Pulsed Fourier (7) Flashcards
What is the difference between CW and pulsed FT-NMR?
CW used before ~1970 ; FT after.
What is the pulse width?
The useful or effective bandwidth
What is the pulse amplitude?
measure of the power with which the pulse is applied and determines the strength of the B1 field
What is an interpulse delay?
The time delay between pulses
What are common values for the pulse width?
less than 10 microseconds
What are the common interpulse delays?
1 to 10 microseconds
What is a rotating frame of reference?
transformed coordinate system that rotates about the z axis at the Larmor frequency
To what do the x, y, and z axes refer?
diagonal, horizontal, vertical in the static frame of reference
to what do the x’, y’, z’ axes refer?
diagonal, horizontal, vertical in the rotating frame of reference
How is the B1 field generated?
pulse amplitude
How does the net magnetic moment change when excited by the B1 field?
experiences a torque that tips off the z’ axis
What are some common pulse lengths?
90 degrees or π/2 rad; 180 degrees or π radians
How does a 180° pulse affect Nα and Nβ?
exchanges the populations
How does a 90° pulse affect Nα and Nβ?
equilibrates and creates phase coherence among dipoles resulting in net transverse magnetization
How does a saturation pulse affect Nα and Nβ?
equilibrates; uniform, random distribution of dipoles around z
How does the rate of precession of the net magnetic moment depend on B1?
if B1is on for 25 μs, then M0 completes only 1/4 revolution around x′axis,
What is the free induction decay?
oscillating signal
How is the FID generated?
RF signal emitted by the nuclei as they return to ground; signal oscillates as a the phase coherence is lost
How is the FID detected?
converted into frequency domain spectra using Fourier
What is a time domain signal?
NMR signal amplitude as a
function of time.
What is a frequency domain signal?
amplitude vs frequency
How are time domain and frequency domain signals related?
time domain converted into frequency domain
What is the typical NMR lineshape?
Lorentzian
how does spin-lattice relaxation increase magnetization along the z axis?
returns thermal equilibrium following perturbation
How is the excess energy dissipated in T1 relaxation?
heat
How does spin-spin relaxation increase magnetization in the x-y plane?
loss of phase coherence
How is a 1D NMR experiment performed?
apply pulse, observe FID signal, transform FID into frequency domain
How is the interpulse delay related to T1?
delay time after 90 degree pulse is 5 * T1
How is signal-to-noise increased in a typical 1D NMR experiment?
need to wait for delay time so spin populations to return to thermal equilibrium
What are multiplex pulse sequences used for?
measuring relaxation times; specialized
