Maximum signal TQS


I have asked something similar but can't figure out how to find the thread Disappointed

Anyway, I am working with optimizing my Gain. In the instructions I was provided, it mentions to try to achieve a signal of 2 10E6 when looking at the EIC. I am running a GC-MS/MS, should this still be the procedure or should I be looking at the maximum signal on my MRM data?


  • Hi,

    The maximum abundance per ion on SQ instruments is approximately 8.4e6, and for TQ 1.7e8. To stay within the linear range of the electron multipler we recommend staying at or below 2.0e6 for SQ and 2.0e7 for TQ.

    This is still maximum abundance per ion, so that number is for the base peak chromatogram and not the total ion chromatogram. I think it is useful to look at the BPC in any mode (scan, SIM, or MRM) though examining each MRM EIC works fine too.

    For MRM analysis methods I find that for typical sample concentration (say 1 ppb-200 ppb of pesticides in QuEChERS extract) a gain of somewhere between 10 and 20 tends to be optimal. Some methods may require higher, but I don't see people using gain factors >40 frequently. I would consider changes to my sample prep if the gain optimization needed a very high gain factor.

    It sounds like you've already been investigating, but for anyone starting out with gain normalization it is best to start with a gain of 1.0 and then adjust the gain accordingly such that the highest concentration standard does not exceed the recommended limits. Depending on the dynamic range required it may not be absolutely necessary to have the abundance of the highest concentration at the maximum abundance allowed: if you can reliably fulfill your method's requirements at the same gain with the lowest concentration you may be able to use a lower gain factor that will conserve electron multiplier lifetime.

    I think it is useful to examine the sample in scan mode at a gain of 1 during MRM method development to get a sense of the relative amount of matrix background compared to other sample preparations (ie different matrix with same sample prep, or different analyses entirely.) This method is also useful for collecting a scan data file either once per sequence or every few sequences to check if any problems are observed during the sequence of MRM acquisitions. For the sample type used for the scan injection you could use a typical sample, matrix blank, calibration standard for matrix matched calibrations--anything that has the expected amount of matrix concentration. If the response of the matrix BPC is greater than the recommended maximum abundance in scan mode with a gain of 1 either more adjustments to the sample preparation procedure should be considered or the instrument will may require more frequent maintenance. If the response of the BPC is below the limits that is good news.

    Best regards,

    Alex Graettinger

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