11 Replies Latest reply on Apr 26, 2018 6:00 AM by james_jenkins

    Sinusoidal baseline.

    ccbarnes5

      We have a baseline that gives a sinusoidal wave that starts about 16 minutes.  We are using an Agilent 6850 GC with a split/splitless injector and an FID.  The inlet is interfaced to a 7679A HS.  The baseline is good up until 15.5 minutes then it starts the sinusoidal response.  This response is consistent from one run to the next.  We've baked the column per the manufacturer's instructions.  We've re-installed the column on the detector end and put in a new inlet liner.  Any ideas? 

       

      chromatogram with sinusoidal baseline.jpg

        • Re: Sinusoidal baseline.
          james_jenkins

          Hi ccbarnes5,

          I have updated the tags for better visibility. Have you tried a new jet in the FID? It could be restricted with carbon. Also when you reply, it would be helpful if you could reply with your method parameters .

          Regards

          James

            • Re: Sinusoidal baseline.
              ccbarnes5

              The column is a DB-1301, 30 X 0.32, 1.0 µm df.  The GC conditions are:

               

                          

                          

                          

                          

               

               

              Curtis [Personal information removed by moderator.]

              • Re: Sinusoidal baseline.
                ccbarnes5

                Oops.  The HS conditions are:

                 

                 

                 

                Curtis [Personal information removed by moderator]

                  • Re: Sinusoidal baseline.
                    james_jenkins

                    At first glance I don't see much to be concerned about here with method parameters other than the gas saver flow is 15ml/min, 5ml below the suggested of 20ml/min. Has there been any maintenance done to the FID, jet change etc?

                    James

                      • Re: Sinusoidal baseline.
                        ccbarnes5

                        No.  There has NOT been any change with the detector; yet.  We’re going to try changing the FID.  I’ll take a look at the collector when the jet is changed.  The issue that’s really throwing us off is that it only happens AFTER the elution of the solvent (Dimethyl acetamide), when the oven temperature gets over 160°C.  AND it’s so consistent.  Subsequent chromatograms can be overlaid and that baseline will look EXACTLY the same over that time period (about 15.5 minutes – 20.0 minutes).

                         

                        Curtis [Personal information removed by moderator.]

                        • Re: Sinusoidal baseline.
                          ccbarnes5

                          Ok. Now we’re cookin’ with gas.  After changing the jet we now have an improved chromatogram:

                           

                           

                                          BUT, we still have the sinusoidal issue.  I’ve asked the analyst to remove the collector and sonicate for a few minutes in acetone and MeOH.  HOWEVER, I was told we saw this ‘sinusoidal’ effect with this method on more than one instrument.  Can the dimethyl acetamide be affecting the stationary phase to produce this?

                           

                           

                          Curtis [Personal information removed by moderator.]

                            • Re: Sinusoidal baseline.
                              james_jenkins

                              I checked with one of our column chemists and he doesnt feel that dimethyl acetamide should cause you any concerns. The amplitude flucuation is low, doesnt appear to be electronic. It maybe something else coming off. One test to try would be an injection without sample or solvent. Essentially a blank run from the headspace with a vial of just air to see what happens.

                              James

                                • Re: Sinusoidal baseline.
                                  old-skuhl

                                  Def second the blank injection idea.  See if systemic or chemical.  Almost looks like something that builds up...  like the bulk of the material comes off but the remainder builds up pressure, then releases, builds up, then releases.. etc.  Perhaps a dead volume issue in your system plumbing?

                                    • Re: Sinusoidal baseline.
                                      ccbarnes5

                                      I agree.  It looks very much like a pressure increase then sudden release repeating over and over.  But WHY during this one particular period?  Why don’t we see it throughout the chromatogram?  This pattern is seen only over this particular time/temperature range.

                                       

                                      Curtis Barnes

                                      [Personal information removed by moderator]

                                        • Re: Sinusoidal baseline.
                                          old-skuhl

                                          Because it has to reach that temp/pressure in order for the material to appear in the first place.  Hence the reason you don’t see it prior to your large peak.  Once you’ve met that T/P “sweet spot” for this material, it is continually building up, or off-gassing, or whatever it’s doing.  If you don’t inject anything but monitor the signal as you gradually increase the oven temp manually, do you still see it?  You don’t even have to do a blank injection for this.  Just set your oven at the temp where your main peak there comes out, then gradually nudge it up.  What do you see?  If it’s something in your system, you’ll start seeing junk come off.  Though if you have a “pocket” somewhere in your plumbing, it could still be systemic but related to the chemical simply because you have to introduce it for the material to build up.

                                           

                                          If you run a series of sequential blanks, do you see the same intensity or does this diminish over time?  I had a “similar” situation where I had a contaminant I could ID but not determine the source.  My first injection had a HUGE peak, but all following runs had a significantly smaller peak, but was VERY consistent.  It ended up being residue from new gas lines that built up in my system overnight, over the weekend—while my system was dormant and cool, but released when I fired it up Monday morning.  The steady peak I saw afterward was what was able to build up in my system during the cool-down btwn runs.

                                           

                                          Have you introduced any new plumbing/gas/etc recently?

                                           

                                          [Personal information removed by moderator]

                            • Re: Sinusoidal baseline.
                              james_jenkins

                              Hi ccbarnes5,

                              I am marking this post as assumed answered due to age. If this isn't the case, please feel free to post again. If any of the suggestions were helpful or helped you resolve the issue, please mark as correct answer.

                              Regards

                              James