Retention Time Shift

[Stillwater59]

I am not a GC expert, so could use some assistance. We analyze our reactions using a DB1701 GC column on a Shimadzu GC 2010 Plus instrument with FID. Up until the issue started, we used a 17 method that has a temperature ramp of 20C/min and then keep a hold for about 5 min. The compound of interest for the method eluted as 2 peaks at 12.5 and 13.5 minutes.

This week, the peaks for the same compound shifted almost 10 minutes and began eluting at 22 and 23 minutes at 270. There has not been a change to the method at all in this time period, and nothing out of the norm has been run on the column. The solvent we are using is dioxane and methanol mixture. The other weird thing is that none of the other peaks on the chromatogram have shifted at all, just this compound. Changed the septum, insert, and put in a different DB1701 and it didnt help. Any ideas?

[chromatographer1]

Your reaction product changed.

It is no longer the product it was.

Rod

[Stillwater59]

I would have thought that, but I have a standard of that product and it matches the new peak times....

[chromatographer1]

Either the original results were wrong or these are.

Since you eliminated the column, either the hardware/software has changed or the pneumatics.

The method has changed or the Oven profile has changed. Or the GC is not heating the oven, or the injector.

Hard to believe that carrier flow would make that much a difference, but perhaps.

You should eliminate one by one the possibilities until you find the physical variable that has changed.

happy hunting,

Rod

[GasMan]

I have seen something like this happen on lowox type columns where peaks in the middle of the run had varying retention times, whereas there was no change to peaks at the beginning or the end of the run. In our case, it was due to water changing the column chemistry. It is possible that you have something in your sample or solvents that is changing the chemistry of the column, and putting in a new column will not help. Generally, I would expect a change in temperature or flow to affect all peaks in the run, or at least peaks that come out later in the run.

Have you started to use new solvents or have you changed something in your process. It is always good to look back at any changes made to your process or procedures.

Gasman

[chromatographer1]

But he tried a new column and had the same results.

I could see if there was a major possibly heavy component in the sample which coated the phase at the beginning of the run which strongly retained the analytes (except for a few), a non-volatile acid for example, this could explain his results for a base containing analyte.

As usual though the poster wants to keep the details private. I also can understand why in a commercial situation.

Gasman may have an insight to the problem.

best wishes,

Rod

[Peter Apps]

I am not a GC expert, so could use some assistance. We analyze our reactions using a DB1701 GC column on a Shimadzu GC 2010 Plus instrument with FID. Up until the issue started, we used a 17 method that has a temperature ramp of 20C/min and then keep a hold for about 5 min. The compound of interest for the method eluted as 2 peaks at 12.5 and 13.5 minutes.

This week, the peaks for the same compound shifted almost 10 minutes and began eluting at 22 and 23 minutes at 270. There has not been a change to the method at all in this time period, and nothing out of the norm has been run on the column. The solvent we are using is dioxane and methanol mixture. The other weird thing is that none of the other peaks on the chromatogram have shifted at all, just this compound. Changed the septum, insert, and put in a different DB1701 and it didnt help. Any ideas?

There is a discrepancy here between the temperature programme rate and elution temperatures (unless there is a very long isothermal hold at the beginning). At 20C/min the peaks used to elute around 260C hotter than the start temp. Even with a quite low start temp of 60C that is an elution temp of 320C, which is high for the column, and higher than the 270C (?) given as the elution temp with 10 min extra retention.

A near doubling of retention makes me suspicious that a setting for the type of carrier gas (H2 vs He vs N2) has been changed on the GC - this is usualy not set in the method and so might escape notice). Why the other peaks did not shift ?? - if they were a long homologous series with variable peak sizes from run to run it might be difficult to see whether thy had moved or not.

Peter

[AICMM]

Here, here Peter. Also look at your column dimensions in this quest. When you note that none of the other peaks shifted, are these all early peaks or are some of them much further in the chromatogram? If early, then you would not see the effect as they are only lightly retained...

Best regards,

AICMM