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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.


Plots of the maximumcentral-transition single-quantum coherencegenerated from unit triple-quantum coherence using an optimized singlepulse (blue line), SPAM (orange line), FAM-II (green line), or FAM-N(red line) pulses as a function of (a) the quadrupolar coupling constant, CQ, and (b) the inherent rf nutation rate, ω1/2π. Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ηQ = 0, ωR/2π= 12.5 kHz, and (a) ω1/2π = 114 kHz or (b) CQ = 1.2 MHz. For SPAM, the optimization programwas limited to two pulses, with the second having ω1/2π = 10 kHz.
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fig3: Plots of the maximumcentral-transition single-quantum coherencegenerated from unit triple-quantum coherence using an optimized singlepulse (blue line), SPAM (orange line), FAM-II (green line), or FAM-N(red line) pulses as a function of (a) the quadrupolar coupling constant, CQ, and (b) the inherent rf nutation rate, ω1/2π. Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ηQ = 0, ωR/2π= 12.5 kHz, and (a) ω1/2π = 114 kHz or (b) CQ = 1.2 MHz. For SPAM, the optimization programwas limited to two pulses, with the second having ω1/2π = 10 kHz.

Mentions: Figure 3a compares the maximum single-quantumcoherence generated (in a simulation) from unit triple-quantum coherencefor some of the different types of conversion pulses, as a functionof the quadrupolar coupling constant (CQ). The simulation was carried out for a single 87Rb (I = 3/2) nucleus with ηQ = 0, at B0 = 14.1 T, subject to a pulse with ω1/2π = 114 kHz and with ωR/2π= 12.5 kHz. The conversion efficiency for a single pulse and for FAM-IIis easy to obtain from the FAM-N optimization process (as shown inFigure 2). To simulate a SPAM conversion pulse,the program described above was limited to just two pulses, the secondof which was set to have a much lower rf nutation rate (∼10kHz in this simulation). It is clear from Figure 3a that the efficiency of a single high-power pulse for theconversion of triple- to single-quantum coherences decreases significantlyas the quadrupolar coupling increases. The efficiency of SPAM is greaterthan that of a single pulse for all values of CQ considered, with maximum efficiency observed when CQ is ∼0.3 MHz. For a FAM-II conversionpulse (with two pulses) maximum efficiency is achieved at higher CQ values (∼0.7 MHz). Although the efficiencyof FAM-II decreases at higher values of the quadrupolar interaction,it is more efficient then either SPAM or a single pulse above ∼1MHz, but less efficient than SPAM at lower CQ. The FAM-N pulse has the highest efficiency of the methodsconsidered here at higher values of CQ, with significant improvements over a single pulse for values above1 MHz. Note that the efficiencies of FAM-N and FAM-II are identicalbelow ∼0.9 MHz, as in this regime it is found that N = 2, i.e., that FAM-II is the most efficient FAM-typepulse at low CQ values. Figure 3b shows a similar plot of conversion efficiency,but with CQ now fixed at 1.2 MHz and therf nutation rate varied. The conversion efficiency using a singlepulse increases as the rf nutation rate increases, and similar behaviorsare also observed for both SPAM and FAM-II. FAM-II exhibits an enhancementof a factor of ∼2 over that achieved with a single pulse formost of the parameter space considered. The conversion efficiencyobtained using FAM-N is the highest observed at each of the rf valuesconsidered, and remains fairly constant, rising slightly as the nutationrate increases. Note that at high values of the nutation rate theFAM-N and FAM-II pulses are formally identical.


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)

Plots of the maximumcentral-transition single-quantum coherencegenerated from unit triple-quantum coherence using an optimized singlepulse (blue line), SPAM (orange line), FAM-II (green line), or FAM-N(red line) pulses as a function of (a) the quadrupolar coupling constant, CQ, and (b) the inherent rf nutation rate, ω1/2π. Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ηQ = 0, ωR/2π= 12.5 kHz, and (a) ω1/2π = 114 kHz or (b) CQ = 1.2 MHz. For SPAM, the optimization programwas limited to two pulses, with the second having ω1/2π = 10 kHz.
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Related In: Results  -  Collection

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fig3: Plots of the maximumcentral-transition single-quantum coherencegenerated from unit triple-quantum coherence using an optimized singlepulse (blue line), SPAM (orange line), FAM-II (green line), or FAM-N(red line) pulses as a function of (a) the quadrupolar coupling constant, CQ, and (b) the inherent rf nutation rate, ω1/2π. Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, with ηQ = 0, ωR/2π= 12.5 kHz, and (a) ω1/2π = 114 kHz or (b) CQ = 1.2 MHz. For SPAM, the optimization programwas limited to two pulses, with the second having ω1/2π = 10 kHz.
Mentions: Figure 3a compares the maximum single-quantumcoherence generated (in a simulation) from unit triple-quantum coherencefor some of the different types of conversion pulses, as a functionof the quadrupolar coupling constant (CQ). The simulation was carried out for a single 87Rb (I = 3/2) nucleus with ηQ = 0, at B0 = 14.1 T, subject to a pulse with ω1/2π = 114 kHz and with ωR/2π= 12.5 kHz. The conversion efficiency for a single pulse and for FAM-IIis easy to obtain from the FAM-N optimization process (as shown inFigure 2). To simulate a SPAM conversion pulse,the program described above was limited to just two pulses, the secondof which was set to have a much lower rf nutation rate (∼10kHz in this simulation). It is clear from Figure 3a that the efficiency of a single high-power pulse for theconversion of triple- to single-quantum coherences decreases significantlyas the quadrupolar coupling increases. The efficiency of SPAM is greaterthan that of a single pulse for all values of CQ considered, with maximum efficiency observed when CQ is ∼0.3 MHz. For a FAM-II conversionpulse (with two pulses) maximum efficiency is achieved at higher CQ values (∼0.7 MHz). Although the efficiencyof FAM-II decreases at higher values of the quadrupolar interaction,it is more efficient then either SPAM or a single pulse above ∼1MHz, but less efficient than SPAM at lower CQ. The FAM-N pulse has the highest efficiency of the methodsconsidered here at higher values of CQ, with significant improvements over a single pulse for values above1 MHz. Note that the efficiencies of FAM-N and FAM-II are identicalbelow ∼0.9 MHz, as in this regime it is found that N = 2, i.e., that FAM-II is the most efficient FAM-typepulse at low CQ values. Figure 3b shows a similar plot of conversion efficiency,but with CQ now fixed at 1.2 MHz and therf nutation rate varied. The conversion efficiency using a singlepulse increases as the rf nutation rate increases, and similar behaviorsare also observed for both SPAM and FAM-II. FAM-II exhibits an enhancementof a factor of ∼2 over that achieved with a single pulse formost of the parameter space considered. The conversion efficiencyobtained using FAM-N is the highest observed at each of the rf valuesconsidered, and remains fairly constant, rising slightly as the nutationrate increases. Note that at high values of the nutation rate theFAM-N and FAM-II pulses are formally identical.

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.