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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.


Pulse sequence and coherencetransfer pathway diagram for a phase-modulatedsplit-t1 shifted-echo triple-quantum MASexperiment. The conversion of triple- to single-quantum coherencescan be carried out using a range of different types of pulses as shown.For the 87Rb experiments performed in this work k = 9/16, k′ = 7/16, and k″ = 0. The final pulse in the sequence (and theadditional pulse for SPAM) is chosen to be selective for the centraltransition.
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fig1: Pulse sequence and coherencetransfer pathway diagram for a phase-modulatedsplit-t1 shifted-echo triple-quantum MASexperiment. The conversion of triple- to single-quantum coherencescan be carried out using a range of different types of pulses as shown.For the 87Rb experiments performed in this work k = 9/16, k′ = 7/16, and k″ = 0. The final pulse in the sequence (and theadditional pulse for SPAM) is chosen to be selective for the centraltransition.

Mentions: Despite its extensive use, MQMAS does suffer from inherentlypoorsensitivity, owing to the need for filtration through multiple-quantumcoherences. This is a particular problem if significant quadrupolarbroadening is present, or for cases where only low radio-frequency(rf) field strength is available. Several methods have been developedto improve the efficiency of MQMAS, with particular attention focusedon the “conversion” pulse, i.e., the conversion of triple-to single-quantum coherences, as shown in Figure 1. DFS (double frequency sweep),11 FAM (fast amplitude modulated),12,13 SPAM (softpulse added mixing,)14,15 and HS (hyperbolic secant)16 pulses have all been shown to yield improvedefficiency over the use of a single high-power pulse for this coherencetransfer. Although significant sensitivity gains are possible, achievingthese in practice becomes more difficult as the complexity of thepulses used increases. SPAM offers a very simple approach to increasedsensitivity (involving the addition of a central-transition selective90° pulse after the high-power conversion pulse) and is thereforeeasy to optimize and to implement.14,15,17 More significant improvements can often be obtainedusing FAM pulses, which consist of a number of independent oppositelyphased high-power pulses, that can be applied in a “windowed”(FAM-I)12 or “windowless”(FAM-II)13 manner. In the latter case,the number of pulses applied is often restricted to two, leading alsoto a reasonably straightforward experimental optimization process.However, it has been demonstrated that in some cases better sensitivitycan be achieved by using a larger number of pulses, all of which varyin duration.13,18 However, this significantly increasesthe difficulty of the experimental multidimensional optimization,particularly for cases where sensitivity is limiting. The DFS approachintroduced by Kentgens and Verhagen can produce impressive gains insensitivity but can be more challenging to optimize in practice thansome of the simpler approaches.11,19


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)

Pulse sequence and coherencetransfer pathway diagram for a phase-modulatedsplit-t1 shifted-echo triple-quantum MASexperiment. The conversion of triple- to single-quantum coherencescan be carried out using a range of different types of pulses as shown.For the 87Rb experiments performed in this work k = 9/16, k′ = 7/16, and k″ = 0. The final pulse in the sequence (and theadditional pulse for SPAM) is chosen to be selective for the centraltransition.
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Pulse sequence and coherencetransfer pathway diagram for a phase-modulatedsplit-t1 shifted-echo triple-quantum MASexperiment. The conversion of triple- to single-quantum coherencescan be carried out using a range of different types of pulses as shown.For the 87Rb experiments performed in this work k = 9/16, k′ = 7/16, and k″ = 0. The final pulse in the sequence (and theadditional pulse for SPAM) is chosen to be selective for the centraltransition.
Mentions: Despite its extensive use, MQMAS does suffer from inherentlypoorsensitivity, owing to the need for filtration through multiple-quantumcoherences. This is a particular problem if significant quadrupolarbroadening is present, or for cases where only low radio-frequency(rf) field strength is available. Several methods have been developedto improve the efficiency of MQMAS, with particular attention focusedon the “conversion” pulse, i.e., the conversion of triple-to single-quantum coherences, as shown in Figure 1. DFS (double frequency sweep),11 FAM (fast amplitude modulated),12,13 SPAM (softpulse added mixing,)14,15 and HS (hyperbolic secant)16 pulses have all been shown to yield improvedefficiency over the use of a single high-power pulse for this coherencetransfer. Although significant sensitivity gains are possible, achievingthese in practice becomes more difficult as the complexity of thepulses used increases. SPAM offers a very simple approach to increasedsensitivity (involving the addition of a central-transition selective90° pulse after the high-power conversion pulse) and is thereforeeasy to optimize and to implement.14,15,17 More significant improvements can often be obtainedusing FAM pulses, which consist of a number of independent oppositelyphased high-power pulses, that can be applied in a “windowed”(FAM-I)12 or “windowless”(FAM-II)13 manner. In the latter case,the number of pulses applied is often restricted to two, leading alsoto a reasonably straightforward experimental optimization process.However, it has been demonstrated that in some cases better sensitivitycan be achieved by using a larger number of pulses, all of which varyin duration.13,18 However, this significantly increasesthe difficulty of the experimental multidimensional optimization,particularly for cases where sensitivity is limiting. The DFS approachintroduced by Kentgens and Verhagen can produce impressive gains insensitivity but can be more challenging to optimize in practice thansome of the simpler approaches.11,19

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.