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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.


Related in: MedlinePlus

Plots showinghow the maximum central-transition single-quantumcoherence generated from unit triple-quantum coherence using a FAM-Npulse optimized at specific conditions varies with a change in (a)the inherent rf nutation rate, ω1/2π, and (b)the quadrupolar coupling constant, CQ.Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, withηQ = 0, ωR/2π = 12.5 kHz andin (a) CQ = 1.2 MHz with ω1/2π = 25, 50, 75, 100, 125, and 150 kHz, and in (b) ω1/2π = 114 kHz with CQ =0.3, 0.6, 1.2, 2.4, and 4.8 MHz. The maximum efficiency obtained usinga single pulse is shown by the black dotted line.
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fig6: Plots showinghow the maximum central-transition single-quantumcoherence generated from unit triple-quantum coherence using a FAM-Npulse optimized at specific conditions varies with a change in (a)the inherent rf nutation rate, ω1/2π, and (b)the quadrupolar coupling constant, CQ.Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, withηQ = 0, ωR/2π = 12.5 kHz andin (a) CQ = 1.2 MHz with ω1/2π = 25, 50, 75, 100, 125, and 150 kHz, and in (b) ω1/2π = 114 kHz with CQ =0.3, 0.6, 1.2, 2.4, and 4.8 MHz. The maximum efficiency obtained usinga single pulse is shown by the black dotted line.

Mentions: Despite the optionsoutlined above, it is instructive to considerhow the efficiency of FAM-N varies if the pulses are applied underdifferent conditions to those for which they were initially optimized(i.e., to investigate the robustness of the method to variation inexperimental or sample parameters). Figure 6a plots how the efficiency of a specific FAM-N pulse (optimized atthe inherent rf nutation rate shown) varies as the nutation rate atwhich the sequence is carried out changes. For example, the brightred line shows how the (simulated) efficiency of a FAM-N pulse optimizedusing ω1/2π = 100 kHz, ωR/2π= 12.5 kHz, CQ = 1.9 MHz, and ηQ = 0 varies when applied at different values of the rf fieldstrength. It can be seen that good performance is obtained over areasonable range of nutation rates (i.e., 90% of the maximum performanceis achieved over a range of 62 kHz). This suggests that small errorsin the rf calibration measurement (or inaccuracies in any estimation)should have little impact on the efficiency of the FAM-N pulse. However,in cases where sufficient signal can be obtained, it is also possibleto experimentally optimize the B1 fieldstrength used for the computer-generated FAM-N pulse to ensure maximumefficiency is obtained. Figure 6a demonstratesthat this process is not vital, and for samples where this is notfeasible in a reasonable time scale, an estimate of the inherent rfnutation rate is sufficient for successful implementation of FAM-N.One point to note from Figure 6a is that, althoughmaximum efficiency at a rf nutation rate of, for example, 100 kHzis achieved by using the FAM-N pulse generated with that value ofthe rf as an initial input, the maximum efficiency of each FAM-N pulseis actually found when that same pulse is applied at slightly highernutation rates (e.g., for the FAM-N pulse generated using ω1/2π = 100 kHz maximum efficiency for that specific pulseis found when ω1/2π = 110 kHz). This is mostlikely a result of the fact that small variations in rf nutation ratedo not significantly affect the type/length of optimized pulse, butthat the higher B1 field strength appliedcan result in a slightly higher general conversion efficiency.


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 showinghow the maximum central-transition single-quantumcoherence generated from unit triple-quantum coherence using a FAM-Npulse optimized at specific conditions varies with a change in (a)the inherent rf nutation rate, ω1/2π, and (b)the quadrupolar coupling constant, CQ.Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, withηQ = 0, ωR/2π = 12.5 kHz andin (a) CQ = 1.2 MHz with ω1/2π = 25, 50, 75, 100, 125, and 150 kHz, and in (b) ω1/2π = 114 kHz with CQ =0.3, 0.6, 1.2, 2.4, and 4.8 MHz. The maximum efficiency obtained usinga single pulse is shown by the black dotted line.
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fig6: Plots showinghow the maximum central-transition single-quantumcoherence generated from unit triple-quantum coherence using a FAM-Npulse optimized at specific conditions varies with a change in (a)the inherent rf nutation rate, ω1/2π, and (b)the quadrupolar coupling constant, CQ.Simulations were performed for a single 87Rb (I = 3/2) nucleus at B0 = 14.1 T, withηQ = 0, ωR/2π = 12.5 kHz andin (a) CQ = 1.2 MHz with ω1/2π = 25, 50, 75, 100, 125, and 150 kHz, and in (b) ω1/2π = 114 kHz with CQ =0.3, 0.6, 1.2, 2.4, and 4.8 MHz. The maximum efficiency obtained usinga single pulse is shown by the black dotted line.
Mentions: Despite the optionsoutlined above, it is instructive to considerhow the efficiency of FAM-N varies if the pulses are applied underdifferent conditions to those for which they were initially optimized(i.e., to investigate the robustness of the method to variation inexperimental or sample parameters). Figure 6a plots how the efficiency of a specific FAM-N pulse (optimized atthe inherent rf nutation rate shown) varies as the nutation rate atwhich the sequence is carried out changes. For example, the brightred line shows how the (simulated) efficiency of a FAM-N pulse optimizedusing ω1/2π = 100 kHz, ωR/2π= 12.5 kHz, CQ = 1.9 MHz, and ηQ = 0 varies when applied at different values of the rf fieldstrength. It can be seen that good performance is obtained over areasonable range of nutation rates (i.e., 90% of the maximum performanceis achieved over a range of 62 kHz). This suggests that small errorsin the rf calibration measurement (or inaccuracies in any estimation)should have little impact on the efficiency of the FAM-N pulse. However,in cases where sufficient signal can be obtained, it is also possibleto experimentally optimize the B1 fieldstrength used for the computer-generated FAM-N pulse to ensure maximumefficiency is obtained. Figure 6a demonstratesthat this process is not vital, and for samples where this is notfeasible in a reasonable time scale, an estimate of the inherent rfnutation rate is sufficient for successful implementation of FAM-N.One point to note from Figure 6a is that, althoughmaximum efficiency at a rf nutation rate of, for example, 100 kHzis achieved by using the FAM-N pulse generated with that value ofthe rf as an initial input, the maximum efficiency of each FAM-N pulseis actually found when that same pulse is applied at slightly highernutation rates (e.g., for the FAM-N pulse generated using ω1/2π = 100 kHz maximum efficiency for that specific pulseis found when ω1/2π = 110 kHz). This is mostlikely a result of the fact that small variations in rf nutation ratedo not significantly affect the type/length of optimized pulse, butthat the higher B1 field strength appliedcan result in a slightly higher general conversion efficiency.

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.


Related in: MedlinePlus