MRM Inter-Channel CrossTalk

[Triple Quad]

On a Waters Quattro Micro API what are the shortest inter-channel delay and inter-scan delay times you would use without worry about crosstalk?

Is 0.02 seconds for each delay setting for a 2 channel MRM method with a 0.5 second dwell time typical? Is that too fast for the timers to refresh and reset?

[Kostas Petritis]

This will depend on the number of your compounds. If you have too many compounds you can not afford such high dwell times. For example if you had 10 compounds with 0.5 sec of dwell time you would had >5 sec of cycle time so probably wouldn't accumulate enough points per chromatographic peak for good intergrations (especially if your peaks were very sharp).

If you are referring to your analysis of two compounds (that you mention earlier) that would have been just fine. Most of the times, you only have to worry of crosstalk if your Q3 selected masses are identical or very similar.

20 ms of delay setting is plently (in general people use 5 ms). Another thing that people use (especially for the older instruments) is to add a ghost transition between the compounds of interest. A ghost transition is a transition that is way off your transitions of interest and allows the mass spectrometry to purge all remaining ions from your compounds of interest... If in your case you just have a couple of masses, you shouldn't worry to much about it as your pause time is quite high...

[Triple Quad]

Thanks again Kostas!

Yes, I have two transitions that I'm concerned with at the moment.

Perhaps I should have asked about cycle times? Is there a "rule of thumb" cycle time that is too fast that correlates to increased sensitivity or decrease?

Perhaps I should keep it simple and use settings that will get me 12 points per peak and be happy.

[Kostas Petritis]

Cycle time can affect your quantitation accuracy, dwell times can affect your sensitivity. New generation triple quadrupoles can go pretty low dwell times (i.e. 25-50 ms) without minimal effects on sensitivity. They are also much more resistant to cross-talking...

Again for older instruments and for mutliple component analysis, if the chromatography allowed it (i.e. if there was a part in the middle of the chromatogram without any peaks) you could create two events with different transitions (e.g. 10 transitions followed in the first part and 12 transitions on the second). In this way you could afford higher dwell times...

[Triple Quad]

Cycle time can affect your quantitation accuracy, dwell times can affect your sensitivity. New generation triple quadrupoles can go pretty low dwell times (i.e. 25-50 ms) without minimal effects on sensitivity. They are also much more resistant to cross-talking...

Again for older instruments and for mutliple component analysis, if the chromatography allowed it (i.e. if there was a part in the middle of the chromatogram without any peaks) you could create two events with different transitions (e.g. 10 transitions followed in the first part and 12 transitions on the second). In this way you could afford higher dwell times...

Would you mind elaborating on that a bit?

Let me try to reason it out. Cycle time affects quantitation, dwell time affects sensitivity. A dwell time leading to cycle times that are too long can cause missed data point aquisitions thus effecting sensitivity.

I'm not not seeing the relationship to cycle time relationship to quantitation except through the sensitivity loss or gain realized by longer-shorter cycle times. Shorter cycle times will capture more data points, whereby the proper cycle time will be sure to capture the maximum of a quickly changing signal.

Let me try this...I have two analytes that I am looking for with two transitions for each. At the moment I have the two transitions for the two analytes that I am looking for incorporated into one MRM function. Since, I have the two analytes nicely separated, I probably should be using separate MRM functions for each analyte to improve quantitation and sensitivity? I'm talking hypothetically at the moment since my present sensitivity and quantitation are both very good.

I have the following equation:

Cycle time = [(N transitions) * (dwell time)] + [(N transitions)*(interchannel delay)] + interscan delay

[Kostas Petritis]

Cycle time will affect quantitation if you do not have enough data points per chromatographic peak for good integration.

Cycle time depends not only on the dwell time but also from the amount of transitions so practically cycle time will not directly affect your sensitivity, however very small dwell times will.

With only two compounds there is no mean to decrease your dwell time too much as you will get very noisy base line/chromatographic peaks. Depending on your chromatography, 1-2 points every sec should be OK. Manage your dwell times accordingly in order to get such cycle times (or similar). As I said, it will also depend on your MS generation...