Speaker
Description
A 9.4 T cryogen-free system developed by Cryogenic Limited has been used for the high resolution solid-state magic angle spinning (MAS) and liquid-state NMR experiments. The temporal magnetic field distortion due to the cold head operation was 2 Hz peak-to-peak on the resonance frequency 400 MHz [1]. This distortion occurred at the frequency of 1.7 Hz in our experimental setup.
In solid-state NMR measurements, the distortion appears as additional line broadening that is too small to be visible in ordinary MAS experiment. To see the effect, the amplitude of the distortion was enhanced by a factor of 40 above its natural value. Notably, if the NMR signal only lasts a small fraction of the cold head period, for instance, 100 ms, the effect of the cold head can be fully removed by synchronizing the signal acquisition with the cold head operation. In this case the distortion amplified by 400 times remained invisible. This approach was successfully demonstrated through our experiments in recording the solid state multi-quantum 2D spectra of 87Rb in RbNO3. The spinning speed was 11 kHz.
In liquid-state NMR measurements, however, the signal acquisition time normally covers several cold head periods. In this case, the additional peaks separated by the cold head frequency appear in the NMR spectra. The sweep coil of the room-temperature (RT) shim set was used for the correction (or amplification as above) of the cold head distortion. Our experimental observations showed that the amplitude of the distortion needs to be reduced by a factor of 10 to stop being important in liquid-state NMR.
In addition, we have demonstrated that applying reference deconvolution post-acquisition software processing [2] helps remove the field instability artifacts from NMR data, in both 1D and 2D NMR. We have also developed a method to stabilize the magnetic field in a short period of time following a field ramp [3], enabling high resolution measurements at different fields every day. We have measured four double quantum (DQ) filtered COSY spectra in ethyl acetate at 50, 100, 155 and 400 MHz. These spectra were measured at different fields in four consecutive days.
Currently, we are developing a dedicated liquid-state NMR system based on a cryogen-free magnet with an improved design. This next-generation system aims to eliminate the need for field corrections or post-acquisition processing, thereby improving both measurement accuracy and operational efficiency.
References
1. E. Kryukov et al, SS NMR, 113(2021)101732
2. A. Karabanov et al, JMR, 353(2023)107494
3. E. Kryukov et al, SS NMR, 109(2020)101684
