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

Question

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?

james_jenkins · Accepted Answer

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

nickbharden · Answer

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.

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

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