The key technique in solid-state NMR for obtaining highly resolved spectra is called Magic Angle Spinning (MAS): The sample is rapidly spun around an axis which is inclined at the angle of 54.7° with respect to the magnetic field. This mechanical sample spinning removes most of the anisotropic interactions which otherwise would cause strong line broadening and result in significant signal overlap. For comparison, in solution state NMR, highly resolved spectra are obtained without the need of mechanical sample rotation because the Brownian motion of the dissolved molecules usually fully removes any anisotropy.
MAS is implemented by placing the sample material in a so-called rotor (Figure 1), which is a hollow cylinder, usually made of ceramic material, equipped with a turbine cap. A MAS NMR probe contains a so-called stator (Figure 2) which consists of an air-bearing system to enable low-friction spinning of the rotor and a drive system to spin the rotor to the desired angular frequency by injection of a gas flow into the rotor’s turbine cap.
For an NMR measurement, the rotor with the sample substance is inserted into the stator inside the NMR probe, and gas is supplied to the bearing and drive systems to spin the rotor. The spinning speed is measured optically and precisely controlled by regulating the drive and bearing pressures. Bruker’s proprietary MAS rotor/stator systems are known for their exquisite performance when it comes to maximum speed and rotational stability, and have become a de-facto standard in many NMR labs.
The optimum rotation frequency depends on several factors, most importantly on the nucleus of interest and sample properties like molecular mobility or paramagnetism. Technically, the maximum achievable rotation frequency depends on the strength of the rotor ceramics which are subjected to significant stress from centrifugal forces and the fact that the rotor’s circumference speed must be below the speed of sound in the bearing gas. Rotors, therefore, are available in various diameters to provide the maximum sample volume for the required spinning speed as shown in the table below.
|Rotor Diameter [mm]||Max. MAS Speed [kHz]||Rotor Volume [µl] (Rounded)|
A second key technique in solid-state NMR is Cross-Polarization (CP), which is a spectroscopic technique in which nuclear magnetization is transferred from one type of nucleus to another via heteronuclear dipolar interactions. Typically, magnetization is transferred from protons to X nuclei such as 13C or 15N, which – due to the high abundance of protons and the resulting high polarization of the proton spin system – results in greatly enhanced sensitivity. The transfer of the magnetization is achieved by irradiating the sample with special RF pulses which are commonly called “contact pulses”. Bruker’s CPMAS probes have been optimized for RF performance such that optimum contact pulses are possible. Bruker’s CPMAS probes are known for their superior performance under all CP conditions, most importantly zero quantum, double quantum or double CP transfers.
Bruker’s CPMAS probes are available for standard bore (SB) and wide bore (WB) magnets.
Bruker’s newest CPMAS probes (Figure 3) capitalize on the renowned iProbe platform which incorporates several proprietary, patented technical solutions to achieve best performance, high reliability and superior usability. Each CPMAS iProbe is equipped with motors for automatic tuning, automatic matching, and automatic adjustment of the magic angle. Algorithms for tuning and matching, for the adjustment of the magic angle as well as for automatic shimming have been included in Bruker’s TopSpin software and enable both remote operation and full automation of solid-state NMR experiments.
CPMAS iProbes are compatible with Bruker’s MAS Shuttle, which transfers the NMR rotor with the sample substance into the stator inside the CPMAS iProbe. In combination with a sample case, this facilitates fully automatic and remote sample changes. It is not necessary to remove the NMR probe from the magnet for a sample change.
In Bruker’s CPMAS iProbes, special technical measures are taken to maximize the range of sample temperatures at which solid-state NMR experiments can be performed and to minimize temperature gradients along the rotor. For instance, in CPMAS iProbes, the bearing gas is temperature controlled, and the gas flow paths in the probe and in the magnet bore have been carefully optimized to enable a sample temperature of up to 200 °C.
Installing the iProbe inside the magnet – and removing the probe from the magnet when a different probe needs to be installed – is particularly simple and convenient due to a dedicated quick lock mechanism which connects the probe to the room temperature shim stack.
Bruker’s family of HCN CPMAS probes has been developed for biosolids applications such as the study of complex insoluble protein systems. The tuning range of the proton channel can be extended to include 19F. HCN probes are also available in a special “low-EF” variant. These probes are equipped with special radio frequency coils which combine maximum CP performance with low losses in dielectric samples. This helps to minimize sample heating and is thus particularly well suited for the study of proteins and other temperature sensitive substances.
Magic angle spinning at very high spinning rates enables new opportunities and new application fields in solid state NMR, since spectral lines originating from protons become sufficiently narrow at such high speeds. The high sensitivity which is achieved with proton detection enables investigations of large samples at natural abundance. Combined with the absence of a general limitation of molecular size in solid state NMR, the additional availability of sidechain protons opens new horizons for structural investigations.
Bruker’s 0.7 mm CPMAS probes are capable of spinning samples at a rate of 111 kHz and are specifically tailored for investigations of structure and dynamics of large and biologically important proteins. Application examples include the study of the membrane protein AlkL or the enzyme human carbonic anhydrase II. Some examples of highly resolved spectra of biological samples are shown in Figures 4 - 6.
Figure 6: Proton detected INEPT-based 13C HSQC correlation from a fully protonated solid biological sample (SH3 domain of chicken alpha-spectrin).
Bruker’s family of HX, HFX and HXY CPMAS probes have been developed for material science applications. Typical applications include research on catalyst materials and research on polymers, where order, cross-linking and other local connectivity characteristics can be observed.
Bruker’s HX, HFX and HXY CPMAS probes are available for a large number of different X-nuclei, both in the low-gamma and high-gamma regimes. Pharmaceutical applications profit from Bruker’s probes with 19F capability. In such pharmaceutical applications, for instance, it is possible to distinguish the different polymorphic forms of an active pharmaceutical ingredient, which enables quality control of starting materials as well as monitoring of the production process. Bruker’s CPMAS probes for material science applications are available for all rotor diameters shown in the table above, including 0.7 mm with 111 kHz spinning speed. Studies of systems with highly anisotropic features (e.g. paramagnetic environments) can benefit dramatically if studied using very fast MAS rates.
An application example is depicted in Figure 7 which shows the results of a MQMAS experiment on the aluminophosphate framework material AlPO4-14. The processed 2D spectrum can be used for the determination of the crystal structure, or – as shown in Figure 7 – impurities in the sample can be detected (red circle). Solid-state NMR techniques, e.g. experiments based on heteronuclear couplings such as HECTOR, can also be used to investigate the connectivities in the network of Al and P atoms in this material.
Figure 8 illustrates how material science experiments can benefit from higher MAS spinning rates. In this particular example, a lanthanide luminescent complex was investigated. Lanthanide luminescent materials have very interesting photophysical properties and have thus attracted attention for applications such as bio-imaging or for solid-state displays. Figure 8 shows how experiments on such highly paramagnetic samples can benefit from the fastest spinning speed. While spectral resolution is fully absent at low spinning speed (30 kHz), the different signals can be clearly identified at 111 kHz.
Figure 7: MQMAS spectrum of an AlPO4-14 sample. The spectrum shows four regular sites and one impurity (marked with the red circle) and illustrates a potential quality control application.
Figure 8: High-power, short-pulse excitation spectra of lanthanide luminescent complex at three different MAS spinning rates (30, 60 and 111 kHz) recorded on a 300 MHz spectrometer. This comparison shows how fast spinning can help in the analysis of compounds with large paramagnetic field contributions.