Conversion of Standard Gas Mixture used for calibration

Discussions about GC and other "gas phase" separation techniques.

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Dear All,

I am looking for your experience and skills.

We are using certified gas mixture cylinders for the calibration of our GCs, I am looking for a good practice to convert the concentration of any cylinder gas mixture from its unit to the desired unit. For example, we have a cylinder with LNG components in mol% and I want to convert all the concentrations to ppmv or ppmw! The components are H2, CO2, H2S, N2, CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, nC5H12 and C6H14, the total concentration is 100 mol%.
First of all, condensed-phase ppm is not the same as gas phase ppm. You can't make it so unless you know what mass/volume of water in which you are liberating the gas - then you have to determine how much is actually trapped in the solvent. Difficult problem.

I've been able to wrap my mind around gas-phase parts per million by thinking of it as µmole analyte/mole of ideal gas. I know that it can end up that µL analyte/L of gas can also be parts-per-million under some set of conditions but I prefer to think of concentration as some sort of mass units. Could be a volume but it's related to mass through its density so it's essentially mass.

We worry a lot about acetaldehyde (AA) in CO2 and how it might affect the condensed-phase concentration of AA in water. 1 ppm AA in the CO2 is 1 µmole AA/mole of gas.

Let's say we're carbonating water at 3.0 volumes of CO2 (as it would be at atmospheric pressure) in a 12 oz. (355 mL of liquid) can. The volume of the can is 0.378 L (overflow volume, not the volume of liquid in the can) so 3x0.378 = 1.134 L of CO2 at atmospheric pressure in the can.

PV=nRT
n = PV/RT

1 atm x 1.134 L/0.0821/293 = 0.0471 mole of CO2 (ideal gas law, 20 °C)

Assume 100% of the AA is trapped in the water in the container:

1 µmole AA/mole x 0.0471 mole/355 mL water x 44 µg AA/µmole AA =
0.006 µg AA/mL in the water

1 ppm in the gas ends up - worst case - as 6 ppb in the liquid (assuming it's all trapped in the water during the preparation of the carbonated water).
abuzuhaira wrote:
Dear All,
... For example, we have a cylinder with LNG components in mol% and I want to convert all the concentrations to ppmv or ppmw! ...

Making it simple and sufficient for your needs (not precised): mol% equals vol%.
Thus multiply mol% by 10 000 to get concentration in ppmv.
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