Chromatography Forum

Hi, I'm a graduate student and new to GC (about 1 year).
Currently, I'm working on to develop a method that can detect the trihalomethanes(THM) and haloacetic acids(HAA) at the same time by headspace SPME-GC/MS.

But I'm facing a problem as the title says... so here are the detail of my question.
Originally, I didn't add the internal standard(IS) to my sample. After three injections, RSD come up to be around 20% for each chemical.
As a result, I add the IS to improve the RSD. However, the RSD is getting worse with the adjust of IS! The result that is not adjusting with the IS looks much better!

I don't know what to do, I've checked the liner and septa, I think they look fine.
Please give me some advice!
Thank you so much.
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Instrument:
I'm using Agilent 7890A GC-MS/MS with a DB-5MS column (60 m x 0.25 mm id x 0.25 um).

You have to think about how internal standardization works. Essentially, when you take a ratio of two measurements (analyte and IS), any *correlated" errors (errors that have a similar effect on the analyte and the IS) will tend to cancel out; the ratio will be more reproducible than the individual measurements. However, *uncorrelated* errors will accumulate so that the ratio is less reproducible than the individual measurements.

Using an internal standards made things worse, so you have to look for sources of error that will affect the analyte and the IS independently. Things like excessive baseline noise (poor S/N ratio), integration setting issues, etc.. Also possibly varying detector response (if the analyte and the IS do not elute close together).

Thank you for the reply.

It did make me confused that the IS I used show the different characteristics with my analyte. And I even try to replace it with another one, but the result ended up the same.

The first IS eluted at the second place, and the second one was the last to come out.
I'm just wondering if these two IS I chose are both affected by the amount of my analyte (Since some of HAAs might degrade with time or during the analyzing process, but I have modified the parameters to avoid this as possible), thus they will adsorb onto SPME at different amount.

I think the S/N ratio looks fine (SIM mode), and other things I need to think more about them. Thanks again!

Thank you for the reply.

It did make me confused that the IS I used show the different characteristics with my analyte. And I even try to replace it with another one, but the result ended up the same.

The first IS eluted at the second place, and the second one was the last to come out.
I'm just wondering if these two IS I chose are both affected by the amount of my analyte (Since some of HAAs might degrade with time or during the analyzing process, but I have modified the parameters to avoid this as possible), thus they will adsorb onto SPME at different amount.

I think the S/N ratio looks fine (SIM mode), and other things I need to think more about them. Thanks again!

In the EPA method 552.2 and 552.3 for the HAA analytes they methylate the analytes. How does your chromatography look for the HAA analytes? Do you have any tailing of the peaks? THM are pretty easy to chromatograph, but not sure about non-methylated HAA.

In the EPA method 552.2 and 552.3 for the HAA analytes they methylate the analytes. How does your chromatography look for the HAA analytes? Do you have any tailing of the peaks? THM are pretty easy to chromatograph, but not sure about non-methylated HAA.

Thanks for the reply.
I derivatized HAA with Dimethyl sulphate (DMS) to become methylated HAA.
Basically, they are the same compounds as in the EPA method.

Yes! I had peak tailing problem in my chromatography. But the tailing happened at the analyte that elute earlier... (My whole program was 30 minutes, but analyte that came out before 25 minutes would have tailing problems.)

I actually tried to solve these problems by the methods below:
1. lower the temperature of inlet (lower 50 degree Celsius): peak remain the same
2. lower the rate of the ramp: peak remain the same and they elute later
3. Change liner and septa: peak remain the same
After discussing with my advisor, it seemed to be not a big problem for him, and also my classmate thought so. So I did not try to solve the tailing problem anymore.

But after listening to your advice, I did some study and I think that might be the problem! I'll try other way to solve it. For example, increase the velocity of the gas...

Thank you so much!