Chromatography Forum

Hello Everyone,

I am new to Mass Spectrometry and recently started working on Mass Specs. I am working on a 4500 QTRAP.
My Question is Concerning Duty Cycle. I can understand the duty cycle concept from a triple Quadrupole perspective, However, in Qtrap I am not able to understand how much duty cycle will an MRM-IDA-EPI / EMS-IDA-EPI Method will take. Also, it will be a great help if somebody can help me understand the calculation.

Thanks in Advance
Semina

Hi,

I must say you are aiming pretty high by starting with these complex acquisition modes while being new to mass spec! But anyway, here is some kind of answer to your question: first I assume that by duty cycle you mean cycle time. The duty cycle is quite an ambiguous term since it is often used to describe the rate of transmission of ions in a time-of-flight instrument. EMS-IDA-EPI is a typical DDA experiment where the instrument scans in the trap and as soon as a peak reaches a certain thresold, the instrument switches to MS/MS to produce a product ion spectrum. MRM-IDA-EPI performs similarly but using MRM instead of EMS as first event. You can set a certain number of "simultaneous" MS/MS (=EPI) events, which will have an impact on the cycle time. In theory, you could calculate the cycle time by summing the time needed to perform a full scan plus the time taken by each MS/MS events, but this is not the end of the story: the instrument needs time to switch from one event to the next one (interscan delay) and to perform the computational process for precursor ion selection. The interscan delay value should be in principle known, but the time needed for the decision process is not and can only be measured empirically. See our recent paper that explains how to do it.

https://analyticalsciencejournals.onlin ... /mas.21715

Hi Gaetan,

Thanks for the detailed reply, as well as for the link to the publication. It will take me some time to "digest" the concepts, but it is beneficial.

Hi Gaetan,

Thanks for the paper and it was very helpful in understanding DDA criterions. However my mind is still stuck with scan times in Qtrap? In a Qtrap there is a setting of "Total Scan time with Pauses" How is this value calculated?

Thanks
Semina

This is the time spent on a particular "period" as called in Analyst. For instance if you perform 2 MRM experiments on a QTRAP 6500, one in positive mode with 10 transitions of 20 ms each (=200 ms) and the other in negative mode with 2 transitions of 20 ms each, the first period (positive mode) will (probably, haven't verified) have a total scan time including pauses of 200 ms + 50 ms for pauses, and the second period (negative mode) 40 ms + 10 ms for pauses. Then you sum both periods and add the polarity switching delay (2 times 15 ms) and you get a total cycle time of 250+50+30 = 320 ms. The total cycle time is what interests us, not the total scan time within a period. Naturally, if you have only one period, the total scan time and the cycle time will be identical.

Hi Gaetan,

Thanks. This makes it clear for me.