Retention time problem

Good morning,

we have a GC-MS with a DB-HeavyWax Column. Yesterday we changed the column for a new one (the same). The retention times of our samples shifted 8-11 min. Now, the peaks appear 8-11 min later. We are using a flow of 1.8 mL/min, if we try to do a retention lock, the pressure would be so high. The new column is the same model as the old one. We don't understand why we have this change in the RT's. Column : 123-7132.

 

Thank you!

 

Emilian

Top Replies

  • First of all, thank you for replying my post!

     

    We have checked if the column is the same and also we have checked the entire method and it is all ok. We are really surprised about what is happening.

    Emilian

  • Good Morning ,

    A column change for the same type of column shouldn't have RT shifts that extreme. I would check that it is indeed the same column and catalog number. I would also verify that you have flow and that there were no method changes that occurred during install.

    Regards

    James 

  • Hi  

    To solve this problem you can try:

    1- reinstall the column because bad instill can cause this problem

    2- check leaks in the valves,septum(change it if necessary),injector,sample line 

    we face this (retention time shift about 2 min.) we solve this problem by reinstall the column.

     

    Regards

    Dheya

  • We have already reinstalled the column and checked all the leaks and the retention time has not changed.

  • Other considerations: Has the old column been in service for a long time and has it routinely been pushed to the edge of its max temp? Has the column been trimmed at either end a few times during its tenure? Did the earliest runs of this method on the old column match the RTs you are used to seeing, or are they closer to what you see with the replacement column?

     

    Some columns are WAY longer than they need to be to get a certain separation done because the method developer wanted to be able to use the same column for a while. This is often intended to include regular trimming of either or both ends whenever chromatography suffers because of one end of the column getting either fried or dirty over time. If you do this enough, you can experience significant changes in RTs while still meeting your SST requirements.

     

    In other words, establish whether you *really* have a problem, then if you do, try repeating the column manufacturer's QC test run on it, reinstalling it or whatever.  In general though, RTs will fall over time with any column and they do so to a larger extent with stationary phases that are thicker and more amenable to heat/oxygen/contamination damage.

  • The earliest runs of the old column match the RTs I am used to see, not the actual ones, so it is not a problem of being old or not...

    We have measured the old and the new column and practically they have the same length. 

     

    Thank you for answering!

     

    Emilian

  • I'm suggest to install another new column if you have or reinstalled old column and see the RT, if not shifted that mean the problem in new column(there's an industrial defect).

    if RT still appear 8-11 min later that  mean the problem not in a new column.

     

    Regards

    Dheya

     

       

  • Hi,

     

    As I already mentioned before, I'm not a wiz at GC. I just try to read a lot.

     

    I do have retention times shift during my injections but this is due to a slow degradation of the column because of the type of compounds injected. When I change column, the retention times are completely off.

    It is not worrying because the relative retention times remain unaltered. Check those for both columns.

    Maybe you (we) should stop worrying about retention times and consider, instead, relative retention times or, even better, Kovats retention index.

     

    After installing the column according to the 'Column Installation Guide' that came with it, check the 'Column Performance Summary', provided as well, and compare it with the one of the older column. If differences exist in rt for the same compounds (assuming GC conditions are the same), columns are not the same to begin with.

    Try to reproduce the injections of the standards, used in the summary, at the same chromatographic conditions of course. If you have big differences, maybe you should talk with the column manufacturer.

     

    Finally, in GC, I believe we should be talking about average carrier gas linear velocity instead of flow. Did you see if the velocity of the carrier gas was the same for both columns?

  • I would check the split vent flow with a flow meter.  Also I would try power cycling the gc.  

  • Just to be careful, count the number of loops on the old one and the new one.

    Also read in the manual how to calibrate the column length.

     

    and 1.8 ml/min is high flow into an Agilent GCMS ion source. It will work in the orthogonal (SS,Inert,Extractor,CI) source but not be optimum.  The axial HES source does not like that high flow at all. 

     

    If you think you need that high a flow to get your peaks out in a timely manner, either your column is too long or your oven ramp is not not aggressive enough early enough, or both.   

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