Method for NMR spectroscopy
A method for performing magnetic resonance spectroscopy on solid samples containing nuclei of interest with spin quantum number I subjects the sample to a static magnetic field. The sample is spun at the magic angle and broad-band excitation of transverse magnetization of the nuclei of interest is effected by applying a first train of rotor-synchronized rf-pulses, having a carrier frequency, to the nuclei of interest with a pulse duration 0.1 μs<τ p <2 μs, the first train of rf-pulses comprising k·n pulses extending over k rotor periods τ rot with n pulses per rotor period τ rot , wherein n is an integer n>1. Uniform excitation of a great number of spinning sidebands or families of sidebands that arise from large first-order quadrupole or hyperfine interactions is enabled and signal intensity is thereby improved.
1. A method for performing magnetic resonance spectroscopy on a solid sample containing nuclei of interest having spin quantum number I, the method comprising the steps of:
a) subjecting the sample to a static magnetic field;
b) subjecting the sample to magic angle spinning, thereby averaging out first-order components of second-rank tensor interactions; and
c) exciting, in a broad-band manner, a transverse magnetization of the nuclei of interest by applying a first train of rotor-synchronized rf-pulses, having a carrier frequency, to the nuclei of interest with a pulse duration 0.1μs<τ p <2μs, the first train of rf-pulses comprising k·n pulses extending over k rotor periods τ rot with n pulses per rotor period τ rot , wherein n is an integer n> 1 .
2. The method of claim 1 , wherein the pulse duration τ p is equal for all k·n pulses of the first train of pulses.
3. The method of claim 1 , wherein the nuclei of interest are nuclei with a nuclear quadrupolar moment having spin quantum number I=1, 14N nuclei or 2D nuclei.
4. The method of claim 1 , wherein the nuclei of interest are quadrupolar nuclei having spin quantum number I=3/2, 5/2 or 7/2.
5. The method of claim 1 , wherein the nuclei of interest have spin quantum number I=½, are 13C or are 15N.
6. The method of claim 5 , wherein the nuclei of interest are comprised in paramagnetic samples, spectra of the nuclei of interest thereby being broadened by hyperfine interactions with unpaired electrons.
7. The method of claim 1 , wherein a reconversion of transverse magnetization into longitudinal magnetization is carried out by applying a second train of short rotor-synchronized rf-pulses, the first train and the second train being separated by a variable evolution interval t 1 .
8. The method of claim 7 , wherein the first and the second train are equal with respect to a number n·k of pulses, a number k of rotor periods, a number n of pulses per rotor period and the pulse duration τ p .
9. The method of claim 7 , wherein excitation of the nuclei of interest by the first train of short rotor-synchronized rf-pulses is carried out subsequent to the following steps:
a1) exciting transverse magnetization of spy nuclei with spin quantum number S=½; and
b1) waiting an excitation interval τ exc , either without applying rf-irradiation or with applying a multiple-pulse recoupling sequence to recouple dipolar interactions between the spy nuclei S and the nuclei of interest I, wherein the first train of short rotor-synchronized pulses is applied with a suitable carrier frequency immediately after the excitation interval τ exc .
10. The method of claim 9 , wherein, after the second train of rotor-synchronized rf-pulses, reconversion of magnetization of the nuclei of interest I to transverse magnetization of spy nuclei S is achieved during a reconversion interval τ rec .
11. The method of claim 9 , wherein the evolution interval t 1 is split into two equal intervals by a refocusing 180° pulse applied to S spins of the spy nuclei in order to refocus a frequency offset of the S spins of the spy nuclei with respect to the rf carrier frequency.
12. The method of claim 11 , wherein durations of two halves of a split interval t 1 are incremented in steps of τ rot /2n to achieve a greater bandwidth in an indirect dimension and to reduce losses due to signal decay in the interval t 1 .
13. The method of claim 7 , wherein the evolution interval t 1 is incremented in a manner of heteronuclear single—or multiple-quantum two-dimensional correlation spectroscopy.
14. The method of claim 1 , wherein excitation of the nuclei of interest by the first train of short rotor-synchronized rf-pulses is carried out subsequent to the following steps:
a1) exciting transverse magnetization of spy nuclei with spin quantum number S=½; and
b1) waiting an excitation interval τ exc , either without applying rf-irradiation or with applying a multiple-pulse recoupling sequence to recouple dipolar interactions between the spy nuclei S and the nuclei of interest I, wherein the first train of short rotor-synchronized pulses is applied with a suitable carrier frequency immediately after the excitation interval τ exc .
15. The method of claim 14 , wherein step a1) comprises cross-polarization from protons to the spy nuclei.