4 Replies Latest reply on Sep 28, 2017 4:45 AM by mkpbphd

    Is a large drop in Max Gain Factor over a short time normal?

    mkpbphd

      I had been messing around with the custom tune settings on my GC-QQQ trying to pass a DFTPP tune check without completely destroying my sensitivity. While I was looking at my last several tune reports, I noticed that my max gain factor has started dropping pretty dramatically. I know it's normal for it to drop over time but I don't know if it's normal to drop so quick.

       

      In April after an Agilent PM, it was around 7 million.

      In Mid-August after having hardly used it for several months, it was 1.4 million.

      It stayed around 1 million for awhile, and was about 0.9 million on 9/15.

      On 9/18, it dropped to 0.6 million.

      Today, I tried re-tuning from default values (to counter any tweaking from trying to compromise between sensitivity and DFTPP), and it's 0.4 million.

       

      I realize that it's still plenty high, but it seems to be dropping pretty quickly. Does anyone know if that is normal?

        • Re: Is a large drop in Max Gain Factor over a short time normal?
          james_jenkins

          Hi mkpbphd,

          From my experience supporting customers, what you are seeing is normal. The warning criteria for max gain factor is 90 or below, even at that level it may not be a concern depending on what you are running for gain. If your method uses a gain of 20 and the EM voltage is still reasonable, the instrument is functional. I have placed a little more detail below.

           

           

          The Max GAIN Factor is essentially the maximum Gain Factor the EM will be capable of at the

           

          maximum EMV setting of 3000V. It is thus an indicator of the multiplier life and it is expected

           

          to decline through usage. The EM's basically are like batteries and have a finite life in terms of

           

          amp*hrs or coulombs.

           

          The abundances at the time the tune file is saved are based on a Gain Factor of 1, whereas for

           

          the single quad instruments the base peak is adjusted to fall within 250K to 500K. Now, for the

           

          GC QQQ, during tune, the EMV is adjusted to yield enough ion current to adjust peak widths,

           

          mass assignments and do the lens ramps.

           

           

          Detailed Explanation:

           

          The Max GAIN Factor is essentially the maximum Gain Factor the EM will be capable of at the

           

          maximum EMV setting of 3000V. It is thus an indicator of the multiplier life and it is expected

           

          to decline through usage. The EM's basically are like batteries and have a finite life in terms of

           

          amp*hrs or coulombs.

           

          The abundances at the time the tune file is saved are based on a Gain Factor of 1, whereas for

           

          the single quad instruments the base peak is adjusted to fall within 250K to 500K. Now, for the

           

          GC QQQ, during tune, the EMV is adjusted to yield enough ion current to adjust peak widths,

           

          mass assignments and do the lens ramps.

           

          The Gain calibration is pretty simple. First, it establishes the baseline (electronic) noise

           

          statistics (mean and standard deviation) and with this information, a threshold is established. It

           

          is the objective of the threshold to be able to only assess ION pulse events and not the normal

           

          EM dark current. Ideally, a pulse count of approximately 200 to 400 pulses in 16,000 capture

           

          points will ensure the highest likelihood of discrete pulse events. Overlapping pulse events will

           

          be counted as individual events and will yield higher Gain calibration results. Lower pulse

           

          counts will limit the pulse statistics thus limit the Gain certainty. However, deviating outside

           

          this range a bit still will result in a reasonable gain calibration.

          Gain uses ion 222 for the calculation and can be changed in the custom tune area of the GCQQQ tuning section.

           

          If there is sufficient background contamination at m/z 222, it might be difficult to do a Gain

           

          calibration as there often are too many pulses and the calibrations fails. Custom tune and the

           

          Detector tab allows one to change the Gain ion from 222 to something else. I would suggest

           

          looking at that mass in profile mode before selecting it. It should also be noted that the gain

           

          changes with mass and that has to do with the "fixed" gain stage HED which has a different

           

          conversion efficiency as a function of mass. The higher masses typically yield more electrons

           

          from the HED.

           

          The drop you are seeing seems normal to me. Source condition's due to use, vacuum and background all play a factor in the calculation results as well. I like to look at the multiplier voltage as well as the gain if your method uses gain factor. As long as the em is below 2500 and the max gain factor is above your method gain setting, I would continue to use the instrument without worry. I have had calls where customers have replaced a healthy multiplier without much improvement in the max gain factor. I hope this helps.

           

          Regards

          James

            • Re: Is a large drop in Max Gain Factor over a short time normal?
              mkpbphd

              Hi James-

               

              Thank you for that explanation! I knew that it was normal for it to drop over time, but didn't have any reference for how quickly. It is still well above our gain setting and the instrument is performing well, so everything is fine. I just like to keep an eye on anything that might indicate problems in the future.

               

              Thank you again!

               

              Megan

                • Re: Is a large drop in Max Gain Factor over a short time normal?
                  nickbharden

                  To add to James' excellent reply; the gain curve for an EM is steeper when there is water present on the EM, which there most likely was after a PM. This would mean that the calculated "Max Gain Factor" would be artificially high just after a PM and would drop quickly as the moisture pumps away. The air and water check readings on the tune report may give a small indication of how wet the system is but we are talking about water down near the EM which is a long way from the source, so it may not be a good indicator of the amount of water on the EM.

                  1 person found this helpful
                    • Re: Is a large drop in Max Gain Factor over a short time normal?
                      mkpbphd

                      Hi Nick-

                       

                      Thanks for that information! I'm not sure if that's the cause here. It was 8 million when it was installed in December 2015, in August 2016 it was 9 million, March 2017 it was 8 million, April 2017 was 7 million, then August 2017 it was only 1 million. If you look at the tune plots, it's appears to be a rather sudden drop and that's why I wanted to be sure it was normal. I have been performing a lot more analysis on it in the last 6 months (before that it was mostly just a beautiful bench ornament), so I imagine that has more to do with it.

                       

                      This is our first triple quad and the first mass spec that I am personally in charge of (my other instruments are mostly GC-ECD), so I'm still trying to learn what to watch for. It's always better to be able to predict/diagnose a problem early, than to have to wait for the instrument to fail!

                       

                      Thank you, again, for the information!