Pulsed Fourier (7)

Question 1

What is the difference between CW and pulsed FT-NMR?

Answer 1

CW used before ~1970 ; FT after.

Question 2

What is the pulse width?

Answer 2

The useful or effective bandwidth

Question 3

What is the pulse amplitude?

Answer 3

measure of the power with which the pulse is applied and determines the strength of the B1 field

Question 4

What is an interpulse delay?

Answer 4

The time delay between pulses

Question 5

What are common values for the pulse width?

Answer 5

less than 10 microseconds

Question 6

What are the common interpulse delays?

Answer 6

1 to 10 microseconds

Question 7

What is a rotating frame of reference?

Answer 7

transformed coordinate system that rotates about the z axis at the Larmor frequency

Question 8

To what do the x, y, and z axes refer?

Answer 8

diagonal, horizontal, vertical in the static frame of reference

Question 9

to what do the x’, y’, z’ axes refer?

Answer 9

diagonal, horizontal, vertical in the rotating frame of reference

Question 10

How is the B1 field generated?

Answer 10

pulse amplitude

Question 11

How does the net magnetic moment change when excited by the B1 field?

Answer 11

experiences a torque that tips off the z’ axis

Question 12

What are some common pulse lengths?

Answer 12

90 degrees or π/2 rad; 180 degrees or π radians

Question 13

How does a 180° pulse affect Nα and Nβ?

Answer 13

exchanges the populations

Question 14

How does a 90° pulse affect Nα and Nβ?

Answer 14

equilibrates and creates phase coherence among dipoles resulting in net transverse magnetization

Question 15

How does a saturation pulse affect Nα and Nβ?

Answer 15

equilibrates; uniform, random distribution of dipoles around z

Question 16

How does the rate of precession of the net magnetic moment depend on B1?

Answer 16

if B1is on for 25 μs, then M0 completes only 1/4 revolution around x′axis,

Question 17

What is the free induction decay?

Answer 17

oscillating signal

Question 18

How is the FID generated?

Answer 18

RF signal emitted by the nuclei as they return to ground; signal oscillates as a the phase coherence is lost

Question 19

How is the FID detected?

Answer 19

converted into frequency domain spectra using Fourier

Question 20

What is a time domain signal?

Answer 20

NMR signal amplitude as a
function of time.

Question 21

What is a frequency domain signal?

Answer 21

amplitude vs frequency

Question 22

How are time domain and frequency domain signals related?

Answer 22

time domain converted into frequency domain

Question 23

What is the typical NMR lineshape?

Answer 23

Lorentzian

Question 24

how does spin-lattice relaxation increase magnetization along the z axis?

Answer 24

returns thermal equilibrium following perturbation

Question 25

How is the excess energy dissipated in T1 relaxation?

Answer 25

heat

Question 26

How does spin-spin relaxation increase magnetization in the x-y plane?

Answer 26

loss of phase coherence

Question 27

How is a 1D NMR experiment performed?

Answer 27

apply pulse, observe FID signal, transform FID into frequency domain

Question 28

How is the interpulse delay related to T1?

Answer 28

delay time after 90 degree pulse is 5 * T1

Question 29

How is signal-to-noise increased in a typical 1D NMR experiment?

Answer 29

need to wait for delay time so spin populations to return to thermal equilibrium

Question 30

What are multiplex pulse sequences used for?

Answer 30

measuring relaxation times; specialized