8 Replies Latest reply on Feb 12, 2019 5:48 AM by abdur

    Seeing peak towards the end of the run

    oshin-sebastian

      Hello Guys,

       

      I am using Agilent 7890a GC with an FID detector. Last week I saw that there are few ghost peaks in my chromatogram, so I decided to bakeout the column. After baking the ghost peaks disappeared  but now I see and increase in the peak towards the end of the run (please refer to the attached picture). I cleaned the liner, even put a new one and also trimmed the column a little bit. Problem still persists.

      Do you have any idea why this is happening and what would be the solution?

        • Re: Seeing peak towards the end of the run
          abdur

          Hi oshin-sebastian,

           

          If you do a blank run (no injection) do you still see the peak at the end? It's possible that your bakeout wasn't long enough.

            • Re: Seeing peak towards the end of the run
              oshin-sebastian

              Hi Abdur,

               

              I bakeout out for almost 12 hours. The image I attached is of a blank run. I realized that when i do blank run at constant temperature, i don't see this problem. When using a temperature ramp, the baseline drift towards the end of the run.

               

              Best regards,

              Oshin

                • Re: Seeing peak towards the end of the run
                  abdur

                  Hi Oshin,

                   

                  Does the level of the peak change if you do mutliple blank runs in a row?  My thoughts are column bleed, or perhaps an air leak in the column.

                    • Re: Seeing peak towards the end of the run
                      oshin-sebastian

                      Hi Abdur,

                       

                      Yes, with consecutive runs the baseline shift increases and also peaks also move forward a little.

                       

                      Regards,

                      Oshin

                        • Re: Seeing peak towards the end of the run
                          gchaplain

                          It sounds like you column is degrading. As mentioned previously that in the presence of air in the system this can happen more rapidly.

                           

                          What column are you using? Do you know the maximum operating temperature of the column? And also what temperature and it's ramp is method set to towards the end of the run?

                           

                          If the temperature exceeds or is close to the maximum operating temperature of the column it will bleed and stationary phase will leach out making RT shorter on following runs.

                          It's worth noting that C-H bonds break down on average at 320 degrees Celsius, so any bake out above this or close to this value with decompose parts of say a C18 column.

                            • Re: Seeing peak towards the end of the run
                              oshin-sebastian

                              Hi there,

                               

                              Yeah, the column was definitely degrading. I was using CP-Sil PONA CB 100m capillary column. The maximum operating temperature was 250C and I was operating close to that temperature (230C) for very long time. My ramp rate was also quite aggressive towards the end of the run. 50C (hold for 5 min) -> 20C/min to130 (hold for 3 min) -> 50C/min to 230 (hold for 3 min). The last aggressive ramp rate was put in order to make sure all high boiler were flushed out.  As I mentioned, the GC is connected online to a reactor and I need to make at least 3-4 measurements per hour, so 15-20 min max for a GC run.

                               

                              Now I am using a new column from Restek (Rtx-PONA 100m) with a maximum temperature of 350C. My max operating temperature of 230C should be fine for this column I guess. But it's still a struggle to get rid of all the high boilers (Ethylcyclopentane, octane, heptane and aromatics so on) with in a 15 min run. I would appreciate if you have some solutions.

                               

                               

                              Best regards,

                              Oshin

                    • Re: Seeing peak towards the end of the run
                      dmarchini

                      Hi Oishi

                      It could be the column bleed; I've noticed this getting worst if I bake the column in presence of air.

                       

                      I usually change the liner, than run a solvent vial, then I bake for 2 hour, until the baseline get flat.

                       

                      I don't think it is necessary to bake out for longer than 2 hour.

                       

                      If you don't have peaks of interest on the bleed baseline you're doin good ; If you need good peaks on the bleed baseline I suggest to slow the gradient, I mean, it's seems you have like 50°C/5 min ramp or similar, it's too extreme.

                       

                      greetings

                      Dario

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