NMR7 Flashcards
(41 cards)
Why is it important to keep a high field homogeneity?
The radiofrequency and the chemical shift depend on the field strength and if the field strength has slight differences, it will create measurable differences in resonance frequency and chemical shift, depending on where the atom is located in the sample.
How can bad field homogeneity be seen in the NMR spectra?
- It can be seen on the peak, where their “smoothness” will be destroyed by observation of broad tails on every signal. In other words the peaks will get clearer and charper with good field homogeneity.
How can the field homogeneity be improved within a magnet?
By addition of coils.
What is coils/shim coils?
It is small peaces of metal added around the sample area to create field homogeneity and ophold the field magnetization.
How does coils work?
By sending more or less electricity through the coils an inhomgeneity will be created and used to cancel out the inhomogeneity in the magnetic field.
In which direction is it mostly critical/important to add shim coils?
It is mostly important to add coils in the z-direction because this is the longest (the vertical direction of the sample)
What is the field-frequency lock (deuterium lock)?
A method to hold the magnetic field constant (field stability). The lock will constantly monitor the field strength of the magnet and when it observes change in field strength it will try to couterbalance the change. This is done by following the deuterium signal of the solvent over time. If a change appears, a z_0 coil will regulate this imhomogeneity.
How is the temperature controlled in NMR?
By streaming N_2-gas under the sample to cool it down.
What is the formula for the frequency of a spinning device?
- ω=√(1/(L∗c))
L=inductivity
C=capacitance
What is a spinning device in solid state NMR?
A spinning device in solid-state NMR refers to the system that performs Magic Angle Spinning (MAS), where the sample is spun rapidly at a specific angle (54.7°) relative to the magnetic field.
What is the formula for the frequency given from the gyromagnetic ratio?
ω=2πγ=γ∗B
B=magnetic field strength
Insert the correct terms into the following sentence:
The__1__ is the part of the spectrometer that creates the radiofrequency waves and processes the signal from the sample. The radiofrequencypulses are sent by the__2__, and the signals are detected by the__3__
svarmuligheder for alle 3: console, probe, magnet, shim system, lock system.
1) console
2) probe
3) probe
The process where the frequency of the probe is adjusted to the desired resonance frequency, is called:
frequency-aligning, matching, locking, tuning, calibrating
tuning
Adjusting the impedance of the probe to the desired level is called:
impedance-aligning, matching, shimming, locking, tuning, calibrating
matching
The process of making the magnetic field homogeneous throughout the sample, is called:
homogeneity-aligning, matching, shimming, locking, tuning, calibrating
Shimming
When you measure the effect of a gradient, what will the sum of the magnetization vector be if the vectors point in one direction?
The sum of the vectors will show a signal equal to the direction they are pointing.
When you measure the effect of a gradient what will the sum of the magnetization vectors be if the vectors point in different directions?
The sum will be 0 because the signals from the different vectors will cancel eachother out. This is called dephasing.
What is refocusing in NMR?
When you have a dephased vector where no vector sum can be detected due to vectors pointing in different directions. These vectors can be “arranged” to show a signal by adding another gradient.
What is the rule of gradients and creation of signals?
In order to get at signal one gradient has to invert the effect of the other. The sum of the effect of all gradient should be 0 in order to observe magnetization.
How many T/m would 60 G/cm be?
1 Gauss (G) = 10⁻⁴ Tesla (T)
1 cm = 0.001 m
(60×10^-4)/0.001=0.6 T/m
The effect of a gradient depends on (more than one answer possible):
the Larmor frequency
the moon phases
the coherence order
the sign of the gradient
the sign of the magnetization (product operator)
the field strength
the field homogeneity
quality of tuning and matching
the length of the gradient
the phase of magnetization (i.e. whether magnetization is in x or y)
sample temperature
probe temperature
the Larmor frequency
the coherence order
the sign of the gradient
the sign of the magnetization (product operator)
the field strength
the length of the gradient
What is a DOSY-spectra?
It stands for Diffusion Ordered Spectroscopy and is a 2D NMR technique that separates signals based on how fast molecules diffuse in solution. It reveals information about molecular size, aggregation, and binding. Diffusion coefficients are extracted using an inverse Laplace transformation.
Solvents used in NMR are special in that….
they are very pure
they do not contain water
they are not magnetic
they are labelled with 2H
they are labelled with 13C
they are radioactive
they must not be poisonous
they fluoresce
they are metal-free
they are labelled with 2H
What is wobbling in NMR?
Wobbling describes the rapid, random tumbling of molecules in solution. This motion influences relaxation times (especially T₁ and T₂) and dipolar interactions, affecting NMR signal shapes and intensities. It plays a key role in NOE effects and molecular mobility assessments.
