【质谱】TOF-MRM 方法的最大通道数（the maximum number）是多少？
菜鸟博士
TOF-MRM 方法的最大通道数是多少？- 中文版本
环境
Xevo G2-S
Xevo G2-XS
Synapt G2-S
Synapt G2-Si
Synapt XS
Vion IMS QTof
答案
使用三重四极杆仪器时，通常会采集大量通道。但是，无法在 TOF 仪器上进行类似实验，因为 TOF 和三重四极杆检测器的工作方式差异很大。
在三重四极杆仪器上，光电倍增管可测量离子电流。强度（测得的离子流量）不随扫描时间变化。这类似于使用电压表测量电池的电压。如果测量持续 1 秒或 10 秒，则读数相同。
在 TOF 仪器上，离子每隔 50 µs 被推入飞行管一次（粗略估计取决于质量数范围和飞行管长度）。离子移动至检测器，并根据其 m/z 进行分离。扫描时间越长，合并的推送越多，观察到的信号也越强。
因此，在 TOF 仪器上同时采集过多的通道时，占空比会迅速下降，同时出现更多通道则会导致灵敏度变差。通常的建议是避免同时采用 4 个以上的通道。同时采用超过 4 个通道后，TOF-MRM 的灵敏度优势将消失，此时首选标准 MSE 或 HDMSE 测量。
如果被监测的通道具有已知的离散非重叠保留时间，则可以在诸如此类的 Tof-MRM 采集方法中为每个离子对定义特定的保留时间窗口。这样，尽管监控的通道总数大于 4，但同时监控的通道数量将保持在较低水平。
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.
