【質(zhì)譜】TOF-MRM 方法的最大通道數(shù)（the maximum number）是多少？
菜鳥博士
TOF-MRM 方法的最大通道數(shù)是多少？- 中文版本
環(huán)境
Xevo G2-S
Xevo G2-XS
Synapt G2-S
Synapt G2-Si
Synapt XS
Vion IMS QTof
答案
使用三重四極桿儀器時，通常會采集大量通道。但是，無法在 TOF 儀器上進行類似實驗，因為 TOF 和三重四極桿檢測器的工作方式差異很大。
在三重四極桿儀器上，光電倍增管可測量離子電流。強度（測得的離子流量）不隨掃描時間變化。這類似于使用電壓表測量電池的電壓。如果測量持續(xù) 1 秒或 10 秒，則讀數(shù)相同。
在 TOF 儀器上，離子每隔 50 μs 被推入飛行管一次（粗略估計取決于質(zhì)量數(shù)范圍和飛行管長度）。離子移動至檢測器，并根據(jù)其 m/z 進行分離。掃描時間越長，合并的推送越多，觀察到的信號也越強。
因此，在 TOF 儀器上同時采集過多的通道時，占空比會迅速下降，同時出現(xiàn)更多通道則會導(dǎo)致靈敏度變差。通常的建議是避免同時采用 4 個以上的通道。同時采用超過 4 個通道后，TOF-MRM 的靈敏度優(yōu)勢將消失，此時首選標(biāo)準(zhǔn) MSE 或 HDMSE 測量。
如果被監(jiān)測的通道具有已知的離散非重疊保留時間，則可以在諸如此類的 Tof-MRM 采集方法中為每個離子對定義特定的保留時間窗口。這樣，盡管監(jiān)控的通道總數(shù)大于 4，但同時監(jiān)控的通道數(shù)量將保持在較低水平。
What is the maximum number of transitions for a TOF-MRM method? - 英文版
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.
