Why are my results high

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

11 posts Page 1 of 1
Hi
Im trying to determine residual ethanol concentration in extracted product, this has previously been sent to external accredited labs for testing but the company has purchased a HS GC FID to do the testing in-house.
Ive worked a method that seems quick and accurate, meeting validation requirements and a good calibration based on ethanol diluted in water at a range of concentrations. Ive run the previously tested samples and the results are coming out much higher than theirs at around 20-40% difference on comparison.
Without knowing the other labs methods i feel like im flying blind.

What parameter might i need to change?
System - Shimadzu HS-10, Nexis GC-2030, FID.
Rearinj Temp 150c
Rearinj Pressure 99.5kpa
Column flow 1.35 ml/min
Split ratio 40
Velocity 32.7 cm/s
Column temp 50c
FID temp 250c
HS Oven temp 80c
Sample line temp 90c
Transfer line temp 90c
Carrier Gas N2

Thank you
You're leaving out the column and other info.
To quote others on this board, "The more you don't tell us, the more we can't help you."

Also, are you using an internal standard? My current lab favors the USP alcohol methods which are for packed or cap columns and use ACN as an internal standard. With your flow rate, I'm guessing you're using a cap. column.

Also, is your matrix aqueous? If so, you have to be *really* careful about injection parameters and injector & column initial temps to avoid flashing lots of sample into all the places you don't want it.
Thanks,
DR
Image
DR wrote:
You're leaving out the column and other info.
To quote others on this board, "The more you don't tell us, the more we can't help you."

Also, are you using an internal standard? My current lab favors the USP alcohol methods which are for packed or cap columns and use ACN as an internal standard. With your flow rate, I'm guessing you're using a cap. column.

Also, is your matrix aqueous? If so, you have to be *really* careful about injection parameters and injector & column initial temps to avoid flashing lots of sample into all the places you don't want it.


Since it seems to be a headspace method I think the sample flashing shouldn't be a problem, but I do question the parameters on the transfer lines being only 90c, which could be causing condensation problems with the water vapor. I would run them at 110c to 150c to help prevent that.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
DR wrote:
You're leaving out the column and other info.
To quote others on this board, "The more you don't tell us, the more we can't help you."

Also, are you using an internal standard? My current lab favors the USP alcohol methods which are for packed or cap columns and use ACN as an internal standard. With your flow rate, I'm guessing you're using a cap. column.

Also, is your matrix aqueous? If so, you have to be *really* careful about injection parameters and injector & column initial temps to avoid flashing lots of sample into all the places you don't want it.


Since it seems to be a headspace method I think the sample flashing shouldn't be a problem, but I do question the parameters on the transfer lines being only 90c, which could be causing condensation problems with the water vapor. I would run them at 110c to 150c to help prevent that.




Hi, thanks for the replies
Yes cap column Rxi®-1301Sil MS, 30.00 m, 0.25 mm ID, 1.00 µ.
External standards using abolute ethanol diluted in water, however the samples are oil based
Bellyup wrote:
James_Ball wrote:
DR wrote:
You're leaving out the column and other info.
To quote others on this board, "The more you don't tell us, the more we can't help you."

Also, are you using an internal standard? My current lab favors the USP alcohol methods which are for packed or cap columns and use ACN as an internal standard. With your flow rate, I'm guessing you're using a cap. column.

Also, is your matrix aqueous? If so, you have to be *really* careful about injection parameters and injector & column initial temps to avoid flashing lots of sample into all the places you don't want it.


Since it seems to be a headspace method I think the sample flashing shouldn't be a problem, but I do question the parameters on the transfer lines being only 90c, which could be causing condensation problems with the water vapor. I would run them at 110c to 150c to help prevent that.




Hi, thanks for the replies
Yes cap column Rxi®-1301Sil MS, 30.00 m, 0.25 mm ID, 1.00 µ.
External standards using abolute ethanol diluted in water, however the samples are oil based


Do you have a clean oil to use to make standards with? Depending on the solubility of ethanol in the oil you can get different results when using two different matrix for standard versus sample. If ethanol is more soluble in water then it could be why the samples show better recovery, since the standard would tend to retain more of the ethanol during the headspace extraction.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Bellyup wrote:
James_Ball wrote:

Since it seems to be a headspace method I think the sample flashing shouldn't be a problem, but I do question the parameters on the transfer lines being only 90c, which could be causing condensation problems with the water vapor. I would run them at 110c to 150c to help prevent that.




Hi, thanks for the replies
Yes cap column Rxi®-1301Sil MS, 30.00 m, 0.25 mm ID, 1.00 µ.
External standards using abolute ethanol diluted in water, however the samples are oil based


Do you have a clean oil to use to make standards with? Depending on the solubility of ethanol in the oil you can get different results when using two different matrix for standard versus sample. If ethanol is more soluble in water then it could be why the samples show better recovery, since the standard would tend to retain more of the ethanol during the headspace extraction.



Thanks James, yes i believe the difference in matrix will be an issue although i cant imagine the big accredited lab goes to the trouble of doing a oil calibration just for our samples when they will be running multiple samples of various matrixes, I can only get oil that has low ethanol content ie 0.2% and ive tried to make standards from that by spiking it but its quite difficult and time consuming, the results i got from that came out lower than the labs. Just trying to get results which are within range of the others, its inhouse results for production purposes
The simple answer is that you cannot calibrate a headspace method for an oil with standards in a water matrix.

Depending on batch size and sample throughput you could try using known additions, especially since you don't have oil with no ethanol in it. You run each sample as is, and with three levels of added ethanol. Then plot the peak area vs addition and extrapolate back to the y axis to get the concentration in the sample. Known addtions automatically takes into account any variability in the samples that might affect partition of the ethanol into the headspace.

Peter
Peter Apps
Peter Apps wrote:
The simple answer is that you cannot calibrate a headspace method for an oil with standards in a water matrix.

Depending on batch size and sample throughput you could try using known additions, especially since you don't have oil with no ethanol in it. You run each sample as is, and with three levels of added ethanol. Then plot the peak area vs addition and extrapolate back to the y axis to get the concentration in the sample. Known addtions automatically takes into account any variability in the samples that might affect partition of the ethanol into the headspace.

Peter


Thank you Peter :-)
The oven temperature for your headspace samples is 80C; are you doing total evaporation headspace on your samples, i.e.: hot enough to ensure every component and the matrix are in vapor phase? If so, your water will be left behind, meaning more ethanol in the headspace than you'd expect. Admittedly, your matrix won't be total evaporation headspace, but if you run at a sufficiently high temperature and run some matrix spikes...

Run a sample of your oil. Then run it with (say) 50 ppm ethanol. See what your numbers look like.

When you work the numbers manually (versus calculated via software) do the numbers come out right? It may be a math error in your dilution factor.

Other things to double-check: your equilibration time after pressurization; your sample volume (usually 1000 uL on this type of analysis); if your matrix will take it, try HS at 140, sample line at 150, transfer line at 160C, you want lowest to highest from sample to column to ensure nothing condenses.

Is your column running an isothermal temperature profile?
osp001 wrote:
The oven temperature for your headspace samples is 80C; are you doing total evaporation headspace on your samples, i.e.: hot enough to ensure every component and the matrix are in vapor phase? If so, your water will be left behind, meaning more ethanol in the headspace than you'd expect. Admittedly, your matrix won't be total evaporation headspace, but if you run at a sufficiently high temperature and run some matrix spikes...

Run a sample of your oil. Then run it with (say) 50 ppm ethanol. See what your numbers look like.

When you work the numbers manually (versus calculated via software) do the numbers come out right? It may be a math error in your dilution factor.

Other things to double-check: your equilibration time after pressurization; your sample volume (usually 1000 uL on this type of analysis); if your matrix will take it, try HS at 140, sample line at 150, transfer line at 160C, you want lowest to highest from sample to column to ensure nothing condenses.

Is your column running an isothermal temperature profile?




I have tried to research an optimal temps for each parameter for ethanol but every method i find is different, i cant see any consistency, so im running the same sample repeatedly at different temps. Theres a little moisture in the oils, so i do wonder if i need to reduce that initial headspace temp and increase the sample and transfer lines.
I am running isothermal, and also trying to find the best temp, is it based on bp of solvent or ratio with other temp parameters?
I take 0.6g sample into 20ml vial

Thanks
Bellyup wrote:
I have tried to research an optimal temps for each parameter for ethanol but every method i find is different, i cant see any consistency, so im running the same sample repeatedly at different temps. Theres a little moisture in the oils, so i do wonder if i need to reduce that initial headspace temp and increase the sample and transfer lines.
I am running isothermal, and also trying to find the best temp, is it based on bp of solvent or ratio with other temp parameters?
I take 0.6g sample into 20ml vial

Thanks


We got good numbers on ethanol at 35-40 mg of sample in a 10 mL headspace vial, using GC/FID, 10 to 1000 ppm for quantitation, although you could see peaks at the 1 ppm range. I forget the split; maybe something like 20? It's been a while.

Try a column like a 624 Sil MS, same as is used in pharma for residual solvents.

Now that I think about it, the USP method for residual solvents uses a 624 Sil MS column with a real thin layer, something like 1.4 microns, 30 meter column. Check your peaks; are they tailing badly? If so, you're overloading your column. Set your hammer to "delicate": much smaller samples in the headspace vial. Find that sweet spot of linearity, right around 10-100 times your LOD with ethanol under these conditions. Wrap your methods around your samples, not the other way 'round.
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