| 13 Jan 2026
Bimodal Q-band Probehead with Improved Signal-to-Noise Ratio in Pulse EPR
Abstract. In addition to the development of various resonators, the concept of a probehead equipped with an additional low noise amplifier (LNA) is becoming increasingly popular to enhance the sensitivity of EPR spectrometers. The low noise detection amplifier makes it possible to measure pulsed EPR signals with high sensitivity. However, a strong reflected pulse signal can cause saturation and deterioration of the LNA characteristics, which requires protection of the LNA (for example, by using a protection switch in front of the LNA), which in turn reduces the signal-to-noise ratio. To overcome these limitations, we propose using an EPR probehead based on a bimodal cavity with strong isolation between the input and output ports, in combination with a low noise amplifier connected to the cavity output. Experiments demonstrate 4-fold increase in the signal-to-noise ratio (SNR) compared to the reflection mode. Performance of the probe was also compared with the Bruker EN 5170 D2 probe available in our laboratory, which showed an improvement that can be achieved by increasing the SNR by 2 times due to additional LNA and isolation of the detection channel from the input signal, and by 3.3 times due to a larger sample volume in the bimodal probe (~20 µl) at Q-band frequencies compared to the Bruker one (~6 µl).
The developed probehead can be used together with commercial Bruker ELEXYS EPR spectrometers without modification of the microwave bridge.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Magnetic Resonance.
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General comments:
The paper describes performance of a bimodal Q-band resonator and use of a low-noise amplifier to improve the signal-to-noise of pulsed EPR measurements. The comparison will be of interest magnetic resonance spectroscopists,
Specific comments:
but the nature of the resonator is left to the imagination of the reader. No description of the resonator is provided other than “based on the design by James Hyde.” A diagram of the resonator with dimensions is essential to make this paper useful. Did the authors also build a replacement for the Bruker Flexline insert for the CF935, or did they build this new resonator into a standard Bruker Flexline resonator system?
The Bruker resonator with which the new resonator is compared is stated to be an ER5170D2 and that a 2.8 mm OD capillary was used in it. The Bruker resonator listing includes a model ER5107D2 that has a 2 mm diameter. Please explain the differences between these two models.
Please expand the discussion of the results in figure 4 to include the reasons for the widths of the echoes and whether the integrated intensities are more meaningful than the peak amplitudes.
The resonator Q factors should be stated.
Were echoes obtained with standard Bruker Xepr excitation, up-conversion and down-conversion to X-band, signal detection and digitization hardware and software?
In figure 5 the baseline region with lower noise should be explained.