organic vapor pressure = β * Pbar * Ca

Pbar = Atmospheric pressure at analysis conditions, mm Hg
β = 1.333 × 10−7kPa/[(mm Hg)(ppm)]
Ca = Measured vapor phase organic concentration of sample, ppm as propane.


The remaining questions I have are:

Why does this equation work?

What is β?

Is Pbar the local atmospheric pressure, or that plus the pressure added to the vial by the HS analyzer?

If I put in 1,000,000 ppm into the equation, shouldn't it spit out the vapor pressure for propane? But... at what temperature?

[rb6banjo]

At constant temperature and volume, from the gas law, P1/n1 = P2/n2. What you're shooting for is the partial pressure (P2) of propane.

n2/n1*P1 = P2

What you measure is ppm C3H8. That's µmole C3H8/mole gas. n2/n1 is the concentration of C3H8. There are 7.50062 mmHg/KPa. There are also 1,000,000 µmole C3H8/mole C3H8. If you want P2 in KPa, to convert the ppm C3H8 you measure to KPa you have to multiply the measured ppm C3H8 by 1/7.50062x10^6 and the atmospheric pressure in mmHg. That factor is 1.333 x 10^-7.

KPa = 1.333 x 10^-7 x Atmospheric Pressure (mmHg) x ppm C3H8

[MichaelVW]

Thank you so much for the effort you put into answering my question. This was really helpful.

So to simplify units:

pressure of propane (mmHg) = ppm propane / 1,000,000 * atmospheric pressure (mmHg)

Would you agree that atmospheric pressure should be the pressure in the vial, i.e. ~1 atmosphere plus the pressure the HS adds (in this case, 14 psi)? I'm not sure because it doesn't pressurize it during equilibrium, just at the end for 3 minutes to sample.

... then again the whole point of doing this is to find what the vapor pressure will be of the sample in the real world...

[rb6banjo]

That is correct. I believe your gas is going into the vial with the water at atmospheric pressure so that's it.

You only need the 7.50062 mmHg/KPa if your final result must be in KPa.

You are welcome. It turns out that this might be useful for me too. I have an unknown that I'm working on - might be microbiological in nature and perhaps a methanogenic bacterium - that's causing us some issues. Knowing the methane concentration in a gas sample might lead me to the root cause. We'll see!

Nope. The cause of the pressure increase was not due to methane............... Ugh!