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Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR.

Colaux H, Dawson DM, Ashbrook SE - J Phys Chem A (2014)

Bottom Line: This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency.The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation.Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, EaStCHEM and Centre for Magnetic Resonance, University of St. Andrews , North Haugh, St. Andrews KY16 9ST, U.K.

ABSTRACT
The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed "too challenging".

No MeSH data available.


Plot of the variation in the maximum central-transitionsingle-quantumcoherence obtained from unit triple-quantum coherence using a FAM-Npulse as a function of ηQ. Results are shown eitherfor a FAM-N pulse generated using CQ =1.9 MHz and ηQ = 0 (black line) and applied at allvalues of ηQ or for a series of FAM-N pulses optimizedusing CQ = 1.9 MHz and each specific ηQ value (gray line). Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ωR/2π = 12.5 kHzand ω1/2π = 114 kHz.
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fig5: Plot of the variation in the maximum central-transitionsingle-quantumcoherence obtained from unit triple-quantum coherence using a FAM-Npulse as a function of ηQ. Results are shown eitherfor a FAM-N pulse generated using CQ =1.9 MHz and ηQ = 0 (black line) and applied at allvalues of ηQ or for a series of FAM-N pulses optimizedusing CQ = 1.9 MHz and each specific ηQ value (gray line). Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ωR/2π = 12.5 kHzand ω1/2π = 114 kHz.

Mentions: Although similar sensitivity enhancementswere observed using FAM-Nfor all Rb sites in RbNO3, all three do have similar CQ values. However, the value of ηQ differs significantly between the three. Figure 5 plots how the (simulated) efficiency of the FAM-Npulse used above (generated using ω1/2π = 114kHz, ωR/2π = 12.5 kHz, CQ = 1.9 MHz, and ηQ = 0) changes as the valueof ηQ for which it is applied varies. As shown bythe black line, a small decrease in efficiency is observed as ηQ increases. If different FAM-N pulses are generated for eachvalue of ηQ individually, this decrease is not assignificant (as shown by the gray line), but the maximum single-quantumcoherence generated still falls slightly with increasing ηQ. Table 1 compares the predicted efficiencyof different FAM-N pulses (and the corresponding signal obtained usinga single pulse) when applied in simulation to sites with CQ and ηQ values for the three Rb speciesin RbNO3. For a species with CQ = 1.9 MHz and ηQ = 0 the FAM-N pulse generatedusing these NMR parameters gives 306% of the signal of that usinga single pulse (i.e., a 206% enhancement). When this same pulse isapplied at the CQ/ηQ valuesfound for the three Rb sites in RbNO3, a decrease in efficiencyis found (with enhancements between 155 and 177% obtained). The signalobtained increases in all cases when the FAM-N pulse applied has beengenerated for the specific CQ values foreach site and increases further still when the pulse applied is generatedinitially using both CQ and ηQ values. However, in all cases, significant signal enhancementsover a single pulse are obtained.


Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR.

Colaux H, Dawson DM, Ashbrook SE - J Phys Chem A (2014)

Plot of the variation in the maximum central-transitionsingle-quantumcoherence obtained from unit triple-quantum coherence using a FAM-Npulse as a function of ηQ. Results are shown eitherfor a FAM-N pulse generated using CQ =1.9 MHz and ηQ = 0 (black line) and applied at allvalues of ηQ or for a series of FAM-N pulses optimizedusing CQ = 1.9 MHz and each specific ηQ value (gray line). Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ωR/2π = 12.5 kHzand ω1/2π = 114 kHz.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126738&req=5

fig5: Plot of the variation in the maximum central-transitionsingle-quantumcoherence obtained from unit triple-quantum coherence using a FAM-Npulse as a function of ηQ. Results are shown eitherfor a FAM-N pulse generated using CQ =1.9 MHz and ηQ = 0 (black line) and applied at allvalues of ηQ or for a series of FAM-N pulses optimizedusing CQ = 1.9 MHz and each specific ηQ value (gray line). Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ωR/2π = 12.5 kHzand ω1/2π = 114 kHz.
Mentions: Although similar sensitivity enhancementswere observed using FAM-Nfor all Rb sites in RbNO3, all three do have similar CQ values. However, the value of ηQ differs significantly between the three. Figure 5 plots how the (simulated) efficiency of the FAM-Npulse used above (generated using ω1/2π = 114kHz, ωR/2π = 12.5 kHz, CQ = 1.9 MHz, and ηQ = 0) changes as the valueof ηQ for which it is applied varies. As shown bythe black line, a small decrease in efficiency is observed as ηQ increases. If different FAM-N pulses are generated for eachvalue of ηQ individually, this decrease is not assignificant (as shown by the gray line), but the maximum single-quantumcoherence generated still falls slightly with increasing ηQ. Table 1 compares the predicted efficiencyof different FAM-N pulses (and the corresponding signal obtained usinga single pulse) when applied in simulation to sites with CQ and ηQ values for the three Rb speciesin RbNO3. For a species with CQ = 1.9 MHz and ηQ = 0 the FAM-N pulse generatedusing these NMR parameters gives 306% of the signal of that usinga single pulse (i.e., a 206% enhancement). When this same pulse isapplied at the CQ/ηQ valuesfound for the three Rb sites in RbNO3, a decrease in efficiencyis found (with enhancements between 155 and 177% obtained). The signalobtained increases in all cases when the FAM-N pulse applied has beengenerated for the specific CQ values foreach site and increases further still when the pulse applied is generatedinitially using both CQ and ηQ values. However, in all cases, significant signal enhancementsover a single pulse are obtained.

Bottom Line: This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency.The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation.Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, EaStCHEM and Centre for Magnetic Resonance, University of St. Andrews , North Haugh, St. Andrews KY16 9ST, U.K.

ABSTRACT
The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed "too challenging".

No MeSH data available.