In order to get meaningful results from multiple-pulse NMR pulse sequences, it is essential that the 90° and 180° pulses are calibrated at the power levels used in the sequences (see this post for example). The calibrations are usually done on a standard sample in a well tuned and matched probe. The calibrations are typically stored in a file which is called up when setting up particular NMR experiments. It is important to know that these calibrations are correct for the particular sample of interest only when the probe is well tuned and matched. For samples of high ionic strength, it may not be possible to properly tune and match the probe and the 90° and 180° pulses for these samples will be longer than those previously calibrated, resulting in questionable data. In these cases, the pulses must be calibrated on the problematic sample. The figure below addresses the question of how important proper tuning and matching are with respect to the 90° pulse duration. The 1H 90° pulses for a sample HDO in a 500 MHz broadband probe were measured by the fast nutation method for various states of probe tuning and matching. In the left-hand side of the figure, pulses were calibrated for a perfectly matched probe as a function of tuning frequency. One can see that the 90° pulse is at a minimum when the probe is perfectly tuned and increases as the probe is detuned in either direction. In the right-hand side of the figure, pulses were calibrated for a perfectly tuned probe as a function of probe mismatch. One can see that the 90° pulse is at a minimum in a perfectly matched probe and increases as a function of the degree of mismatch (in units of screen divisions on the spectrometer display). It is interesting to note that the 90° pulse duration is more forgiving to mismatch than to errors in probe tuning.
[Question from NMRWiki Q&A forum] Tuning and matching problem on atma command
Tuning and matching problem on atma command
hello nmr wikier I am using Z-gradiant BBI probe on bruker AV-500 since last weak I am facing tuning and matching problem both by atma command and manually atmm command.ON atma command it is abroted in all standard sample and on atmm the coarse button did not arive only fine button active on proton channel while carbon channel work fine and befor reaching in mid of the curve the button reaches at end and curve goes away from center.I have tried by enlarging wbsw by 2o 40 60 but it is not working.I have also tried all command which are in BASH...
nmrlearner
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10-05-2011 08:57 PM
[Question from NMRWiki Q&A forum] Tuning probe failed after a dual probe was replaced with a BBI probe
Tuning probe failed after a dual probe was replaced with a BBI probe
We generally use Dual to run 13C and BBI to run 2D. After changed the probe, the command "edhead" was used to set the probe. Put the sample tube, lock the solvent, and then type "atma" to tune the probe. We always do it like this, but now we can not tune the proton after installed the BBI probe (13C is OK). The dip can not be found by "atma", and "atmm" was also not work on forming a dip. What is the most possible reason for this error? How to solve it and avoid it in the future ? Thanks. (Instrument: Bruker 400 MHz,...
nmrlearner
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08-23-2011 05:31 PM
[U. of Ottawa NMR Facility Blog] Shaped Pulses
Shaped Pulses
Shaped rf pulses are used frequently in modern NMR experiments for selective excitation and more efficient inversion. The figure below shows some of the pulse shapes in the Bruker shape library measured with an oscilloscope on an AVANCE III console. Each 50.3 MHz rf pulse was 1 msec in duration and was measured at the output of the signal generation unit.http://2.bp.blogspot.com/--GjbnC0soco/TeU9TDXS7nI/AAAAAAAAA4c/Kt3DwPKLjx8/s400/shaped_pulses_scope.jpghttps://blogger.googleusercontent.com/tracker/3300702123878659843-4230712340261495616?l=u-of-o-nmr-facility.blogspot.com...
nmrlearner
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05-31-2011 11:41 PM
[U. of Ottawa NMR Facility Blog] FT NMR Spectra Without Pulses
FT NMR Spectra Without Pulses
An FT NMR spectrum is obtained by applying a pulse at the Larmor frequency to a sample in a magnetic field. The precession of the spins induces a voltage in the receiver coil which is recorded as a function of time. The Fourier transform of the time dependent signal is the NMR spectrum. What happens if you do not provide any pulses? You might think that you would not observe a signal - but this is not the case. Even without any pulses there is sufficient noise present to allow incoherent precession of the nuclear spins. This precession can be measured and...
nmrlearner
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04-19-2011 02:44 AM
[U. of Ottawa NMR Facility Blog] Excitation Profiles for Shaped Pulses
Excitation Profiles for Shaped Pulses
Shaped pulses are very commonly used for selective excitation and nonselective inversion in a large number of NMR pulse sequences. The frequency domain excitation profile of a radio frequency pulse is the Fourier transform of the time dependent pulse shape and determines the width, uniformity and phase of the frequency spectrum excited. Since time and frequency are reciprocals of one another, short rf pulses have very wide excitation profiles and long rf pulses have very narrow selective excitation profiles. In a previous BLOG post the excitation...
nmrlearner
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01-21-2011 03:31 AM
Signal enhancement in protein NMR using the spin-noise tuning optimum.
Signal enhancement in protein NMR using the spin-noise tuning optimum.
Signal enhancement in protein NMR using the spin-noise tuning optimum.
J Biomol NMR. 2010 Oct 6;
Authors: Nausner M, Goger M, Bendet-Taicher E, Schlagnitweit J, Jerschow A, Müller N
We have assessed the potential of an alternative probe tuning strategy based on the spin-noise response for application in common high-resolution multi-dimensional biomolecular NMR experiments with water signal suppression on aqueous and salty samples. The method requires the adjustment of the...
nmrlearner
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10-07-2010 10:33 AM
[U. of Ottawa NMR Facility Blog] Fast 90 Degree Pulse Determination
Fast 90 Degree Pulse Determination
Almost all NMR measurements rely on the correct calibration of 90° pulses. This is traditionally done by collecting a series of spectra as a function of pulse duration, finding a null for the 180° or 360° pulse and calculating the 90° pulse by simple division by 2 or 4 in the case of the 180° and 360° nulls, respectively. This determination, although trivial, can be very time consuming. Wu and Otting* have presented a much faster method of determining a 90° pulse based on measuring the nutation of a magnetization vector directly. Continuous nutation is...