10.4 Continuous Wave NMR Instrumentation

Transcription

1 10.4 Continuous Wave NMR Instrumentation coherent detection bulk magnetization the rotating frame, and effective magnetic field generating a rotating frame, and precession in the laboratory frame spin-lattice relaxation and T 1 spin-spin relaxation, dephasing and T 2 the 60 MHz cw NMR spectrometer 10.4 : 1/12

2 Coherent Detection Direct detection of absorption of NMR "photons" from an incoherent source is difficult because of saturation. Instead, the excess spins (N α - N β ) are coherently excited. The precession of the resultant coherent magnetic moment will induce an electric current in a wire coil. This current is at the precession frequency and easily detected. The unexcited spins do not produce a detectable current because the individual nuclei are precessing with random phase and average to zero. A coherent source is defined as one where every photon has an identical phase. In the UV visible a single-mode laser is required to make a coherent electric field. In the radio frequency region, a simple wire coil can be used to generate a coherent magnetic field : 2/12

3 Bulk Magnetization In the absence of radiation, the individual precession moments are randomly oriented along the precession circle. The vector sum of all of the α spins is a vector N α in magnitude pointing in the positive z-direction. The vector sum of all the β spins is a vector N β pointing in the negative z-direction. z, B N α y The resulting sum-vector is pointing along the positive z-axis with a magnitude of N α - N β. It is this bulk magnetization vector that needs to acted upon by the applied rf source. Note that the bulk magnetization is proportional to N. x N β 10.4 : 3/12

4 The Rotating Frame If the sinusoidal magnetic field (10-7 T) is to produce coherent precession, it must do so by moving the nuclei against the strong fixed field, B 0 (7 T), being used to split the spin states. How is this possible? A rotating coordinate system (u,v) can be used. If ν rot = 0, the nuclear magnetic moments appear to rotate at the Larmor frequency, given by γb 0. As ν rot is increased the nuclei appear to precess at a slower rate, e.g. as if they were in a field weaker than B 0. If ν rot = γb 0, the axes are rotating at the Larmor frequency. From the perspective of the rotating axes, the nuclear magnetic moments no longer seem to be precessing. That is, to anything in the rotating frame the nuclei behave as if they were in a zero strength external field. u x z, B v y 10.4 : 4/12

5 Effective Magnetic Field The differing rates of precession are accounted for by the concept of an effective field. ν1 Beff = B0 γ If a magnetic field, B 1, can be made to rotate at the Larmor frequency, B 0 will effectively go away and the nuclear magnetic moments can be made to rotate about B 1. If the Larmor frequency is ~300 MHz for a 7 T magnetic, the nuclear magnetic moment will precess around a 10-7 T rf magnetic field with a frequency of 4.26 Hz : 5/12

6 Generating a Rotating Frame Coils in the z,y-plane carry an electrical current oscillating at the Larmor frequency. The current induces a magnetic field, B 1, in the x-direction which is building and collapsing at the Larmor frequency. This linearly polarized magnetic field can be decomposed into two counter propagating circularly polarized fields. One of these is rotating in the same direction as precession around B 0 and can effect the NMR transition. The other is rotating counter to the precession and can't contribute to the signal (the magnetic field appears twice as strong, i.e. 2B 0 ). x B 1 z, B 0 coils in the y,z-plane y 10.4 : 6/12

7 Precession in the Laboratory Frame 10.4 : 7/12 As the net, bulk nuclear magnetic moment precesses around the rf field, it moves away from the z-axis. In the laboratory frame, this movement is seen as a precession along the surface of a sphere. For the example we have been using, the precession would be 300 MHz in the x,y-plane and 4.26 Hz in the z,x-plane. x µ N z, B 0 y detector coils To detect the induced precession, coils are placed in the x,z-plane with their center along the y-axis. As the precession moves down the spherical surface, the magnetic moment forms larger circles parallel to the x,y-plane. This circular component induces a 300 MHz current in the detector coils. The maximum detected current appears when the bulk magnetic moment is tipped 90. If the rf magnetic field continues to be applied, the vector will continuing working its way down the sphere until it points to -B 0. When this is achieved, all of the excess spins are in the β energy state and an inverted population is achieved.

8 Spin Lattice Relaxation Consider the situation where the rf was applied long enough to make N α = N β, and then the rf was turned off. There is a thermodynamic drive for N β /N α to return to the value predicted by the Boltzmann distribution. In the UV/visible portion of the spectrum, both spontaneous emission and dark processes play a role in thermalizing the two states. In the NMR region spontaneous emission is very unlikely and does not play a significant role. Spin-lattice relaxation is a dark process that transfers energy from the excited nucleus to other magnetic nuclei on the same molecule or in adjacent molecules. The field of the two spin systems must have equal E, but moving magnets have a wide distribution of frequencies (Doppler effect, ~ 1-3 ppm). This process converts β levels into α levels. It is called longitudinal relaxation since the change is along B 0. The relaxation time is called T 1 and has values from 10-2 to 10 2 seconds : 8/12

9 Spin-Spin Relaxation Spin exchange can occur between chemically equivalent nuclei. A(m I = +1/2) + B(m I = -1/2) ---> A(m I = -1/2) + B(m I = +1/2) This process yields no net change in energy, but the phases have been changed depending upon when the exchange takes place during the precession. Since nuclei with random phases are in a huge excess, it is most probable that spin exchange will occur with them. Thus, the coherent, excess spins will gradually assume the random phases of the bulk nuclei. This process is called dephasing : 9/12

10 Dephasing of Coherent Precession When coherent absorption occurs, all of the β nuclei are precessing about the external field with the same phase. As the spins dephase they spread out along the precession circle. This is called transverse relaxation and has the symbol, T 2. Although transverse relaxation doesn't change the value of N α - N β, it destroys the ability to detect the excess spins. For samples in fluid solution, transverse relaxation is faster than spin-lattice relaxation. In a CW instrument the power is adjusted to just balance the loss of the coherent signal : 10/12

11 60 MHz NMR 10.4 : 11/12 For proton resonance at 60 MHz the magnetic field is = B 0 B 0 = T The difference in γ values among elements is so large that only one type of nucleus is examined at a time. The permanent magnetic field is fixed so a different nucleus requires a different rf frequency. Thus, a 1.4 T field requires radiation at 56.5 MHz for 19 F. This requires a different probe and electronics. The exact energy of the NMR transition depends upon the electronic environment around the nucleus. The electrons shield the nucleus, in a structurally dependent way, so that the magnitude of the field at the nucleus is less than expected, B eff = B 0 (1 - σ) where σ is the shielding constant. An approximate maximum proton chemical shift is about 17 ppm, which corresponds to an effective magnetic field of T.

12 The Varian A-60 CW NMR In the cw instrument, the transmitter is fixed at 60 MHz and the magnetic field varied by the sweep coils. For a chemical shift range of 0-17 ppm, the sweep coils would need to add from 0 to T to the fixed magnetic field. This range will bring all chemically shifted protons into resonance with 60 MHz. transmitter coils magnet sample sweep magnetic coils receiver coils magnet The receiver power is measured as the coil field is swept across all the NMR resonances. To maximize the signal, the transmitter power is adjusted so that the absorption and relaxation rates are the same at the strongest absorption. The Varian A-60 spectrometer was introduced in 1961 and was sold until ~ : 12/12