Analysis of gases in liquid samples; response factors

[cbd]

Hello all,

I need to analyze propylene gas dissolved in a liquid (solvent) mixture (at room temp. and 1 atm pressure) and determine its concentration (wt%). I have calculated response factors for my liquid components by making up a known wt% mixture and injecting manual 0.2 microL samples onto the GC (TCD detector). My problem is calculating a response factor for propylene on TCD. Is it possible for me to calculate a RF for propylene based on gas-phase measurements?

I have injected a 200 microL gas-phase sample (manual injection) of pure propylene (from Tedlar bag). Can I use the area from these injections to calculate the weight % of propylene in the liquid samples? Could I calculate a response factor for propylene in the liquid phase by dividng the area of the gas-phase injection by 1000 (ie. 200/0.2)?

Any other ideas or suggestions on how to calibrate for dissolved gases in liquid mixtures?

Finally, once I have response factors for all my components in the liquid phase, can I use those to analyze an unknown gas mixture comprised of the same components? I will also be injecting the gas samples manually at ambient T and P.

Many thanks for your ideas and help,
Christina

[promochrom]

You only inject 0.2 uL liquid. The error of volume could easily be 10-20% from the true value. When both standard and the sample are introduced using the same syringe, the error could be canceled a lot. If you use the RF from gas to quantify proplene, you may get accurate amount of proplene. Since the volume is uncertain, you still cannot get accurate concentration (concentration=amount/volume).

FID gives very similar RF for hydrocarbons. You may use a FID for the analysis of all the components in the liquid and use a similar compound in the liquid to quantify proplene.

Just some thoughts.

[chromatographer1]

The response for hydrocarbons is fairly uniform once one uses a carbon number greater than 5. Using a relative response of hexane or higher to calculate the response of propylene will result in a sizeable error.

best wishes,

Rod

[GOM]

Hi Christina,

How did you get your propylene into the bag? If you have access to

1. headspace vials with butyl rubber crimp tops
2. a 25ul gas syringe
3. some tubing and a disposable hypodermic needle

then it is possible to make up your mix in the headspace vial, crimp seal and weigh, then dispense your propylene into the vial and reweigh then inject 5ul - making sure that you leave the needle in the port for 5 seconds to ensure that all the solvent is evaporated from the needle.

I am doing a similar exercise at the moment with some propellants to establish response factors. I do have an old paper somewhere with TCD response factor tables. It does state that, despite the difference in designs the values are remarkably similar between instruments and I have to say that mine have been close to them. I'll try to dig it out.

Regards,

Ralph

[cbd]

Thanks for all your replies.

promochrom: Yes, I guess 0.2 microL is quite small. I don't want to overload the column as I am also trying to quantify the largest component in my sample mixture (solvent, ethyl acetate). When I inject my samples I'm usually pretty good at minimizing the error volume (unless I get a propylene "bubble" in my syringe, then my numbers are way off. I normalize my result to 100% to calculate the "corrected" weight precent in my sample. I have also used FID for this analysis, but now I am also looking for water, and other gases such as O2 and CO2 in my sample (products of my reaction). When I did a similar calibration technique for the FID, my area % propylene in the liquid injections where very close to weight % calculations.

Ralph: The Tedlar bag has a valve on it. With bag emptied (under vaccuum) and closed, I open propylene cyclinder with shortest tubing section I can find. After some propylene has flowed through the tubing, I attach the tubing to Tedlar valve and open the valve. I empty the bag manually and refill, to try to remove any air that might have gotten into the bag from the dead space in the valve. Here's the catch with propylene: when I did FID injections with pure propylene from Tedlar bags all I got was a 100% propylene peak. When I did the injection on TCD I get a 3% area peak at ~0.45 min, which I think is air. I have all the equipment you mentioned, but I'm not sure if you are talking about making liquid injections or all gas injections (or combination?) from the vials? I am also working with acetic acid. If I inject into a liquid sample my propylene will be present in both the liquid and the vapour phase. Can you explain your method?

If you find the response factors please let me know; I'ld like to see if I am at least in the ballpark with my method.


Many thanks!
Christina

[GasMan]

Christina,

The response factor in most integrators is amount of compound per unit count of the integrator. Therefore, PROVIDING your detector response is linear, you will get the same number if you inject 200 ul or 0.2ul.

This is all taken care of in the calibration process for your method.

Gasman

[GOM]

Hi Christina,

I found it! J of G.C. February 1967 p68-71. Rosie and Grob (that'll be papa Grob) "....found the same thermal response for hot wire filament......the response values were independant of temperature, carrier gas, flow rate and concentration."

you multiply this weight factor by the area to get the true weight area.

ethyl acetate 0.79
propylene 0.65
no listed value for acetic acid.

What are your other components?

BTW to avoid propylene bubbles place your syringe in the freezer for 30seconds then sample without holding the barrel too much.

Regards,

Ralph

[cbd]

Hi Ralph,

Thanks for the info! My other components are acetaldehyde, propylene oxide, and (trace) water.

Many thanks!
Christina