Chromatography Forum

I work at an ethanol plant and need help with our GC. What is the proper way to calibrate it? The last time a standard was ran was 2015 but the numbers were thrown out and we kept the numbers from 2014.


It was suggested (because service pm guy legally can't tell us what to exactly do) to get a standard GC kit, run it 3-5 times and take an average to plug into one of our excel sheets that calculates RF, MRF, Corrected values, etc. Those numbers calculate our % Vol for Ethanol, Methanol, and Denaturant. This would be done once a month and a check standard once a week since we run one sample on our GC every day.

We tried to run it this way after our yearly pm but our concentration came out 92% for Ethanol and it's supposed to be 96%. So we've ordered some new standards and are going to try again. We have no SOPs for our GC...at all. At least none that I can find on our servers.

Also, how can you tell if it's time to replace the column? My new supervisor has never worked in a lab and says that since we run one practically clean ethanol sample a day then the column should never go bad. It was last replaced in 2014 maybe before. In 2015 the ends were trimmed due to having burnt ends and that was the last time it was touched.

We validated our own method for ethanol in hand sanitizers, up to 72% ethanol (by volume). No, polar columns will not last forever, what column are you using?

Besides, you should always have a brand-new column in the drawer to use for troubleshooting if needed, or if you break the column.

Can you upload your chromatograms to something like Photobucket and then post here?

How are you calculating the percents? Ethanol area divided by total area, or are you using an internal standard? We used internal standard.

Did you tried to reconditioning the column before the analysis?
Even with low numbers of analysis, Ethanol has a high amount of water and could be adsorbed by the column. When you inject a sample, water could react with ethanol resulting in a lower concentration than the expected.

Leonardo.

Did you tried to reconditioning the column before the analysis?
Even with low numbers of analysis, Ethanol has a high amount of water and could be adsorbed by the column. When you inject a sample, water could react with ethanol resulting in a lower concentration than the expected.

Leonardo.

Leonardo - what reaction is possible between water and ethanol ?

Peter

GeneticPanda, first of all, did your sample has water? I supposed yes, but i'm not sure.
Depending on the stationary phase, the water can react with it and increase the baseline and column bleeding.
What column are you using?
Are the peaks wider now?

You can try to reverse the column and see if you get different results.

Leonardo.

Did you tried to reconditioning the column before the analysis?
Even with low numbers of analysis, Ethanol has a high amount of water and could be adsorbed by the column. When you inject a sample, water could react with ethanol resulting in a lower concentration than the expected.

Leonardo.

I have no idea how to do that. The service PM rep might have done that before he left. Our water content in our ethanol is less than 1.000%. I've honestly never looked at the peaks on the chromatogram just the number that it spits out. This is my 2nd time working with a GC. The first being in college and it was a ganky, old GC and even that was basic. So I don't have too much experience.

We're using a Restek capillary column. 100m 0.25mmID 0.5μm. At least that's what the tag says. We have one back up column that is the same. The other two are shorter.

We validated our own method for ethanol in hand sanitizers, up to 72% ethanol (by volume). No, polar columns will not last forever, what column are you using?

Besides, you should always have a brand-new column in the drawer to use for troubleshooting if needed, or if you break the column.

Can you upload your chromatograms to something like Photobucket and then post here?

How are you calculating the percents? Ethanol area divided by total area, or are you using an internal standard? We used internal standard.

We're using a Restek 100m 0.25mmID 0.5μm capillary column. We have one spare and 2 others that are shorter.

Hopefully this upload works and you can view it. I ran it this morning.

We're using an external standard. We have an excel sheet that calculates percents based on the last standard (2014). We ran a calibration once in 2015 and once last month. Both times the numbers were off. My previous supervisor threw out the numbers in 2015 so we kept using the same from 2014. Last month was when the service PM rep recommended running calibration once a month and check standard once a week after the calibration numbers came out off.

Dear GeneticPanda,

we determine the quantitative content of volatile compounds in alcohol products and has accumulated a fairly large experience. Probably I can help you.
The proposed method is to use ethanol as the internal standard.
Firstly, this method provides good repeatability.
Secondly, the coefficients RRF (Relative Responce Factros) to be determined are quite stable values and there is no need to frequently do calibrations https://www.youtube.com/channel/UCXgL2c ... lW1oxOGqtQ .

You can send me svcharapitsa@tut.by the files of the measured chromatograms. I will handle them and send you with the comments.

Best regards,
Siarhei

Dear GeneticPanda,

we determine the quantitative content of volatile compounds in alcohol products and has accumulated a fairly large experience. Probably I can help you.
The proposed method is to use ethanol as the internal standard.
Firstly, this method provides good repeatability.
Secondly, the coefficients RRF (Relative Responce Factros) to be determined are quite stable values and there is no need to frequently do calibrations https://www.youtube.com/channel/UCXgL2c ... lW1oxOGqtQ .

You can send me svcharapitsa@tut.by the files of the measured chromatograms. I will handle them and send you with the comments.

Best regards,
Siarhei

Siarhei,
Pray tell, how is Genetic Panda supposed to calculate ethanol % by using ethanol as an internal standard ?? There are more than enough problems with the current procedures, without you adding to them.

GeneticPanda

Unless you have a current, valid calibration of peak area vs quantity or concentration your results are not fit for purpose. A calibration from 3 years ago is not current, especially since a subsequent one has been ignored. Do the regulatory authorities wherever you are (ethanol often falls under customs and excise) have any guidance on how they want you to validate your methods ? What do your customers say about quality control ?.

Peter

Dear GeneticPanda,

we determine the quantitative content of volatile compounds in alcohol products and has accumulated a fairly large experience. Probably I can help you.
The proposed method is to use ethanol as the internal standard.
Firstly, this method provides good repeatability.
Secondly, the coefficients RRF (Relative Responce Factros) to be determined are quite stable values and there is no need to frequently do calibrations https://www.youtube.com/channel/UCXgL2c ... lW1oxOGqtQ .

You can send me svcharapitsa@tut.by the files of the measured chromatograms. I will handle them and send you with the comments.

Best regards,
Siarhei

Siarhei,
Pray tell, how is Genetic Panda supposed to calculate ethanol % by using ethanol as an internal standard ?? There are more than enough problems with the current procedures, without you adding to them.

GeneticPanda

Unless you have a current, valid calibration of peak area vs quantity or concentration your results are not fit for purpose. A calibration from 3 years ago is not current, especially since a subsequent one has been ignored. Do the regulatory authorities wherever you are (ethanol often falls under customs and excise) have any guidance on how they want you to validate your methods ? What do your customers say about quality control ?.

Peter

I don't have access to regulatory information. I would have to ask my supervisor. No one outside of the lab techs here seem concerned since the tests "look" okay and are within our legal limits. But we know the numbers can be skewed since we're using old calibration numbers. They're going by that old adage, "If it's not broke, don't fix it"...

If you can get your supervisor to follow the service technician's advice it would be good. Since you are ordering new standards it sounds that there is at least some desire to do things properly. Standards of volatiles like ethanol and methanol are not stable if you keep opening the stock bottle to take a bit out - they lose components by evaporation, and absorb water from the atmosphere.

A column should last a long time on one sample a day of nearly pure ethanol, and from what I can see of the chromatogram yours is in good condition.

What kind of inlet do you use ?, and how do you inject your samples ?, manual, autosampler or is the GC connected directly to a product line ? How often do you do inlet maintenance ?

Peter

If you can get your supervisor to follow the service technician's advice it would be good. Since you are ordering new standards it sounds that there is at least some desire to do things properly. Standards of volatiles like ethanol and methanol are not stable if you keep opening the stock bottle to take a bit out - they lose components by evaporation, and absorb water from the atmosphere.

A column should last a long time on one sample a day of nearly pure ethanol, and from what I can see of the chromatogram yours is in good condition.

What kind of inlet do you use ?, and how do you inject your samples ?, manual, autosampler or is the GC connected directly to a product line ? How often do you do inlet maintenance ?

Peter

Not sure of inlet but we use an autosampler. The only in-house maintenance we do to the GC is changing the septum every 100 injections and switching out the gas tanks when they start getting low.

I would expect the service tech to change liners in the annual preventive maintenance, and at one injection a day of clean samples that is probably OK. A year between routine maintenance sounds a lot to people who do a hundred samples in a day. If you can get the calibration sorted out you should be OK.

Peter

Correct, can't use ethanol as internal standard if your sample is ethanol, duh !!! We used n-propyl alcohol or acetonitrile (from a USP procedure) as internal standard generally.

We made up internal standard-ethanol reference mix fresh daily and we calibrated and did system suitability every day.

Not sure of inlet but we use an autosampler. The only in-house maintenance we do to the GC is changing the septum every 100 injections and switching out the gas tanks when they start getting low.

Wow - we changed the septum each day of use. We changed the split liners at least every week; granted, our type of products did contain some solids which did get trapped and would build up on split liner over time.