Increase your injection volume.

Give us your chromatographic conditions! %RSD or accuracy is related to the concentration of the analyte. The lower the concentration (closer to the LOQ) the higher the %RSD (in general).

The smaller the particle size (film thickness), the longer the GC column, the sharper the peak and thus the lower the %RSD (in general!).

Same RSD you obtain for differwnt compounds? You inject your sample manually or using autosampler? Check the syringe.

Beside od instrument performance - you can use an internal standard method.

If you calculate the area ratio of EG to TEG for three injections of the same sample - what will be the %RSD for these values - also 10% ?

Re

GB

Thank you Grzegorz Boczkaj and Bruce Neagle for your help! My injection volume is 0.1 uL with 1:2 split ratio. My flow rate is 5 mL/s with He as carrying gas. My detector is FID and operates at 400 C. I've checked my syringe and it is clean. I inject through an autosampler. I dissolve my EG and TEG in THF or ethanol at 10 times dilution. I am using an internal standard but when I am performing a mock sample test to exam my accuracy some of my mock samples would have 10% more or less concentration than my calculation. Considering I am testing a highly concentrated sample, the error is really big. For the same sample, if I inject it multiple times, I usually get below 1%RSD. It is the quantification that I think I'm having the most problem with. Although my calibration curve says correlation is greater than 0.99 so it came pretty confusing to me.

Thanks again for your help! Hope you have a great weekend!

Grzegorz Boczkaj Bruce Neagle My apologies for not mentioning the temperature. My inlet temperature is fixed at 250C and oven temperature is 50C for 2 min and up to 230C at 10C/min. Thanks again and hope you have a great weekend!

Tao,

As I recall there is no mathematical relationship between the 'correlation coefficient' and the %RSD. In your method it makes sense to dilute your sample (10 fold) and increase your injection volume to 1 uL (instead of 0.1 uL). I assume you obtain 'good' sharp baseline peaks in your present method!

Hi Bruce Neagle,

Thank you very much for your reply. Yes, my peaks are sharp, with a small amount of tailing. My apologies I don't have a GC graph in my hand (I can post one on Monday). Thank you again for your suggestion, I'll definitely try to inject with a higher volume. One thing I found that is if I inject with 15% instead of 10%, I am starting to get fronting of peaks. Would you think if I increase my injection volume, but change my split ratio (such as from 2:1 to 10:1), it would still improve my accuracy?

Thanks again!

Tao

Perhaps. But try my suggestion 1st since fronting or tailing can have many reasons.

Thank you very much, Bruce. I'll definitely try that.

10 times dillution - you use a solution with 5% concetration?

Why so high?

For such a conc. Use split 100:1 or 200:1. Probably also the temp. of injector is too low.

If there ia no need to use such a hich conc. Try to use 1000ppm or lower conc.

Regards

GB

Hi Grzegorz Boczkaj ,

Thank you for your reply. My goal is to determine the concentration of glycol from dehydration plant, which is usually 90% EG or TEG with small amount of contamination. That's why I try to use a 10 or 5% concentration. I tried to use a 10:1 split ratio instead of 2:1 but the %RSD got higher. I'll try a higher injector temperature. My concern is that my column has a maximum temperature of 230C and my injector is already 250C, if I use a higher temperature, would that damage the column?

Thank you very much for your reply!

Tao

Only the part in the injector will be affected. You can use an empty deactivated fused silica as a pre-column.

Post a chromatogram here. The peak is not cutted?

Regards

GB

Hi Grzegorz Boczkaj ,

Thanks again for your reply. Apologies I don't have a chromatogram with me, I will post it tomorrow. My peaks are sharp with a small amount of tailing. I am also using 2-ethyl-haxanol as the internal standard. Could I use anything else instead and get better results?

Again, thank you so much for your time and help!

Tao

Hi Grzegorz Boczkaj and Bruce Neagle

Please see attached for a GC graph of glycol analysis. I am trying increasing my injection volume now. Is there anything else you see that could be improved?

Thank you very much!

Looks good (nice sharp peak that is baseline separated)! What is your diluent? Do you have differences in viscosity (density) that may affect your %RSD?

generally looks ok, but can you make a zoom to show the peaks near the baseline ? make a zoom for time range 11-22 min, for signal range 0-300 pA.

regards,

GB

Hi Bruce Neagle and Grzegorz Boczkaj ,

Thank you very much for your reply! My diluent is ethanol (I'm thinking of switching to THF for better solubility for possible contaminates from oil sources). Both EG and TEG are quite viscose but they are soluble in ethanol or THF. I have attached a file, on page 2 is the zoom in of the GC graph.

Thanks again!!

Tao

Although using a 1:2 split is not common practice, you are getting good chromatography, good reproducibility, and good calibration. Perhaps the problem is in the preparation of the "mock samples". Can you give us details of this procedure?

Have you checked your syringe? Inconsistent injection volumes can cause poor precision.

Hi Robert L. Veazey ,

Thank you for replying I prepare my sample using methanol or THF with volumetric flasks. For instance, for preparing a 10% sample, I pippet 5mL of EG and 5 mL of 2-ethyl-hexanol(internal standard), then top up the volume to 50 mL. I did the same way with all my standards and kept my internal standard at 10%. Although I do observe that at both 10%, my internal standard is having almost twice the peak area comparing to glycols. Would it help if I reduce the amount of my internal standard?

Thanks again!

Tao

Hi Michael Toner ,

I did check my syringe by injecting an empty injection and it was fine. Should I check my septum and if my column is leaky? If so how can I do that?

Thank you very much for replying!

Tao

To check your syringe inject a standard several times, at least in triplicate, all else being the same if your peak area changes significantly then either your syringe is bad or your injection program is drawing up the sample too rapidly.

Some things are not adding up for me with your data. Flow rate of 5 mL/sec, I assume you mean 5 mL/min. What are your column dimensions? Injection of 0.1 uL at 2:1 split is suspect. Using 10% solutions as you have described would put about 5 ug of analyte on your column which would overload a capillary column and also be outside of the linear range of an FID. You would not have nice peaks as you show nor would you have a good calibration. Check your flows and check for leaks.

Hi Robert L. Veazey ,

Thank you for your reply. My flow could reach my targeted flow although I'll need to double check the information. I do have nice shape of peaks could I rule out leaking from that?

Thanks again!

Tao

Increase your injection volume.