What is the maximum number of transitions for a TOF-MRM method? - WKB92322

ENVIRONMENT

• Xevo G2-S
• Xevo G2-XS
• SYNAPT G2-S
• SYNAPT G2-Si
• SYNAPT XS
• Vion IMS QTof

ANSWER

With triple-quad instruments, it is normal to acquire a high number of transitions. However, similar experiments on a TOF instrument are not possible. The detectors for TOF and triple quad work in a very different manners. 

• On a triple-quadrupole instrument, the photo-multiplier measures an ion current. The intensity (measured flow of ions) does not change with scan time. This is similar to measuring the voltage of a battery with a voltmeter. The reading will be the same if the measurement is done for one second or 10 seconds.
• On a TOF instrument, ions are being pushed in the flight tube every 50 µs (rough estimate depending on mass range and length of flight tube). The ions travel to the detector and separate according to their m/z. The longer the scan time, the more pushes are being combined, and the higher the observed signal.

Therefore, when acquiring too many simultaneous transitions on a TOF instrument, the duty cycle decreases quickly, with more simultaneous transitions leading to poor sensitivity. The general advice is to avoid more than four simultaneous transitions. Above four simultaneous transitions, the sensitivity benefits of TOF-MRM are lost, and standard MSE or HDMSE measurements would be preferable.

If the transitions being monitored have known discrete non-overlapping retention times it is possible to define specific retention time windows for each transition in the tof-MRM acquisition method such. This allows the number of simultaneously monitored transition to be kept low although the total number of transitions monitored is greater than 4.

ADDITIONAL INFORMATION