4 Replies Latest reply on Jul 3, 2018 1:09 PM by mj.hazlett

    Recent reproducibility issues and increase in peak fronting after consecutive injections

    mj.hazlett

      The model number on GC is 6890N (G 1540N). I am using a method with split injection with 50:1 split ratio on hp-5 column to measure concentration of a methanol/water mixture using ipa as an internal standard. The method has been successfully used giving a reasonable standard deviation for several months, and recently the standard deviation increased 3x. I tried replacing the liner, septa, gold base seal, reinstalling column, and the split vent cartridge.  After replacing the liner, septa, gold base seal, and reinstalling column, I started noticing some variation in the retention times of the peaks I was looking for. It was suggested to me to change the split vent cartridge as well so I did so, now the peak response on the FID also decreases pretty dramatically on consecutive injections, and the peak fronting still occurs. 

       

      I have also noticed something else, when running the method when the split vent valve turns on (time 0.2) the error light in the software turns red until maybe time 0.4 and the dialogue box says something about the inlet flow being too low and the column flow rate drops from 1 ml/min to 0.9 ml/min, and then turns green again before the switch turns off. Then when the turns off again the error light turns red again shortly after the valve turns off and then turns green again.  Is this normal for the GC or is this another symptom of the problem?

       

      My next step is to call service, if anyone has any ideas on what might cause this sudden loss of performance please let me know.  And if you need more information let me know.

        • Re: Recent reproducibility issues and increase in peak fronting after consecutive injections
          gchaplain

          Do you have the method parameters for the injection and inlet?

          They maybe incompatable with one another, for example, a large injection volume followed by a high split on a pulsed pressure injection followed by a large jump in inlet purge flow maybe going from one extreme value to another, pushing the inlet limits.

           

          Can your inlet reach 60 PSI? - if not then there is a leak on that inlet.

          How long is the column into the inlet? - check with length fits the tolerances, this may contribute to the peak change

          Does the fronting an reproducability get worse with more injections, one after another then later on in the day it's "reset"?

            • Re: Recent reproducibility issues and increase in peak fronting after consecutive injections
              mj.hazlett

              This is a manual injection on a split, single taper, glass wool, deactivated, low pressure drop liner using a 70 mm manual injection needle where I inject 0.5 ul of sample.  The inlet parameters are below:

                 Pressure:  4.94 psi (On)

                 Split ratio:  50:1

                 Split flow:  49.9 mL/min

                 Total flow:  53.4 mL/min

                 Gas saver:  On

                 Saver flow:  20.0 mL/min

                 Saver time:  2.00 min

                 Gas type:  Nitrogen

               

              I performed leak tests and didn't see any evidence of a leak. The column is installed 5 mm past the ferrule into the inlet as described by manuals.  The fronting gets worse with more injections - sometimes I don't see any fronting at all until multiple injections, and then the next day again there is no fronting until multiple injections.  At the same time the area of the peaks decrease with multiple injections and the ratio of peak areas are not consistent (no pattern, just all over the place)

            • Re: Recent reproducibility issues and increase in peak fronting after consecutive injections
              james_jenkins

              I think we need to see the method parameters. The 0.2 valve switch is short, that may be to compensate for vapour volume. Water in your sample, depending on percentage can be very difficult to contain inside the liner as it has vapour volume that can easily expand beyound the liner and cause carryover and other issues. The method parameters as a whole may give us a better picture.

              Regards

              James