Solid-State NMR Accessories

In Magic Angle Spinning (MAS), the rotor containing the sample substance is driven by a turbine. To enable the impressively high rotation rates of many kHz, gas bearings are used to support the rotor. Bruker’s MAS unit, now in its third generation, controls the gas flow to the drive turbine and the gas bearings. The MAS III unit, shown in Figure 1, also performs the spin rate measurement, and manages several other important functions such as rotor insertion and eject.

The MAS III unit is known for its seamless integration into Bruker’s spectrometer environment. For instance, depending on which type of probe is connected to the spectrometer, the correct drive and bearing pressure profiles are automatically selected to ensure smooth spin-up and stable rotation. In addition, the MAS III unit is equipped with a range of features which enhance reliability. For instance, the external gas and power supplies are monitored and in case of an outage, the MAS rotation is safely stopped such that rotor damage is avoided. To ensure sufficiently long autonomy, a gas buffer and an internal battery are included.

Download: MAS III Datasheet

Bruker’s MAS Shuttles, shown in Figure 2, transport MAS rotors to and from the sample volume inside the NMR probe. The shuttles are compatible with Bruker’s CPMAS and HRMAS iProbes. In combination with Bruker’s SampleCase, they facilitate automated operation, remote operation and help to increase throughput. The video below shows how Bruker’s MAS shuttle enables fully automated solid-state NMR.

Bruker offers a variety of different solutions for variable temperature (VT) control of solid-state NMR samples. The most used device is the BCU II Smart Cooler, shown in Figure 3. The BCU II provides chilled VT gas – typically dry air or nitrogen – at a minimum temperature of -80 °C and a maximum flow rate of 60 l/min to the NMR probe. Inside the NMR probe, the VT gas is then heated such that the sample temperature maintains the desired value. The BCU II uses compressor refrigerators to generate the required cooling power. The BCU II is recommended for NMR experiments with a sample temperature above 230 K and is typically used with room-temperature CPMAS probes, HR MAS probes and MAS CryoProbes.

When lower sample temperatures are required, a liquid nitrogen heat exchanger can be used to pre-cool the VT gas. Bruker offers a carefully engineered LN2 heat exchanger, shown in Figure 4. The heat exchanger can be installed inside a dewar filled with liquid nitrogen which serves as a cold source. The VT gas flow is routed through the heat exchanger where it is cooled. The liquid nitrogen heat exchanger is recommended for NMR experiments with a sample temperature above approximately 180 K. The heat exchanger package includes electronics required for temperature control.

Even lower sample temperatures are typically encountered in solid-state DNP experiments, or when LTMAS probes are used. For such applications, Bruker offers an LTMAS cooling cabinet to facilitate sample cooling to approximately 95 K. The LTMAS cooling cabinet, shown in Figure 5, also uses liquid nitrogen as a cooling source. To maximize cryogenic efficiency and to minimize temperature gradients along the rotor containing the sample substance, the LTMAS cooling cabinet provides cold VT gas, as well as cold gas streams for the MAS turbine and the rotor bearings.

Since pressurized nitrogen gas is used to drive the MAS turbine and to the supply the bearings with gas, special precautions need to be taken to avoid droplet formation in these gas streams when the gas is cooled to temperatures close to its dew point. Such droplet formation could lead to rotational instabilities, and – in the worst case – to damage of the MAS equipment. Bruker’s LTMAS cabinet thus uses an advanced heat exchanger concept which combines high thermal efficiency with superior stability and reliability of the MAS rotation.

When compromises on the minimum sample temperature can be made, DNP and LTMAS probes can also be used with a BCU II or with the LN2 heat exchanger. A special adapter, shown in Figure 6, has been developed for this purpose.

In solid-state NMR, external RF-filters are often used to remove unwanted noise and artifacts from the signal, to improve channel separation, and to enhance sensitivity. Bruker offers a wide variety of high-performance filters with the high-power ratings required for solid-state NMR.

One common type of electrical filter used in solid-state NMR is the low-pass filter, which allows signals below a certain frequency to pass through while attenuating signals above that frequency. Low-pass filters are useful for removing high-frequency noise and other artifacts from the NMR signal. Some low pass filters are equipped with traps which absorb undesired harmonics. Another type of electrical filter used in solid-state NMR is the band-pass filter, which allows signals within a certain frequency range to pass through while attenuating signals outside of that range. Bandpass filters are important in triple-resonance NMR experiments, e.g. HXY experiments, where the X-frequency needs to be blocked from the Y-channel and vice versa. In contrast to band-pass filters, band-stop filters attenuate signals within a certain frequency range while allowing signals outside of that range to pass through.

Diplexers are passive devices which multiplex two signals with different frequency bands from two separate ports onto a third port. Diplexers are used to connect two RF channels on the console to one RF channel in the NMR probe. For example, this is done on Bruker’s WB HFX solid-state NMR probes, where 1H and 19F are both connected to a combined 1H/19F type N-port on the solid-state probe by means of a diplexer.

The high isolation of Bruker filters allows the discrimination of very small NMR signals even in the presence of strong emission signals nearby (up to 120 dB isolation for bandpass filters). These exceptional characteristics are compulsory for high-quality NMR and exceed what is encountered in most other industrial RF applications.

Bruker’s filter manual contains details on the various filters which are available and lists their respective frequency ranges:

Bruker’s modular rotor test stand, shown in Figure 8, is used in conjunction with Bruker’s MAS probes to ensure that the rotor has properly been packed with the sample substance and spins stably and reliably, before inserting that rotor into a MAS probe. The rotor test stand consists of a base station and can be equipped with different modules for different rotor diameters. The rotor test stand is equipped with a port for VT gas – this is particularly important for temperature sensitive samples which can suffer when rotation tests are being performed without temperature control. The rotor test station is supplied with bearing and drive gas from the MAS III unit.