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  Microcoil MAS
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[Background]

--- What is a microcoil? ---
A microcoil stands for a relatively small NMR coil having a volume of on the order of microliters. Roughly speaking, coils with diameters less than a millimeter are categorized into microcoils. You may have a feeling you are dealing with quite a tiny stuff, but you can manually wind a microcoil without much difficulty. Since the sensitivity of NMR measurements is relatively high when the sample volume is very small, a number of reports have been published regarding microcoil NMR measurements in volume-limeted liquid samples. Another interesting feature of a microcoil is that such strong rf irradiation is available that has so far been beyond hope, thus attracting interest also in solid-state NMR.

--- Magic Angle Spinning (MAS) inside a microcoil ---
Aiming at realizing magic-angle spinning (MAS) experiments using a microcoil, we have developed a new hardware. Our idea is to insert a capillary sample tube sticking out of an existing, commercially available rotor into a microcoil, as depicted in Fig. 1.

Note 1: Other groups had got the same idea and published nice papers on microcoil MAS[1-2].

Note 2: Another approach for realizing microcoil MAS has been proposed recently, in which a microcoil is embedded inside a rotor, and the rotor is spun together with the microcoil. This impressing technique is called as Magic Angle Coil Spinning (MACS) and has been published in Nature[3].



Figure 1. (a) An idea sketch by K.T. (drew in Apr 2004), and (b) the revised drawing.



[A coin-shaped microcoil MAS probe] [4-5]

--- Making a tiny probe as well as the coil! ---
In order to realize microcoil MAS based on the idea in Fig. 1 or Fig. 2(a), we decided to make a tiny probe as well as the coil, so that the probe could be attached to an existing MAS probe or MAS module with the minimum modification that allows microcoil MAS experiments as well as the conventional MAS experiments. After some trial and error, a coin-shaped probe shown in Fig. 2(b) has been developed. A microcoil with a diameter of 0.8 mm is put in hole drilled at the center of the disk-shaped circuit board, which can be attached to a Varian 4 mm spinning module (Fig. 2(c)-(d)). Then, the capillary sample tube (o.d.: 0.5 mm) stuck through a modified rotor cap is inserted into the microcoil, and stable spinning at up to 15 kHz was found to be possible.


Figure 2.


We have also successfully developed a doubly-tuned circuit at 470 MHz and 100 MHz without extending the board size.

[Application to MQMAS] [4]

--- Utilizing strong rf irradiation ---
Since a moderate power amplifier suffices to create very strong rf field inside a microcoil, a microcoil can open new applications in solid-state NMR. We are interested in applying microcoil MAS for the study of structure and properties of inorganic materials throuhgh NMR measurements of quadrupolar nuclei, which span ca. 70% of NMR active nuclei in the periodic table of atomic elements (see Fig. 3). The dynamics of the nuclear spin>1/2 depend on the quadrupolar interaction, which holds important ing information as to the local structure around the nucleus but often leads to spectral complexity, making data analyses formidable.

This, however, used to be the case until 1995, when a technique called as MQMAS was put forth[6].

--- Solid-state NMR of inorganic materials ---
The impact of the MQMAS (Multiple-Quantum Magic-Angle-Spinning) technique, put forward in 1995, was tremendous, making high-resolution solid-state NMR of half-integer quadrupolar nuclei possible with a conventioal solid-state NMR apparatus equipped with a MAS probe, and igniting a number of its application to inorganic solid materials.

Figure 3. (Red) quadrupolar nuclei. (Blue) Nuclei with spin 1/2. (Grey) Magnetically inert nuclei having spin zero. Note that this diagram does not take isotopes into account.



--- The problem is rather low efficiencies of MQ excitation/conversion ---
However, the sensitivity of the MQMAS measurement is rather low, which arises from the facts that
  • MQMAS requires rf irradiation under MAS to create multiple-quantum (MQ) coherence, and then to convert the MQ coherence into the observable single-quantum (SQ) coherence, and that
  • The process of MQ-excitation and MQ-to-SQ-conversion are quite inefficient.
It has been known that you need as strong rf irradiation as possible so as to optimize the MQ-excitation and MQ-to-SQ-conversion efficiencies. That is why we have been motivated to study the MQMAS sensitivity under strong rf irradiation with a microcoil!

--- Microcoil MQMAS ---
We have studies the efficiencies of MQ-excitation and MQ-to-SQ-conversion for 23Na spins in a polycrystalline sample of Na2(SO)3 for various rf intensities using the microcoil MAS probe shown above. We also propose an approximation formulas for the MQ-excitation and MQ-to-SQ-conversion, which is valid when the rf irradiation is stronger than the quadrupolar frequency. Using this formula, we could make a qualitative account for what we have observed, and propose the optimal combination of rf intensity and pulse width for a given quadrupolar coupling constant. Interested readers are encouraged to refer to our paper[4].

[References]

[1] H. Janssen, A. Brinkmann, E.R.H. van Eck, J.M. van Bentum, A.P.M. Kentgens, Microcoil high-resolution magic angle spinning spectroscopy, J. Am. Chem. Soc. 128 (2006) 8722-8723.

[2] K. Yamauchi, T. Asakura, Development of microMAS NMR probehead for mass-limited solid-state samples, Chem. Lett. 35 (2006) 426-427.

[3] D. Sakellariou, G. le Goff, J.-F. Jacquinot, High-resolution, high-sensitivity NMR of nanolitre anisotropic samples by coil spinning, Nature 447 (2007) 694-698.

[4] M. Inukai, K. Takeda, Studies on multiple-quantum magic-angle-spinning NMR of half-integer quadrupolar nuclei under strong rf pulses with a microcoil, Concepts in Magnetic Resonance 33B (2008) 115-123.

[5] K. Takeda, Coin-Sized Probes for Solid-State NMR, 46th ENC, 10-15 April 2005, Providence, RI, USA (poster presentation).

[6] A. Medek, J.S. Harwood, L. Frydman, Multiple-quantum magic-angle spinning NMR: a new method for the study of quadrupolar nuclei in solids, J. Am. Chem. Soc. 117 (1995) 12779-12787.


[Contributed by]
  Munehiro Inukai
  Kazuyuki Takeda

 

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