With the introduction of the Zeno trap on the ZenoTOF 7600 system, sensitivity improvements on the order of 4 to 25x can be seen for MS/MS analysis, because of the optimization of duty cycle.1 When concentrations are low and peak intensities are limited, this bump in sensitivity is definitely desirable. But what about when peak intensities are already high, and MS/MS data already look great without the Zeno trap? Won’t the signal enhancement just saturate the detector? Zeno threshold to the rescue!
The Zeno threshold is an algorithm built into SCIEX OS software that activates and deactivates the Zeno trap on-the-fly during acquisition, based on signal intensity.
For targeted quantification experiments, this allows the MS/MS signal to be enhanced at the low end, to lower LLOQs without sacrificing linear dynamic range at the high end (when the signal enhancement isn’t needed). Even though the Zeno threshold control is hidden in the default view during method setup, it is always available when using the Zeno trap in an MRMHR experiment. The settings can be seen by showing the advanced parameters.
From the user input perspective, the Zeno threshold is simply set as a signal toggle. Below this threshold, the Zeno trap is activated; above the threshold, the Zeno trap is deactivated. During Zeno IDA acquisition, the signal intensity is determined by the triggered precursor sum intensity. When a high abundant precursor ion is selected for MS/MS that is above the threshold, the Zeno trap will not be activated. Conversely, if a low abundant precursor is selected, the Zeno trap will be activated to obtain a much more sensitive MS/MS.
During Zeno MRMHR acquisition, the signal intensity is determined from the base peak of the MS/MS spectrum from the previous scan. During Information Dependent Acquisition (IDA or DDA), the signal intensity of the precursor ion from the survey scan is used.
The software defaults have been thoroughly investigated so it is unlikely that you will need to change the default Zeno thresholds, but they are available under advanced settings just in case!
It is important, however, to perform the automatic Zeno calibration procedure during instrument tuning, to maintain linearity. With the Zeno threshold, data acquired when the Zeno trap is deactivated is automatically rescaled based on theoretical Zeno trap behavior and the real calibration data acquired during this tuning.
The Zeno threshold allows the full benefits of the sensitivity enhancements afforded by the Zeno trap to be leveraged when signal is limited, without increasing the potential for detector saturation and sacrificing linear dynamic range when signal is abundant.
- Qualitative flexibility combined with quantitative power. SCIEX technical note, RUO-MKT-02-13053-A.