Acquisition Method Error - Open Lab CDS

Hi all, 

I am using using Open Lab CDS and am having an issue with setting up the SIM acquisition method. I have selected 74m/z as the unique ion to this compound group. For the fatty acids which do not include 74m/z as part of their spectra, I would like to select their characteristic ion to monitor. I am able to do this for the majority of the compound list, but for some when selecting plot ion I receive the following error "The number of signals displayed in the real time plot is 13 and is limited to 12." When I deselect this ion, the error is resolved. Any ideas on how to resolve this issue? 


  • Yes. Don't select that many ions to plot in real-time plot as shown on the screen during the run. The ions will all be saved in the data file for you to see there.  Most only plot a very few ions.   

    You have many SIM groups.  If your retention time changes, you'll have to fix the times for all of them.  You may want to consider condensing them a bit?  Like 43 and 43.7 together with 6 ions, and maybe 51 and 51.9 together with 6 ions.   Like that...

    It's important to balance the number of ions and the dwell time per ion to allow enough data points to make useful peaks.  If you have few ions, like the three or four shown, then you can use longer dwell times than some do.  You want to have at least three to four SIM ion cycles per second so that you get at least ten to twelve or more data points for each chromatographic peak. Mathemeticians say it takes that many point per peak to make one that can be integrated properly and used for quantitation.

  • Thank you! From what I understand, once I process my data I will obtain the full SIM plot (even though I did not select them for the real time plot)?

    Okay, thank you for that suggestion, I will condense some of the SIM groups.

    For dwell times, if I have 6 ions I would use a dwell time of 50ms, but with 3-4 ions I could increase the dwell time to 60-70ms? Is there a formula or method I should use to calculate the dwell time required for a given set of ions. 

    Thanks in advance

  • For only six ions.  50 ms/ion x 6 ions=300 ms is about 3.33 SIM ion cycles per second.  70 ms/ion x 6 ions = 420 ms is about 2.4 SIM ion cycles per second. There is system overhead per SIM ion cycle, so the actual numbers would be a bit less. 

    If your earliest eluting/narrowest chromatographic peaks are six seconds wide, there would be either 19.98 or 14.4 SIM ion cycles across the peak.  Mathematicians say that we need at least 10 to 12 data points to properly define a peak and make it sort of easy for the system to integrate, and so 14.4 is sufficient.  More ions means less dwell time per ion up to a point.  I've not had good success with dwell times shorter than, oh, 5 to 8 ms/ion ea for a big list of ions. 

    A good example can be seen on page 52  of , as found here: (+) Collection of 5977B GC/MS Resources - Wiki - GC/MS - Agilent Community

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