bcmb/chem 8190 - university of...
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NMR Instrumentation
BCMB/CHEM 8190
Biomolecular NMR
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Instrumental Considerations - Block Diagram of an NMR Spectrometer
B0
Magnet
Probe
Lock
Receiver
Transmit
Computer
Sample
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Modern NMR Magnets are Super Conducting Solenoids
• Materials: NbTi < 10T, NbSn > 10T
• Max Field (2000): 23.5 T (900 MHz)
• Advantages: high field, stability, homogeneity > 1 : 109
B0 i
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Shim Coils
B0
Z
Inherent field profile
B0(z) = B0 + a1 z + a2 z2 + ….
Design coils to produce –a1 z, -a2 z2, etc ai are set by adjusting current in each of 16-40 coil sets – x, y, z, powers and cross terms
i i
z a linear z coil
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Room Temperature Shim Assemblies
From Modern NMR Techniques for Chemistry Research, Andrew E. Derome
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Shimming Origin: old electromagnets used shims under pole pieces
Interactive shimming
B0
Z
ν0
Auto shim: - based on amplitude of lock signal Gradient shim: observing effect of imposed field gradient –
can deconvolute field inhomogeniety
Adjust z2
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A Deuterium Lock Stabilizes the Field
X axis (absorptive)
Y axis (dispersive)
ω0 ω0
Drift of field will produce positive and negative signals Depending on direction when dispersive signal is observed
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Detection of NMR Signals: Probe Coils
E x
y
M
B’
E ∝ dB’/dt ∝ dM/dt ∝ γB0M0 = γ3B02h2I(I+1) / (12π2kT)
S/N ∝ (γC / γH)3 = 64-1
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NMR Probe
x y
M coils: to B0
dual role: -generate B1 -detect dMy/dt
solenoid coil: specialized applications
Helmoltz coil: normal applications
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Coil Design: Needs/Implications Requirements: 1). Maximize B1 from applied rf current
2). Maximize signal from M of sample
r
s
look at 2
So: -minimize coil size to increase S -maximize sample volume to increase M -these demands conflict one another (compromise)
surface area of coil
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Tuned Radiofrequency (RF) Circuits Improve Efficiency
Example: parallel LC (inductance/capacitance) circuit
tune
i R
-the circuit is resonant (tuned) when the impedance is purely resistive (χL= χC)
-the tuned resonance frequency is υ0
-Quality factors (Q) measure how much current is stored vs dissipated
– cryoprobes reduce resistance and loss Q ~ 1000
χL
χC
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What does an NMR Probe Look Like?
This is for a 7T magnet – 13C observe at 75 MHz
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Probes are delicate – glass, teflon, ceramic
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Receiver Functions
1. Amplify signals: 10 microV -> 1 V Three stages 30dB/stage dB = 10 x log(Pout/Pin) = 20 x log (Vout/Vin)
2. Shift Frequency: 100 MHz -> 1 KHz convenient digitization in the “rotating” frame often uses a “double-balanced mixer”
Signal in
Ref in
Signal out (low pass filtered)
Vout = a0Vin + a1Vin2 + …
2cos(ωs)cos(ωr) = cos(ωs-ωr) + cos(ωs+ωr)
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Quadrature Detection Allows distinction of +/- Frequencies
Sig
Ref + 90
Ref
Real out
Imaginary out
x y
x y = =
My
My
Mx Two detectors
One detector
Actually split signal And use two reference Frequency phases
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Why use Quadrature Detection? To put ωref in middle of spectrum.
ω0
No Quad
ω0
With Quad
Reduced band width of audio filter increases S/N by 1/√2